Beyond Fish Pathogens: A Comprehensive Overview of Aeromonas salmonicida

Abstract

Aeromonas salmonicida is an age-old fish pathogen widely distributed in seawater and freshwater environments that causes significant economic losses to the global aquaculture industry. Genetic mutations and the emergence of thermophilic strains are factors in the continuous expansion of A. salmonicida’s host range. Beyond infecting fish, A. salmonicida poses a potential threat to mammalian and human health. This review synthesizes recent global research advances concerning A. salmonicida, encompassing strain characteristics, genomic features, virulence factors, and pathogenic mechanisms, as well as the clinical manifestations in infected fish and mammals, and discusses prevention and treatment methods. Particular emphasis is placed on evaluating the potential prophylactic roles of Chinese herbs and bacteriophages against A. salmonicida infection. Furthermore, the review provides perspectives on future research directions, diagnostics, and disease management, informed by contemporary domestic and international studies on this pathogen.

1. The Classification of A. salmonicida

A. salmonicida is a Gram-negative short bacillus belonging to the Aeromonas genus of the Aermonadaceae family, which is often widely distributed in marine and freshwater environments [1]. At present, there are five internationally recognized subspecies: salmonicida, smithia, achromogenes, masoucida, and pectinolytica [2,3]. There are notable variations in the outer core structures among the different subspecies of LPS. Specifically, the subspecies salmonicida, masoucida, and smithia exhibit identical structures, whereas the achromogenes subspecies lacks the Gal residue. Additionally, the outer core of the pectinolytica subspecies closely resembles that of Aeromonas hydrophila [4]. These structural differences may be associated with host adaptation and virulence characteristics of the subspecies, thereby offering a molecular foundation for the classification of A. salmonicida. Furthermore, as shown in Figure 1, among these subtypes, only the strains isolated from salmonids fish and classified as subsp. salmonicida will be designated as “typical” A. salmonicida, while the remaining four subtypes, which include strains isolated from non-salmonid hosts or environmental samples, will be collectively termed “atypical” strains [5,6,7].

In addition, A. salmonicida can be divided into psychrophilic and mesophilic categories based on temperature [8]. Research shows that the first four subspecies are pathogens of various cold-water fish, and they are considered a strictly psychrophilic subspecies, while pectinolytica subsp. grows efficiently at 37 °C and is considered a mesophilic subspecies [9], capable of infecting mammals and birds [10]. However, endogenous transposon mutagenesis can transform cold-loving strains into mesophilic ones and vice versa [11]. Furthermore, researchers have divided A. salmonicida into 14 types based on differences in certain nucleotide sequences of the specific virulence gene, vapA (A-layer) [12]. However, the vapA gene, which encodes the A-layer protein, exhibits significant instability in certain strains and is prone to endogenous transposition mutations [13]. There remains ongoing debate regarding the validity of the current classification of A. salmonicida. Moreover, as research on this bacterium advances and strain genes continue to evolve and mutate, it is likely that additional subspecies will emerge, thereby challenging traditional classification methods.

2. Plasmids

In recent years, the rapid advancement of genome sequencing technology has led to an increasing number of strains of A. salmonicida, which has therefore undergone complete genome sequencing [14,15,16,17,18,19,20]. This development provides a crucial foundation for the precise investigation of genomic characteristics, and a deeper understanding of their pathogenic mechanisms and drug resistance profiles, and the formulation of new prevention and control strategies. The analysis of the genome from A. salmonicida reveals that the circular chromosome sequences exhibit high similarity among different strains within this subspecies [18,20]. The similarity among non-typical strains is 99.55% ± 0.25%, and the similarity between typical strain A449 and Aeromonas hydrophila is 99.17%. The copy number of insertion sequences (ISs) is the main factor in the genomic differences between non-typical and typical strains. A. salmonicida is greatly influenced by the plasmid group. Compared to non-typical strains, typical strains carry more plasmids and are associated with virulence genes [21]. Therefore, the phenotype and classification of A. salmonicida are closely linked to its plasmids.

