Natural Antimicrobial Compounds in Veterinary Medicine: Focus on Companion Animals

Abstract

Companion animals, including dogs and cats, share close living environments with humans, making antimicrobial stewardship essential to prevent zoonotic transmission of resistant pathogens. The overuse and misuse of conventional antibiotics in veterinary medicine have accelerated the emergence of multidrug-resistant (MDR) microorganisms, prompting the need for alternative strategies. Natural compounds, such as antimicrobial peptides (AMPs), phytochemicals, chitosan-based polymers, and nutraceuticals, offer promising solutions due to their broad-spectrum activity, low resistance potential, and additional health-promoting properties. This review provides a comprehensive analysis of recent advances of the aforementioned compounds for companion animals, including their mechanisms of action, applications in feed and nutraceuticals, and therapeutic use in dermatological, gastrointestinal, and systemic infections. We discuss the current challenges related to bioavailability, safety, standardization, and regulatory frameworks, as well as future perspectives for integrating these agents into veterinary practice. Emphasis is placed on clinical evidence in dogs and cats, highlighting how natural antimicrobials can contribute to sustainable infection control and antimicrobial resistance mitigation under the One Health paradigm.

1. Introduction

Antimicrobial resistance (AMR) has been recognized as one of the most pressing global health challenges of the 21st century [1]. It threatens human and animal health, food security, and the environment, leading to severe economic and public health consequences [2,3]. The One Health approach highlights the strong connections between human, animal, and environmental health in the context of AMR, recognizing that resistance genes can be shared across species and ecosystems [4].

Antibiotics are very often administered in companion animals and owners themselves require their usage: this can happen due to the fact that the number of pets has been growing substantially over the last decades and people asks for the same level of care and cure for their pets as would be expected for a family member [5,6,7]. Considering the affectionate relationship and the shared lifestyles, habits, and spaces, it is hard to explain to owners that companion animals represent a crucial point in the transmission of AMR through direct contact, bites, scratches, and licks [7,8,9,10]. It must be also underlined that, over recent years, the use of antibiotic drugs has been indiscriminate: overuse and misuse have greatly enhanced AMR, causing strong selective pressure with reduced or absent sensitivity to several antimicrobials [11,12]. This has compromised the therapeutic success of antimicrobials, wasting money and time for pet owners, decreasing their confidence in veterinary medicine, and leading to an increase in mortality rate for sick patients [3,11]. Several investigations have been focused on the most worrying resistant bacteria shared among pets and humans; according to the most recent literature search, methicillin-resistant Staphylococcus pseudintermedius (MRSP), methicillin-resistant Staphylococcus aureus (MRSA), Enterococcus faecium and faecalis, and ESBL are the most diffused and shared resistant strains [10,13].

The facilitation and amplification of AMR in pets and owners is related to the increase in long life expectancy, the presentation of similar pathologies, and to the administration of similar therapeutic protocols using the same classes of antibiotics. These factors, associated with the above-presented considerations, highlight the importance of concerns about the use of the highest-priority critically important antimicrobials (HP-CIAs) that should be reserved for life-threatening human pathologies caused by bacteria [14,15]. In recent years, there has been a widespread misconception that the problem of AMR was primarily linked to antibiotic use in food-producing animal livestock. Today, however, companion animals are also acknowledged as contributors to the spread of this phenomenon [10,16,17,18]. This sector-specific perspective is also reflected in legislation: for example, the European Commission Decision 2013/652/EU [19] focuses on assessing the risk of the transmission of commensal and pathogenic bacteria from food-producing animals to humans, whereas no equivalent regulations exist for pets [20]. Once again, a One Health approach emerges as the only effective strategy for addressing the serious global threat of AMR, emphasizing the improved management of vulnerabilities such as the need for globally harmonized and standardized legislation to regulate antibiotic use and monitor antibiotic resistance [9,13]. It is well-known that no brand new antibiotics will be marketed in the next years to treat veterinary patients; the sole strategy going forward is to use antibiotics in a prudent and rational way: veterinarians should limit the use of antibiotic drugs only in case of clearly diagnosed infections sustained or complicated by bacteria, choosing first the drugs labelled for the target species and the specific pathology, precisely considering the spectrum of action [6,21]. According to the most recent European Legislation, if no antibiotic drugs are available in veterinary medicine, the veterinarian can prescribe an off-label antibiotic according to the cascade rule, including antimicrobial drugs for human use or prescribing a galenic formulation [6,22]. Often in clinical practice, veterinarians may decide to set up empirical therapy because of the life-threatening conditions of a patient. In these limited cases, it could be better to begin the treatment using a narrow-spectrum antibiotic, giving preference to the drugs that have a low impact in selective pressure [7,15,22,23,24].

Veterinarians play a key role in the correct management of antibiotics and education of people, considering the lack of awareness among the pets’ owners about the possible risks of sharing resistant pathogens [16,17,25]. Moreover, owners require more and more alternative and natural approaches for themselves and also for their pets, thus it is not surprising the increasing request of alternative therapies [26].

Among the most promising alternatives are natural antimicrobial compounds, which include antimicrobial peptides (AMPs), plant-derived phytochemicals, polysaccharides like chitosan, and nutraceuticals. These substances exhibit diverse mechanisms of action (such as membrane disruption, biofilm inhibition, quorum sensing interference, and immune modulation) making them attractive candidates for controlling infections while minimizing resistance development [26,27,28].

In the companion animal sector, the application of natural antimicrobials currently spans in various domains:

• Therapeutic use: topical treatments for skin infections, otitis externa, oral infections, and wound management.
• Nutritional strategies: inclusion in pet diets as functional feed additives or nutraceuticals to promote gut health, modulate microbiota, and prevent gastrointestinal infections.
• Preventive applications: coatings for veterinary devices, dental implants, and hygiene products.

Despite their potential, several challenges hinder widespread adoption, including variability in natural compound composition, bioavailability issues, safety concerns at higher doses, and lack of standardized regulatory frameworks [29,30].