Plasmids can be divided into R plasmids, virulence plasmids, and small plasmids. Among them, R plasmids carry multiple antibiotic resistance genes. The first R plasmid was discovered in the A. salmonicida Ar-32 strain, which was resistant to several drugs such as sulfathiazole, chloramphenicol, and streptomycin [22]. Aoki et al. [23] named the plasmid pAr-32. However, Sørum H et al. found that pRAS1 and pAr-32 might be produced by two independent integrons inserted into the same site of the universal IncU plasmid (respectively, the In4-like integron and the In6-like integron) [24]. The 11.6 kb fragment of pRAS1b has a very close resemblance to the 1 type integron and Tn1721 transposon sequence of the pRAS1 plasmid. The integron contains the antibiotic resistance genes dfr16 and sul1, while the transposon contains tetA [17]. pRAS2 was found in A. salmonicida subsp. salmonicida strain 1682/92, and carries the transposon Tn5393c [25], which is resistant to streptomycin, sulfanilamide, and tetracycline. Currently, pRAS3 has five variants (pRAS3.1, pRAS3.2, pRAS3.3, pRAS3.4, and pRAS3-3432). The first three pRAS3 plasmids contain the tetA and tetR genes, conferring resistance to tetracycline [25,26]. pRAS3-3432 carries an insertion element, which is only found in Porcine Chlamydia [27]. Additionally, there are three plasmids that are not compatible with the IncU group, namely pASOT, pASOT2, and pASOT3. pASOT and pASOT2 are homologous with pRAS1. The plasmids have a 1-class integron and can withstand tetracycline [28,29]. What is remarkable is the large plasmid pAsa8. This plasmid contains a region that harbors multiple resistance genes (it may confer resistance to tetracyclines, florfenicol, chloramphenicol, sulfonamides, ampicillin, ciprofloxacin, streptomycin, kanamycin, and quaternary ammonium compounds), which is the product of multiple integration events of mobile genetic elements. Different mobile elements, including the Tn1721 transposon and a complex type 1 integrator [30], contain all resistance genes. This reinforces the belief that the A. salmonicida may be a reservoir of mobile genetic elements that carry resistance genes [30,31,32].

The smallest virulence plasmid is pAsa7, which carries a gene encoded for chloramphenicol acetyltransferase and confers chloramphenicol resistance [33]. It is very similar to the integron pAsa2 of A. salmonicida, but the evolutionary relationship between the two is still unclear at present [34]. PASA5 contains the genes encoding functional TTSS, including three effectors: AopH, Ati2, and AopO [20]. The structural genes at the TTSS locus of pAsa5 can be inactivated to eliminate bacterial virulence, as discovered by Dacanay et al. [35]. Moreover, pAsa5 is highly sensitive to temperature; if the environmental temperature reaches 25 °C or above, the plasmid will be inactivated [36]. PAsa5-3432 is the first plasmid reported for this bacterium that carries T3SS and multiple antibiotic resistance genes. In 2017, another plasmid, pAsa9 (76.7 kb), was also discovered. This plasmid shares an ancestor with pAsa5 and has a 40 kb highly similar sequence, yet their insertion sequences (ISs) are not the same [37].

Apart from the plasmid related to antibiotic resistance, A. salmonicida also has four small plasmids, namely pAsa1, pAsa2, pAsa3, and pAsal1. Among them, pAsa2 carries the ColE1 type replicon, while pAsa1 and pAsa3 carry the ColE2 type replicon [38]. The existence of these plasmids is not associated with bacterial virulence [39]. The fourth small plasmid, pAsal1, is a derivative of pAsa3 and is cytotoxic, carrying the gene encoding the effector protein AopP [40,41].

The genome of A. salmonicida contains many mobile elements, including insertion sequences (ISs), genomic islands (GEIs), transposons, and prophages [11,42]. These mobile elements enhance the mutational ability of A. salmonicida and further improve the adaptability of the bacteria to the host and environment [43,44,45]. This may be a significant factor contributing to the increase in host species infected by A. salmonicida, the emergence of multidrug resistance in recent years, and the pathogenicity of wild-type and mutant strains.

The genomic characteristics of A. salmonicida indicate that the chromosomal sequences between different strains have a high similarity. The plasmid group differences (typical and atypical) mostly cause phenotypic differences, with typical strains having more plasmids related to virulence. To sum up, the antibiotic resistance spectrum and virulence phenotype of A. salmonicida are shaped by plasmids, and as carriers of mobile genetic elements, they contribute to the spread and evolution of resistance genes.

3. Virulence Factors and Pathogenic Mechanisms

The virulence factors of A. salmonicida play a significant role in the infection process. An in-depth exploration of the mechanisms of action of these virulence factors will provide a theoretical basis for the development of new prevention and control strategies. Based on their functions, they can be divided into seven categories: exotoxins, extracellular enzymes, A-layer protein, secretion systems, iron ion uptake systems, drug resistance genes, and quorum sensing [14]. Previous studies have mainly focused on the pathogenic role of a single virulence factor. However, at present, it is observed that the pathogenic mechanism of Aeromonas is the result of the interaction of multiple virulence factors, and the difference in the number of virulence genes is an important factor leading to the variation in its pathogenicity [46,47,48].