This review aims to provide a comprehensive overview of advances in natural compounds having antibiotic properties for companion animals, with an emphasis on evidence-based applications, mechanistic insights, and translational perspectives for sustainable veterinary practice.

2. Natural Antimicrobial Compounds: Classes and Mechanisms

A wide array of bioactive molecules derived from plants, animals, and microorganisms can demonstrate antimicrobial properties. Their efficacy is mediated through various mechanisms, often acting on multiple targets, which limits the likelihood of resistance development compared to conventional antibiotics [31].

Various innovative strategies are under investigation to complement or replace conventional antibiotics. These include the repurposing of existing antimicrobial drugs, as well as the use of natural compounds like plant-derived essential oils, probiotics or substances exhibiting both antibacterial and anti-inflammatory effects. Moreover, novel technologies such as silver and sulfur-based nanoparticles, iron-chelating agents, and topical antiseptics (e.g., chlorhexidine, olanexidine, and sodium hypochlorite) have shown promise for localized infections [32]. The role and the possible applications of the different compounds in the field of companion animal medicine will be explained in further sections. Nevertheless, readers will appreciate that few clinical studies have been performed in recent years and that the majority of the available information is derived from review and in vitro studies (Figure 1). In order to ease our navigation across the different references and compounds, please refer to

Table 1. The different references used in the present review, arranged according to the compound that was the focus of the investigation.

Compound	Reference Number
Alkaloids	[33,34,35]
Antimicrobial peptides	[27,29,31,36,37,38,39,40,41,42,43,44,45,46,47,48,49]
Biotics	[30,50,51,52,53,54,55]
Catechins	[56]
Cranberry	[50,57]
Curcumin	[58,59,60]
Ensolisin	[61,62]
Essential oils	[26,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80]
Flavonoids	[81,82,83,84]
Lactoferrin	[46,62,85,86,87,88,89,90,91,92,93,94,95]
Lactoperoxidase	[90]
Lysozime	[88,94,96,97]
Photodynamic compounds	[98,99]
Saponins	[100,101]
Spirulin	[102]
Tannins	[103,104,105,106,107,108,109,110,111,112]
Totarol	[113,114,115]

.

2.1. Antimicrobial Peptides (AMPs)

Antimicrobial peptides represent a promising approach for treating several pathologies since they exert rapid and broad-spectrum actions against bacteria, fungi, viruses, and some parasites, primarily by disrupting microbial membranes and modulating immune responses [36]. AMPs were discovered in 1939 by Dubos [116] who showed that an antimicrobial agent extracted from the soil Bacillus strain was efficient in treating mice affected by pneumococcal infections. A few years later, the first properly named AMP was discovered by Dubos and Hotchkiss [117] who decided to call this substance gramicidin. From that moment up to 2024, 5000 AMPs have been extracted from eucariots and prokariots [36].

The most accredited AMPs’ mechanism of action is the interaction with negatively charged bacterial membranes through electrostatic forces, leading to pore formation and cell lysis. Nevertheless, other mechanisms have been proposed like the inhibition of intracellular targets, such as DNA, RNA, and protein synthesis, or interference with biofilm formation [37,38].

It has been recognized that AMPs can regulate pro-inflammatory reactions, inducing anti-inflammatory effects, in addition to the recruitment and stimulation of proliferation of immune cells, and the promotion of wound-healing processes [118]. In human medicine, clinical applications are reserved for orthodontics, wound-healing procedures, and ophthalmology; however, even if all applications seem to be successful and promising, only three AMPs are currently FDA-approved (i.e., gramicidin, daptomycin, and colistin) [38]. The efficacy of these three AMPs against Streptococcus pneumoniae, Pseudomonas aeruginosa, Aspergillus spp., and Candida albicans (C. albicans) is recognized [39].

In veterinary medicine, the use of AMPs has increased in recent years, particularly after the European Union banned the use of animal growth promoters in feed in 2006. Consequently, the search for new antibacterial substances or compounds has become a major focus of research. Many AMPs show potential for application in poultry, swine, and ruminant farming, as well as in aquaculture, due to their ability to enhance production performance [40,41], boost immunity, and promote intestinal health. Additionally, some AMPs exhibit a stronger inhibitory effect on bacterial inflammation when used in combination with antibiotics [42,43]. The success of clinical applications seems to pose AMPs as an integrative or substitutive therapy to antibiotics, but some limitations have to be mentioned. It has been proven that AMPs administered in high concentrations tend to induce cytotoxicity to human cells: this effect is largely influenced by the charge of the peptide causing hemolysis [44]. Moreover, they may cause allergic reactions when administered and their unstable chemical structure will be influenced by the presence of polar fluids such as serum, leading to a decrease in antibacterial efficacy. Finally, the costs of production are not negligible, limiting their availability on the market [36].

The interest in using AMPs in companion animals is mainly driven by a need to increase the efficiency of killing MDR pathogens; here, the non-specific mechanisms of action are considered, and practitioners exploit their wider versatility against bacteria, fungi, and viruses [36,119].

Clinical applications of AMPs in companion animals are listed as follows:

• Cathelicidins and defensins: these are the primary AMPs utilized in dogs and cats. Canine beta-defensins (cBDs) and cathelicidins play roles in skin and mucosal defense [45].
• Lactoferricin and derivatives: lactoferricin is compound derived by lactoferrin hydrolysis. It has been demonstrated that it has potent antibacterial activity and has been evaluated for use in canine otitis and skin infections caused by Pseudomonas aeruginosa and Staphylococcus spp. Moreover, oral supplementation with lactoferrin has been associated with improved immune response and reduced incidence of gastrointestinal infections in dogs [85,86,87].
• Synthetic analogues: advances in peptide engineering have enabled the design of synthetic AMPs with enhanced stability and reduced cytotoxicity, paving the way for future veterinary applications [46].

Recent findings highlight the growing relevance of MDR Acinetobacter baumannii (A. baumannii) associated with respiratory, urinary, and wound infections in companion animals. Rühl-Teichner et al. [120] investigated the inhibitory effects of three AMPs (i.e., Bac7 (17), PAsmr5-17, and PaβN) on A. baumannii isolates collected from humans, dogs, and cats. The study demonstrated that MDR strains from companion animals shared similar resistance and virulence profiles with human isolates, indicating the possible interspecies transmission of predominant international clones. Both Bac7(17) and PAsmr5-17 significantly inhibited bacterial growth and biofilm formation without substantial differences between human- and animal-derived isolates, whereas PAβN showed limited activity [120].

2.2. Enzymatic Proteins with Antimicrobial Properties

Enzymatic antimicrobial proteins—often called enzybiotics—offer promising therapeutic alternatives in veterinary medicine, especially for MDR infections in companion animals. Among the most investigated are endolysins, bacteriophage-derived lytic enzymes that degrade bacterial cell walls. In a study by Nakamura et al. [61], Lys-phiSA012 demonstrated strong lytic activity in vitro against Staphylococcus (Staph.) pseudintermedius, Staph. schleiferi, and Staph. intermedius isolates from canine skin and ear infections, including multidrug-resistant and methicillin-resistant strains; the enzyme significantly reduced colony counts, outperforming the parent phage phiSA012 in many tests. Such findings suggest that endolysins like Lys-phiSA012 could be developed as topical or localized therapies for canine skin conditions where systemic antibiotics are less reliable. In addition to endolysins, endogenous AMPs with enzymatic or quasi-enzymatic functions have been characterized in dogs. A study by Santoro and Maddox [27] showed that canine β-defensin cBD103 and cathelicidin cCath have rapid bactericidal and fungicidal effects: both acted against Staph. pseudintermedius (both methicillin-resistant and susceptible strains), Staph. aureus, Pseudomonas aeruginosa (P. aeruginosa), and yeast Malassezia pachydermatis, with killing observed within 2 h. These peptides also reduced biofilm formation, which is a significant factor in chronic skin infections and treatment failure [27].

Furthermore, modulation of host immune response is also observed. In studies with synthetic AMPs in canine keratinocyte models, peptides such as cBD, cBD103, and cCath reduced cytotoxicity and inflammation induced by P. aeruginosa infection and its lipopolysaccharides (LPSs). This suggests a dual benefit: direct antimicrobial action and mitigation of inflammatory damage to host tissues [47]. While much of the work has been conducted in vitro, these results provide proof-of-concept for translation into clinical practice. For example, in the case of canine pyoderma, synthetic AMPs (such as uperin 3.6, CAMEL, protegrin-1, temporin A, pexiganan, citropin, aurein) have been tested for minimal inhibitory and bactericidal concentrations against Staph. pseudintermedius and Staph. aureus isolates [48]. Uperin 3.6 showed particularly low MICs for Staph. pseudintermedius, supporting the potential utility of AMPs as topical agents in treating skin infections where resistance to traditional antibiotics is a concern [48].

Some challenges remain before enzymatic antimicrobials can be widely adopted in companion animal veterinary practice: stability of the enzymes in vivo, delivery (topical vs. systemic), types of formulations, safety profiles, and potential immune reactions.

Enzymatic antimicrobial proteins (prominently lysozyme, lactoperoxidase—LPO- and lactoferrin—Lf-) are integral components of mucosal and glandular innate immunity in companion animals and represent attractive alternatives or biologically based adjuncts to conventional antibiotics in veterinary dermatology and oral/aural medicine. Lysozyme, a low-molecular-weight muramidase that hydrolyzes the β-1,4 glycosidic bond in peptidoglycan, is widely distributed in canine and feline tissues and secretions (like serum, saliva, tears, milk, and mucosal serous glands) and it is localized in neutrophils and monocytes in dogs, supporting both bacteriolytic and immunomarker roles in health and disease [96]. In dogs, analyses of saliva and the salivary proteome confirmed measurable lysozyme activity alongside other non-immunoglobulin defense factors, and serum lysozyme activity has been used as a biomarker in clinical studies [97] and potential clinical applications [88,89].

The lactoperoxidase system (LPO + thiocyanate (SCN⁻) + H₂O₂ → hypothiocyanite, OSCN⁻) exerts broad antimicrobial effects by enzymatically generating reactive, bacteria-inhibitory products at mucosal surfaces [89]. In milk, the system is well characterized across mammals and contributes to microbiological homeostasis [90]. Activation of the LPO system reduces bacterial counts and limits psychrotrophic growth in milk and provides non-specific antimicrobial protection in the oral cavity; canine saliva contains LPO activity and related peroxidase components, which likely participate in first-line defense at the oral and aural surfaces [90].

Lactoferrin, an iron-binding glycoprotein abundant in colostrum, milk, and many mucosal secretions, combines nutritional immunity with direct microbicidal actions (membrane destabilization, interference with bacterial adhesion and biofilm formation) and immunomodulatory effects (modulation of cytokine responses, enhancement of host cell defenses). Lactoferrin is produced by epithelial cells in multiple species, including companion animals, and is stored in neutrophil granules for rapid release at sites of infection; its multifunctional properties make it an appealing candidate for topical or local adjunctive therapy in cutaneous, otic, and oral staphylococcal infections [91].

Topical formulations (like creams, sprays, wound dressings) or otic preparations exist and are marketed delivering stable lysozyme, LPO-activated systems (with controlled H₂O₂/SCN⁻ provision), or lactoferrin-enriched gels to reduce pathogen load, inhibit biofilms, and modulate inflammation. Nevertheless, several challenges must be addressed before routine clinical use:

• Stability and proteolytic susceptibility of proteins on skin and in exudates.
• Delivery and formulation to achieve effective concentrations at the infection site (including penetration of biofilms).
• Activity in complex biological matrices (e.g., proteases, ionic strength and pH of exudate).
• Safety/immunogenicity with repeated topical applications.