3.1. Exotoxin

Exotoxins are extracellular virulence factors. Their main component is a protein that can be divided into three categories according to its affinity and mode of action on cells: aerolysin (aer), hemolysin (hly), and enterotoxins [49]. These toxins have been identified in A. salmonicida [50,51]. Among the various virulence factors, A. salmonicida carrying hly and aer tends to be highly pathogenic [52,53,54,55]. Aer is a single-molecule polypeptide encoded by the aerA gene, which is highly hemolytic, cytotoxic, and enterotoxic [56]. Aer binds to specific glycoprotein receptors on the host cell membrane, destroys the cell membrane structure, and causes cell lysis. The aerA deletion strain is significantly less virulent than the original strain [57]; hence, it is often used as an indicator of pathogenicity. Hly, a virulence factor encoded by DNA, acts by binding to the cell membrane receptor to form a copolymer that is inserted into the cell membrane in the form of pipes or pores. This changes the permeability of the cell membrane and causes its dissolution and destruction, leading to hemolysis. In addition, virulence factors can induce cellular and humoral immunity in fish [58,59]. Li et al. [60] found that the Hly protein has strong immunogenicity and can be used as an important antigen for the development and preparation of subunit vaccines. In addition, Chen et al. [61] isolated a strain of A. salmonicida SRW-OG1 (a mesophilic strain) from Epinephelus coioides. The aerA and hlyA genes of this strain were significantly upregulated when cultured at 28 °C and 37 °C compared with at 18 °C. Moreover, the hemolytic activity and virulence of extracellular products (ECPs) were higher than for those cultured at 18 °C. Meanwhile, the silencing of aerA and hlyA would lead to a significant decrease in hemolytic activity and virulence. The main pathogenic feature of enterotoxins is the induction of diarrhea. After acting on the cell membrane, the cell membrane is cleaved and damaged, and the secretion of intestinal epithelial cells is promoted, causing intestinal tripping effusion leading to diarrhea. Some enterotoxins are also cytotoxic and can induce apoptosis. Kroniger et al. [62] discovered that some hly proteins of A. salmonicida increase their expression not only under iron-limiting conditions but also significantly under elevated culture temperature. It is apparent that certain virulence factors of A. salmonicida are both regulated by iron and closely linked to culture temperature.

3.2. Extracellular Enzyme

The pathogenicity of A. salmonicida and some protein products are secreted to the extracellular space, such as elastase (ELA), extracellular temperature-labile protease (EPR), serine protease (AHP), glycerophospholipid cholesterol acyltransferase (GCAT), etc. [63]. GCAT is the main extracellular enzyme that causes fish furuncle, and can combine with AspA (a serine protease) to exert toxicity or with lipopolysaccharide (LPS) to form GCAT-LPS, which is highly toxic [49,64]. In vitro experiments revealed that although GCAT exhibited cytotoxicity to rainbow trout erythrocytes, the contribution of GCAT monomers to the toxicity of pathogens was limited. Vipond et al. [65] found that the GCAT gene knockout mutant did not show a loss of pathogenicity in the challenge experiment, suggesting that its role in disease occurrence might not be decisive. In addition, the precursor of GCAT is made up of two polypeptides that are connected by a disulfide bond. During activation, proteolysis occurs, and the epitope structure can effectively stimulate the immune system to generate protective responses, especially antibodies against its enzymatic activity, which can significantly enhance the immune protection effect [66,67]. This provides a foundation for the development of immunotherapy and vaccines targeting GCAT.

3.3. A-Layer Protein

The A-layer is a self-assembled paracrystalline tetragonal array structure encoded by the vapA gene, which coats the bacterial surface. The N-terminal hydrophobic domain of this structure attaches to the core region of LPS, creating a physical barrier that prevents the bacteria from being damaged by complement attack and phagocytosis [6,68]. It has been determined by research that VapA makes up 60% of the total bacterial outer membrane proteins, which is significantly higher than that of other membrane proteins [68]. Furthermore, the A-layer is an important protective antigen of A. salmonicida. The vaccine efficacy of the strain that carries the natural A-layer is significantly better than that of the A-layer-deficient strain. For instance, in the Atlantic cod model, the protection rate of the vaccine containing the A-layer is as high as 40% compared to the strain lacking the A-layer; meanwhile, when the purified A-layer protein is re-adhered to the surface of the A-layer-deficient strain, the induced immune protection is comparable to that of the natural A-layer strain [69,70]. Additionally, the vapA gene sequence demonstrates both genetic diversity and host adaptability. As an illustration, the typical subspecies (A. salmonicida subsp. salmonicida) exhibits a single A-type, while other subspecies and non-typical strains show significant heterogeneity [12,13]. Moreover, the VapA types of strains derived from different hosts are different; for example, the isolates from Atlantic cod (Gadus morhua) mostly belong to A-type IV, while those from salmonid species are mainly A-type I [12]. Additionally, A-layer-deficient strains show increased susceptibility to the bacteriophage T7-Ah. However, their adsorption efficiency is not directly correlated with the presence of the A-layer, suggesting that LPS may serve as the phage’s primary receptor [71,72]. Hence, the A-layer is not only a core virulence factor of A. salmonicida, but its genetic plasticity also drives the evolution of host adaptability, which provides molecular targets for the development of precise vaccines and phage therapies [69,73].

3.4. Secretory System

In 2004, Yu et al. [74] confirmed the existence of the Type Three Secretion System (TTSS) in Aeromonas for the first time. Similar to most Gram-negative bacteria, A. salmonicida has multiple secretory systems. Table 1 displays the types and mechanisms of the secretory system. Currently identified secretory systems include types II (T2SS), III (T3SS), IV (T4SS), and VI (T6SS) [36]. Among the various secretion systems, the T3SS is the primary pathogenic system of A. salmonicida, encoded by a large 150 Kb plasmid [14,75,76]. Its key core components include ascV, aopB/aopD, and exsA [1,77]. T3SS is a complex nanoinjection device that can directly inject effector proteins into host cells, thereby interfering with the host’s immune response and facilitating the survival and reproduction of bacteria within the host. The effector proteins secreted by A. salmonicida T3SS mainly include AexT, AopH, Ati2, AopP, AopO, AopN, and ExsE. They exert their pathogenicity mainly by disrupting cytoskeletal proteins, inhibiting the NF-B pathway, and suppressing the formation of pseudopodia in macrophages [77]. Calcium ion concentration and contact with host cells can affect the expression of these effector proteins [78,79]. Also, some effector proteins in A. salmonicida have been found to cause cytotoxicity and apoptosis in host cells [1]. In conclusion, the T3SS of A. salmonicida disrupts host defense by precisely regulating the secretion of effector proteins, but its genetic instability (IS-mediated recombination) leads to fluctuations in the virulence of natural populations [80]. Therefore, developing vaccines targeting T3SS effectors will provide a novel strategy for the prevention and control of furunculosis.

The T2SS and T4SS play the role of transporting. However, it should be noted that T4SS can facilitate horizontal gene transfer among bacteria, which is conducive to the spread of antibiotic resistance genes among bacteria [81]. The T6SS, a key virulence determinant, critically regulates bacterial pathogenicity [82,83]. In A. salmonicida, T6SS includes 19 genes, 16 of which are located in chromosomes and 3 in plasmids [14]. Cai et al. [84] found that after knockdown of the structural protein Hcp of A. salmonicida T6SS, the adhesion, growth, biofilm formation, extracellular product secretion, and virulence of the strain were reduced to varying degrees.

Table 1. The types and mechanisms of the secretory system.

Secretory System	Mechanism of Action	References
T2SS	Transport virulence factors, such as aerolysin, amylase, and protease.	[14,85,86,87]
T3SS	The main pathogenic system of A. salmonicida.	[14,75,76]
T4SS	The original genes that undergo genetic transfer among bacteria.	[81]
T6SS	Injection of different types of toxin proteins into the host.	[88,89]

Table 1. The types and mechanisms of the secretory system.

Secretory System	Mechanism of Action	References
T2SS	Transport virulence factors, such as aerolysin, amylase, and protease.	[14,85,86,87]
T3SS	The main pathogenic system of A. salmonicida.	[14,75,76]
T4SS	The original genes that undergo genetic transfer among bacteria.	[81]
T6SS	Injection of different types of toxin proteins into the host.	[88,89]