These obstacles are surmountable via established pharmaceutical strategies (i.e., enzyme stabilization, peptidomimetics, controlled-release matrices) and by combining enzymatic agents with other adjuvants (e.g., dispersin B) or conventional antibiotics to achieve synergistic and resistance-limiting effects. The cumulative veterinary and comparative mammalian literature therefore supports the conduction of focused research to develop and clinically evaluate enzybiotics as evidence-based adjuncts for the management of bacterial skin, ear, and oral infections in dogs and cats [62,92]. These natural enzymes exhibit antimicrobial effects through mechanisms including enzymatic degradation of bacterial cell walls and sequestration of essential nutrients like iron [93,94,95]. Lactoperoxidase generates hypothiocyanite ions with bactericidal activity, commonly used in oral health formulations for pets [89] and lactoferrin binds iron, depriving bacteria of this essential nutrient, and exhibits direct bactericidal and antiviral activity [93,121,122].

2.3. Plant-Derived Phytochemicals

The antimicrobial activity of medicinal plant extracts derives from their wide array of secondary metabolites. These compounds are intermediate or end products of plant metabolism, not essential for primary life processes, but play a protective role against bacteria, fungi, protozoa, and viruses [123]. Secondary metabolites encompass quinones, alkaloids, lecithins, polypeptides, flavones, flavonoids, coumarins, terpenoids, essential oils, and tannins [124]. The use of plant-derived compounds is steadily increasing, primarily aimed at treating parasitic diseases and dermatological disorders [63,125].

Essential oils (EOs), such as those extracted from oregano (Origanum vulgare), thyme (Thymus vulgaris), and cinnamon (Cinnamomum verum), exert antimicrobial effects mainly through disruption of microbial membranes and inhibition of enzymatic systems [64,65]. Among EOs, tea tree oil (TTO) is particularly effective against Streptococcus, Enterococcus, and Staphylococcus species, and also exhibits antifungal activity against yeasts and dermatophytes [66]. However, some studies have reported that TTO may have sensitizing potential when used undiluted or at high concentrations, or upon oxidation of its main components, terpinene-4-ol and α-terpinene [67]. In that study, the TTO concentration within the blend did not elicit any allergic reactions, indicating good biocompatibility of the overall formulation. Furthermore, TTO appears to possess strong bactericidal activity against MDR bacteria such as MRSA, carbapenem-resistant Klebsiella pneumoniae, A. baumannii, and P. aeruginosa [68].

Other essential oils, including those from Salvia officinalis, Eucalyptus officinalis, and Lavandula officinalis, are effective in controlling the growth of cocci and bacilli commonly implicated in otitis externa. Salvia officinalis also exhibits antifungal properties, counteracting Candida spp. proliferation [69], while lavender oil has shown potential in managing parasitic infestations, including mite-related conditions [70]. The anti-inflammatory activity of Eucalyptus extracts is mainly attributed to their high content of 1,8-cineole, which suppresses the production of pro-inflammatory cytokines [71]. Additionally, Rosmarinus officinalis essential oil demonstrates both antinociceptive and antibacterial properties, being active against Streptococcus spp., Staphylococcus spp., P. aeruginosa, and C. albicans, all of which are commonly involved in ear inflammatory processes [69].

Essential oils are increasingly studied as complementary or adjunctive agents in the treatment of dermatologic, otic, and parasitic infections in dogs and cats [26]. For example, a study assessing nine commercial EOs (including oregano, thyme, lavender, basil, rosemary, and clary sage) against pathogens from canine and feline otitis externa found that Origanum vulgare and Salvia sclarea exhibited the strongest antibacterial activity, though their efficacy varied depending on the bacterial or fungal species [72].

Furthermore, individual EO components such as thymol, carvacrol, and cinnamaldehyde, as well as whole EOs like oregano and clove, showed significant bactericidal and fungicidal activity in vitro against isolates from canine otitis externa, including Staph. pseudintermedius and Malassezia pachydermatis [73]. Essential oils also show potential for ectoparasite control: citronella, clove, peppermint, ginger, and Zanthoxylum limonella essential oils demonstrated significant in vitro efficacy against ticks and fleas collected from dogs, and topical trials at appropriate dilutions showed acceptable skin tolerance [74]. In clinically applied formulations, an essential-oil-based shampoo was compared to a miconazole/chlorhexidine formulation in cats with spontaneous microsporiasis; this trial demonstrated that the EO shampoo had efficacy, even if full clearance times and relapse rates varied among animals [75]. Despite promising in vitro and limited in vivo data, safety concerns limit widespread use: TTO, when applied topically at inappropriate doses, has been associated with neurotoxicity (e.g., weakness, ataxia, tremors) in both dogs and cats [76]. Taken together, these studies indicate that essential oils have real potential in veterinary clinical use for dogs and cats, especially for skin and ear infections or parasite control, but require rigorous dose–response characterization, demonstrated in vivo efficacy, and careful safety assessment prior to routine clinical adoption.

Hydrolyzable tannins (HTs) and condensed tannins (CTs), two principal classes of plant-derived polyphenols exemplified in products from Castanea sativa (sweet chestnut) and other botanicals, exert a spectrum of antimicrobial actions that are increasingly being investigated for therapeutic and adjunctive use in companion animals. Mechanistically, these actions converge on three interrelated targets: microbial cell envelopes and associated enzymes, microbe–host and microbe–substratum adhesion (including biofilm formation), and the structure and function of the gut microbial community. The preclinical and translational literature supports the feasibility of oral administration in monogastric species including dogs and cats. At the molecular level, HTs (for example, ellagitannins and gallotannins abundant in chestnut extracts) and CTs (proanthocyanidin polymers) differ in hydrolytic stability and protein-binding profiles, properties that determine their interaction with bacterial surfaces: both classes can complex with cell wall and membrane proteins and lipids, increasing permeability, causing leakage of cytoplasmic contents, and producing ultrastructural damage visible by electron microscopy, with consequent growth inhibition and, at higher concentrations, bactericidal effects [103,104].

In addition to direct membrane effects, tannins inhibit extracellular microbial enzymes (e.g., proteases, glycosidases) and metabolic pathways necessary for replication and virulence, thereby reducing pathogen fitness without necessarily selecting for classical antibiotic resistance mechanisms; chemical modification studies further indicate that tannin polymer size, galloylation, and degree of oxidation modulate these inhibitory effects, explaining why chestnut-derived HT preparations can differ in potency from quebracho or mimosa CT extracts [105].

Beyond direct bacteriostasis or bacteriolysis, a salient antimicrobial mechanism of tannins is the impairment of bacterial adhesion and biofilm development, processes central to colonization, persistence, and chronic infection. Tannins bind to adhesins and surface appendages (i.e., pili and fimbriae) or to host/abiotic receptors, creating steric hindrance and masking molecular determinants required for attachment. This has been shown across diverse taxa and is functionally relevant for prevention of surface colonization and biofilm maturation in vitro, with consequent reductions in toxin delivery and invasiveness [105,106].

Importantly for companion animals, where enteric infections and dysbiosis commonly underlie diarrhea, mucosal inflammation, and pathogen overgrowth, these anti-adhesive properties mean that diet- or water-delivered tannin formulations can reduce the effective inoculum and disrupt early stages of pathogen establishment, supporting clinical endpoints such as reduced diarrhea duration and lower fecal pathogen shedding, as observed in livestock and experimental models [106,107]. Tannins also act indirectly by modulating gut microbial ecology in a dose- and structure-dependent manner and that can be harnessed to shift communities away from pathogenic blooms while preserving or promoting beneficial taxa. In vitro digestion and animal-feeding studies demonstrate that chestnut HTs and CTs can decrease the abundance of enterotoxigenic Escherichia coli (E. coli) and other opportunistic Proteobacteria while altering short-chain fatty acid profiles and ammonia/volatile metabolite production; meanwhile, in canine and feline fecal microbiota models, chestnut tannins produced distinct compositional and metabolic shifts (such as reduction in certain proteolytic and putrefactive metabolites) that are plausibly beneficial for intestinal health [108,109]. These microbiota effects are compounded by host-mediated outcomes: tannins’ astringent and anti-inflammatory activity can decrease local mucosal exudation and create a less permissive niche for pathogens, and microbial biotransformation of HTs (to ellagic acid, urolithins, etc.) can generate metabolites with their own bioactivity, further modulating host–microbial interactions [110,111].

From a translational and safety perspective, multiple studies report that properly characterized chestnut tannin preparations are orally tolerated in monogastric species and can be formulated for feed or water administration. Experimental work in livestock demonstrates reductions in diarrheal disease duration and pathogen burdens with chestnut HT supplementation, and in vitro/in vivo pet studies document the modulation of canine and feline fecal microbiota and metabolome without gross adverse effects when used at appropriate concentrations [107,112]. Nevertheless, therapeutic use in dogs and cats requires attention to dose–response relationships and formulation: excessive tannin intake can reduce palatability and nutrient digestibility and, in some contexts, bind dietary proteins and minerals, so veterinary dosing must balance antimicrobial benefits with nutritional safety. These factors represent a constraint that has motivated the development of water-soluble chestnut extracts and controlled-release feed matrices that preserve activity while minimizing antinutritional effects [112].

Totarol, a diterpenoid extracted from Podocarpus totara, exhibits antibacterial, antifungal, and anti-biofilm properties. It has shown potential in veterinary dermatology, dental coatings, and wound healing in dogs [113]. Its potent antibacterial activity is mainly directed against Gram-positive bacteria, including MRSP, a major cause of canine pyoderma [126]. Totarol disrupts bacterial membranes and inhibits biofilm formation, a critical factor in chronic skin infections [114]. In veterinary dermatology, totarol-containing topical formulations have been tested for managing canine superficial pyoderma, showing significant clinical improvements and reduction in bacterial load without inducing resistance [113]. Additionally, totarol-coated dental implants have been proposed as a preventive strategy against peri-implantitis in dogs [115].

2.4. Other Natural Compounds

Saponins, flavonoids, and alkaloids are three major classes of plant secondary metabolites that have demonstrated antimicrobial and modulatory effects on microbial populations; while direct evidence in dogs and cats is more limited than in lab or livestock species, the existing literature and mechanistic insights suggest considerable promise for their use in companion animal health.

Flavonoids are polyphenolic compounds that are widespread in plants, and they exert antimicrobial actions via diverse mechanisms. In canine studies, flavonoids such as quercetin, luteolin, and oligomeric proanthocyanidins (from grape seed extract) have been shown to reduce inflammatory and oxidative responses in peripheral blood leukocytes stimulated by bacterial LPS (from E. coli or Salmonella enteritidis) [81]. These effects are consistent with modulation of the host response to endotoxemia as well as indirect antimicrobial benefit [81]. Structurally, flavonoids can disrupt bacterial cell membranes, interfere with nucleic acid synthesis (e.g., by inhibiting DNA gyrase), impede energy metabolism, and inhibit other features essential to bacterial viability or virulence (e.g., via interference with oxidative stress defense) [82,83]. In addition, flavonoids may synergize with antibiotics, enhancing efficacy against resistant bacteria. Although most of these data come from in vitro studies in non-canine/feline systems, they suggests that flavonoids may help preserve intestinal barrier integrity and reduce pathogen-driven inflammation in dogs, which is often linked to the overgrowth of bacteria or dysbiosis [83].

Saponins are glycosylated compounds, often triterpenoid or steroidal in structure, which are known for their surface-active properties. Their antimicrobial actions include the disruption of microbial cell membranes (often by interacting with sterols in the membrane), causing increased permeability, leakage of cellular contents, and eventually cell death. These glycosides exhibit synergistic effects with other antimicrobials [84]. Studies in non-companion animals have demonstrated that saponin fractions from Melanthera elliptica have MICs in the range of 8–128 µg/mL against various pathogenic bacteria and fungi, often being able to act synergistically with conventional antibiotics [100]. Green tea seed saponins also have shown antibacterial activity both in vitro and in vivo in poultry against E. coli, Salmonella, Staph. aureus, with assays showing damage to the cell wall and membrane, release of intracellular proteins, and reduction in pathogen load in treated animals [101]. These data indicate that, if safety and bioavailability are confirmed, saponins might be exploited in dogs and cats for prevention or mitigation of bacterial infections, for example, in skin, otitis, or enteric contexts.

Alkaloids are nitrogen-containing plant metabolites with varied ring structures. Many of them act directly on microbes or modulate host immunity. In in vitro microbiological studies, aporphinoid alkaloids (such as oliveridine and pachypodanthine) have demonstrated inhibitory activity against foodborne pathogens like Yersinia enterocolitica, with MICs in low micromolar ranges [33]. In a more directly veterinary-relevant context, isoquinoline alkaloid blends have been tested in healthy dogs: in a cross-over feeding trial in beagles, this alkaloid blend at recommended dosage (10–20 mg/kg body weight/day) produced increases in lymphocyte and monocyte counts, some modulation of short-chain fatty acids, and altered fecal ammonia and lactate without compromising nutrient digestibility. Although this study did not directly assess effects on bacterial infection, the changes in immune markers and gut fermentation products suggest that alkaloids can modulate host–microbiome interactions in dogs in ways that could enhance resistance to pathogenic bacteria [34]. It has been demonstrated that berberine, an alkaloid traditionally used in Chinese Medicine, can restore intestinal microbiota homeostasis and regulate the TLR4/NF-κB pathway, thereby controlling inflammatory responses. In a study involving cats affected by intestinal bowel disease (IBD), a novel berberine therapy was proposed that can simultaneously activate the mammalian target of rapamycin (mTOR) complex (MTORC) and autophagy to restore intestinal mucosal barrier function [35].

3. Feed Additives and Nutraceuticals with Antimicrobial Activity in Companion Animals

3.1. Phytogenic Feed Additives in Pet Nutrition

Phytogenic feed additives (PFAs), also referred to as botanicals or phytobiotics, consist of herbs, spices, essential oils, and plant extracts incorporated into diets to improve health, performance, and gut microbial balance [127]. Initially explored in livestock production as alternatives to antibiotic growth promoters, PFAs are increasingly considered in companion animal nutrition to enhance gastrointestinal health and reduce susceptibility to infections without relying on synthetic antimicrobials [127].

Several essential oils, including oregano, thyme, rosemary, and cinnamon, have demonstrated strong antibacterial and antifungal properties in vitro and in vivo. Their primary active compound (i.e., carvacrol, thymol, cinnamaldehyde, and eugenol) targets bacterial membranes, leading to increased permeability and cell death. These compounds also exert anti-biofilm effects, which are critical in controlling chronic infections and oral health disorders in dogs and cats [77].

In addition to essential oils, polyphenolic-rich plant extracts (e.g., green tea catechins, grape seed proanthocyanidins) have been studied for their antimicrobial and antioxidant benefits. Polyphenols act by disrupting microbial membranes, chelating essential metals, and interfering with bacterial virulence factors, including quorum sensing [84]. Green tea extract containing epigallocatechin gallate (EGCG) has been investigated as a dietary supplement in dogs for its gut-modulatory and antimicrobial effects against Clostridium perfringens and E. coli [56]. EGCG exhibits broad-spectrum antimicrobial activity and antioxidant properties, based on the ability to disrupt bacterial membranes, to inhibit urease activity, and EGCG can reduce halitosis in dogs [56].

PFAs are generally recognized as safe (GRAS) when used at appropriate dosages; however, high inclusion rates of certain essential oils may cause gastrointestinal irritation, hepatotoxicity, or alterations in palatability [77,78]. Therefore, proper formulation and standardization are fundamental for effective and safe integration into pet diets.

3.2. Probiotics, Prebiotics, and Synbiotics

Probiotics, prebiotics, and synbiotics (collectively termed “biotics”) are increasingly utilized in companion animal nutrition to modulate the gut microbiome, enhance gastrointestinal health, and support systemic immunity. While their application in dogs and cats is expanding, the scientific evidence supporting their efficacy and safety remains a subject of ongoing research. Modulating the intestinal microbiota through probiotics, prebiotics, and synbiotics represents a key nutraceutical strategy to reduce pathogenic colonization in companion animals [50,51].

Probiotics are live microorganisms that confer health benefits by competing with pathogens for adhesion sites, producing antimicrobial metabolites (e.g., bacteriocins, organic acids), and modulating the immune system [52]. Common probiotic strains used in dogs and cats include Lactobacillus acidophilus, Enterococcus faecium, and Bifidobacterium animalis [53]. These strains can inhibit pathogenic E. coli and Salmonella species, reduce diarrhea incidence, and enhance gut barrier function [53].

In dogs, specific probiotic strains have shown promise in managing gastrointestinal disorders. For instance, certain probiotics have been reported to decrease the clinical severity of inflammatory bowel disease in dogs, suggesting their potential therapeutic role in chronic enteropathies. Prebiotics, such as fructooligosaccharides (FOSs) and mannanoligosaccharides (MOSs), are non-digestible carbohydrates that selectively stimulate beneficial bacteria, indirectly suppressing pathogens [50]. MOSs also bind bacterial fimbriae, preventing the adhesion of E. coli and Salmonella to intestinal epithelial cells [54].

Synbiotics combine probiotics and prebiotics to synergistically improve the survival and implantation of live microbial dietary supplements in the gastrointestinal tract. A study involving healthy dogs and cats demonstrated that a synbiotic formulation containing multiple probiotic strains and prebiotic fibers led to the increased abundance of beneficial gut bacteria without adverse effects, indicating its potential as a dietary supplement to support gut health [54,55].

Clinical studies in dogs have shown that probiotic-supplemented diets can significantly reduce the duration and severity of acute diarrhea and improve immune response after vaccination. Similar benefits have been reported in cats, including improved fecal quality and reduced enteropathogen load [54].

3.3. Functional Nutraceuticals with Antimicrobial Properties

Beyond PFAs and probiotics, several functional ingredients have been investigated for antimicrobial potential in pet nutrition. These nutraceuticals often exert synergistic effects when combined with other bioactive compounds, improving overall gut health, immunity, and pathogen control.

• Spirulina (Arthrospira platensis) is a microalga that is rich in phycocyanin and bioactive peptides. Spirulina enhances immune response and exhibits antimicrobial effects against Gram-negative bacteria [102].
• Curcumin (Turmeric extract) shows anti-inflammatory and antimicrobial properties, although bioavailability remains a challenge. Feed integration of curcumin in dogs and cats is an area of growing interest, particularly because of its antioxidative and anti-inflammatory properties. Recent studies have shown that dietary curcumin, either alone or in formulations combined with other compounds, can reduce oxidative damage, support joint health, and improve some metabolic and biochemical markers in these species [58]. For example, in Beagle dogs fed a diet supplemented with curcumin (33 mg/kg feed), there was a reduction in lipid and protein oxidation and increased antioxidant enzyme activities. In dogs with diabetes mellitus, long-term curcuminoid supplementation (250 mg/day for 180 days) improved oxidative stress markers (e.g., higher glutathione/oxidized glutathione ratio) and affected proteomic profiles associated with insulin sensitivity, without observed adverse effects on liver or kidney parameters [59]. Furthermore, in dogs with osteoarthritis, a co-micronized formulation of palmitoyl–glucosamine with curcumin helped to maintain pain relief when tapering non-steroidal inflammatory drugs (NSAIDs), reducing pain scores and lameness severity over an 18-week period [60].
• Cranberry extract is commonly used in urinary health formulations for dogs and cats due to its anti-adhesive effects against Escherichia coli in the urinary tract [128]. Cats prone to recurrent urinary tract infections (UTIs) benefit from cranberry-derived proanthocyanidins; these were shown to prevent bacterial adhesion to the urothelium in a double-blind, placebo-controlled trial involving 60 cats. This study showed a 35% reduction in UTI recurrence over 6 months in the cranberry group compared to the placebo group [57].

Although it is not a dietary approach, antimicrobial photodynamic therapy (aPDT) represents an innovative, non-antibiotic strategy for treating infections in pets. aPDT has been successfully applied to manage canine otitis externa, feline dermatophytosis, and periodontal infections, often using natural photosensitizers such as curcumin or methylene blue combined with light activation [98]. These treatments have demonstrated high bactericidal efficacy with minimal risk of resistance development [99].

4. Challenges and Limitations in Clinical Translation

While natural antimicrobial compounds hold considerable promise for reducing antibiotic dependency in companion animal medicine, several scientific, technological, and regulatory challenges must be addressed before these solutions can be widely adopted in clinical practice. These challenges span from raw material variability to clinical trial limitations, cost implications, and integration to established therapeutic protocols.

One of the most significant challenges associated with natural antimicrobial agents, particularly plant-derived compounds, is the variability in chemical composition due to differences in plant species, geographical origin, harvesting season, and extraction methods [129]. Essential oils, for instance, can exhibit substantial variation in concentrations of key active constituents such as carvacrol, thymol, and eugenol, which directly affect antimicrobial potency. This inconsistency complicates the formulation of standardized veterinary products with predictable efficacy and safety profiles [79].

Standardization efforts, including chemical fingerprinting and chromatographic profiling, are essential to ensure batch-to-batch consistency in phytogenic formulations [130]. However, such measures increase production costs and require advanced quality control infrastructure, which may limit accessibility for small-scale manufacturers and clinics [131,132].

Natural compounds often face limitations in terms of bioavailability, particularly when administered orally. Many phytochemicals, including polyphenols and curcuminoids, exhibit poor water solubility, low intestinal absorption, and extensive first-pass metabolism, resulting in limited systemic exposure [133,134]. Similarly, peptides such as AMPs are susceptible to enzymatic degradation in the gastrointestinal tract, reducing their oral efficacy [135].

Stability issues also affect essential oils and other volatile compounds, which can degrade during storage or processing due to oxidation and temperature fluctuations [136]. Encapsulation technologies (such as liposomes, nanoemulsions, and polymeric nanoparticles) offer promising solutions for enhancing solubility, protecting compounds from degradation, and enabling controlled release [137,138]. However, these advanced delivery systems increase formulation complexity and cost, potentially limiting their widespread application in the pet care market.

Palatability is a critical factor in companion animal nutrition. Many natural antimicrobials, particularly essential oils and certain plant extracts, possess strong flavors and odors that may reduce food acceptance in dogs and cats [139,140]. This challenge necessitates the use of flavor-masking agents or microencapsulation techniques to improve acceptability without compromising bioactivity. Failure to address palatability can result in poor compliance, rendering the nutraceuticals ineffective in real-world conditions [140].

Although natural antimicrobials are often perceived as inherently safe, this assumption can be misleading. Certain essential oils, such as TTO, can be toxic to cats even at relatively low doses, causing central nervous system depression, ataxia, and hepatic dysfunction [141]. Similarly, high concentrations of phenolic compounds or alkaloids may induce gastrointestinal irritation, hepatotoxicity, or neurotoxicity [142]. The risk is further amplified in animals with comorbidities or those receiving concurrent medications, raising concerns about drug–nutrient interactions [143].

Comprehensive toxicological assessments, including acute and chronic exposure studies, are necessary for each compound and species. However, these studies are costly and time-consuming, which poses a barrier for small companies developing natural veterinary products [144].

Despite a growing body of in vitro studies and anecdotal clinical reports, there remains a lack of large-scale randomized controlled trials (RCTs) evaluating the efficacy and safety of natural antimicrobials in companion animals [80,145]. Most available studies are small, non-blinded, and short-term, limiting the strength of evidence and making it difficult for veterinarians to recommend these products confidently [80,145].

Regulatory authorities and scientific journals increasingly demand high-quality evidence to substantiate therapeutic claims. Therefore, future research must focus on well-designed RCTs with appropriate endpoints, including microbial load reduction, clinical symptom improvement, recurrence rates, and safety outcomes [146]. The regulatory framework for natural antimicrobials in veterinary medicine varies significantly across regions, creating confusion and compliance challenges for manufacturers and practitioners. In the European Union, for example, phytogenic feed additives are classified as sensory additives or flavoring agents rather than antimicrobial agents, restricting their marketing claims [147]. In contrast, the United States classifies most natural compounds as dietary supplements or GRAS substances, provided they meet safety criteria [148]. This lack of harmonization complicates product development and global distribution, as manufacturers must navigate different approval processes, labeling requirements, and claim restrictions. Moreover, the absence of specific veterinary guidelines for nutraceuticals allows unsubstantiated claims, raising concerns about product quality and efficacy [149].

Manufacturers must balance cost-effectiveness with quality to remain competitive without compromising safety and efficacy.

5. Future Opportunities and Synergistic Strategies

Despite the challenges outlined above, the future of natural antimicrobial compounds in companion animal health is highly promising. Advances in formulation technologies, synergistic combinations, and personalized nutrition offer new opportunities to enhance efficacy, safety, and compliance.

Nanotechnology-based delivery systems, including nanoemulsions, solid lipid nanoparticles, and polymeric micelles, have shown potential in improving the solubility, stability, and bioavailability of poorly soluble compounds such as essential oils, curcumin, and polyphenols [150]. Encapsulation also provides controlled release, protecting sensitive bioactives from degradation during feed processing and gastrointestinal transit [151]. These strategies not only improve therapeutic outcomes but also minimize the adverse effects associated with high systemic concentrations and permit the combination of multiple natural antimicrobial agents exploiting a synergistic effect. For example:

• Essential oils and organic acids: Organic acids lower pH, enhancing the membrane-disrupting action of essential oils [152]. This has been successfully proven in poultry and may be potentially translated to companion animal medicine.
• Probiotics and phytochemicals: Probiotics can colonize the gut and produce bacteriocins, while phytochemicals inhibit pathogen adhesion, providing dual protection [49].
• AMPs and plant extracts: AMPs exert rapid bactericidal activity, while phytochemicals target virulence factors and biofilms [153].

These synergistic approaches are particularly relevant for multi-modal therapies, combining topical and nutritional interventions to manage chronic dermatological and gastrointestinal conditions in dogs and cats.

The rise of precision nutrition in companion animals presents an opportunity to integrate natural antimicrobials into tailored diets based on individual health status, microbiota composition, and genetic predispositions, enhancing compliance and therapeutic outcomes. For instance, a dog prone to recurrent gastrointestinal infections could receive a customized diet enriched with specific probiotics, tannins, and polyphenols to modulate the gut microbiota and reduce pathogen colonization [154].

Artificial intelligence (AI) and machine learning algorithms are being applied to identify novel antimicrobial peptides, predict bioactivity, and optimize formulations for maximum efficacy and safety [155]. AI-driven tools can accelerate drug discovery pipelines by screening large peptide libraries and simulating interactions with microbial membranes. Moreover, digital health platforms and wearable devices in companion animals can facilitate real-time monitoring of treatment outcomes, allowing veterinarians to adjust nutraceutical interventions dynamically [156].

Natural antimicrobials align with the One Health paradigm, offering solutions that reduce antibiotic use, limit resistance development, and minimize environmental contamination [157,158]. By replacing or reducing antibiotics in companion animal care, these strategies contribute to global AMR mitigation efforts, benefiting both animal and human health [159].

Sustainability is another driving force, as many natural compounds are derived from renewable plant sources or agricultural by-products, with a reduced ecological footprint compared to synthetic antibiotics [160]. However, sustainable sourcing practices and life-cycle assessments are essential to ensure that large-scale adoption does not lead to overharvesting or habitat degradation.

6. Conclusions

The emergence of AMR as a critical global health concern necessitates innovative strategies to reduce reliance on conventional antibiotics in both human and veterinary medicine. Companion animals, due to their close interactions with humans, represent a key component of the One Health framework, where antimicrobial stewardship is essential to prevent the bidirectional transmission of resistant pathogens.

Natural antimicrobial compounds offer promising alternatives to conventional antibiotics for managing infections and maintaining health in dogs and cats. Nutraceuticals and functional feeds enriched with natural antimicrobials are gaining attention for their preventive and therapeutic roles in gastrointestinal health, oral care, and dermatological conditions. Advances in encapsulation technologies and nanodelivery systems are addressing major limitations such as poor bioavailability, stability issues, and palatability challenges.

Despite these advances, several barriers remain. The lack of robust clinical evidence based on clinical randomized trials and on species-specific toxicological data, in combination with the absence of standardized regulatory frameworks, poses significant challenges to clinical adoption. Future research should focus on well-designed randomized controlled trials, toxicological profiling, and harmonized regulations to ensure product safety, efficacy, and quality.

The incorporation of natural antimicrobial agents into personalized veterinary nutrition, supported by microbiome profiling and AI-driven formulation optimization, represents a transformative approach to preventive healthcare.

In conclusion, while natural antimicrobials cannot completely replace antibiotics in companion animal medicine, they offer valuable adjuncts to current therapeutic strategies. With continued scientific innovation, regulatory support, and veterinarian education, these compounds will play a critical role in shaping the future of antimicrobial stewardship in veterinary practice.