3.5. Iron Ion Uptake System

The acquisition of iron is one of the key conditions for the survival and reproduction of most bacteria within the host, and it has significant influence on the virulence and pathogenicity of bacteria [90]. Therefore, during the long-term evolutionary process, bacteria have developed multiple unique mechanisms for obtaining iron [91]. One of the main ways for A. salmonicida to acquire iron is by synthesizing and secreting iron carriers. As shown in Figure 2, these carriers are capable of obtaining iron from the host’s iron-binding proteins and entering the cell through outer membrane receptor proteins. Studies have shown that A. salmonicida subsp. salmonicida can synthesize two different iron carriers (amonabactin and acinetobactin). Among them, the gene cluster for the synthesis of amonabactin is ubiquitous in all Aeromonas, while the synthesis gene of acinetobactin is restricted to A. salmonicida and all its subspecies [92]. Najimi [93] et al. discovered that iron carrier-mediated iron uptake is an important virulence factor of A. salmonicida. When A. salmonicida grows under iron-limited conditions, it expresses iron-regulated outer membrane proteins, and one of these proteins has been identified as the putative outer membrane iron carrier receptor [94]. Furthermore, Hirst et al. have confirmed that the iron-regulated outer membrane protein is an important protective antigen for Atlantic salmon against scabies [95]. Similarly, Kroniger et al. also discovered that under iron-limited conditions, the outer membrane vesicles (OMVs) of A. salmonicida were enriched with various proteins related to iron acquisition, indicating their potential immunogenicity and promising prospects as vaccine candidates [62]. In addition, pathogenic bacteria can obtain iron by using heme in host cells and tissues through a direct binding mechanism that is independent of iron carriers [96,97]. A. salmonicida has the ability to utilize heme and hemoglobin as iron sources, making heme utilization another way for it to acquire iron [98]. The mechanism for A. salmonicida’s heme uptake is still unclear. According to Lemos and colleagues [99], iron uptake may involve the in vivo expression of heme uptake genes as a common feature for many other bacterial pathogens. As a case in point, when A. salmonicida subsp. salmonicida was cultured in salmonids, the heme receptor was upregulated [94].

3.6. Quorum Sensing

As shown in Figure 3, quorum sensing (QS) is a mechanism by which bacteria regulate gene expression and coordinate group behavior based on changes in population density. In recent years, studies on the QS mechanism of A. salmonicida have gradually increased, providing a new perspective for understanding its pathogenic mechanism. The QS mechanism of A. salmonicida mainly relies on two types of signaling molecules: N-acyl-homoserine lactones (AHLs) and AI-2. The regulation of bacteria’s growth, metabolism, pathogenicity, and biofilm formation is greatly influenced by these signaling molecules [16,100]. For instance, QS signaling molecules of A. salmonicida regulate the expression of related genes, thereby influencing the formation of its biofilm and pathogenicity [100]. The main synthetic genes of AHL in A. salmonicida include luxI and luxR, which play a key role in the generation of QS signals and correspond to communication between bacteria and within bacteria, respectively [101]. Furthermore, the production of AHL signals is positively correlated with the expression of various virulence factors (such as hemolysins and gas toxins), which provides new ideas for the development of control strategies against A. salmonicida [102]. There are significant differences in the synthesis levels of AHL produced by A. salmonicida at different temperatures. An increase in temperature leads to an increase in AHL synthesis, thereby enhancing pathogenicity [103]. AI-2 plays the role of a universal signal molecule in the communication among bacteria. Research has found that the mucin of Atlantic salmon can inhibit the AI-2 level of A. salmonicida and regulate its QS mechanism in an N-acetyllactosamine-dependent manner [16]. This indicates that the QS mechanism of A. salmonicida not only affects its pathogenic ability but is also closely related to the defense mechanism of the host. Apart from AHLs and AI-2, adenosine is also an important quorum-sensing signal molecule in A. salmonicida. Adenosine can regulate the quorum-sensing signals of A. salmonicida and thereby affect the formation of its biofilm and pathogenicity [100].

Interfering with QS can significantly attenuate the virulence of A. salmonicida. For instance, lactone-based compounds act as QS inhibitors, effectively reducing biofilm formation and virulence factor secretion in this bacterium, thereby diminishing its pathogenicity [103]. Additionally, certain plant-derived extracts and compounds, such as furanones and acylated homoserine lactones, have demonstrated potent QS-inhibitory activity [102]. This QS-targeting strategy represents a groundbreaking paradigm in antibacterial control.

4. The Pathogenicity and Clinical Signs of A. salmonicida

4.1. Fish

While A. salmonicida has historically been termed the pathogen of furunculosis in fish, this designation is now recognized as misleading. The majority of infections manifest clinical signs inconsistent with classical furunculosis pathology, and atypical strains often present without furuncles, the nominal hallmark of the disease. Host-specific variations in symptomatology further complicate this classification. Generally, the main symptoms of infection are abscesses, ulceration, and skin ulceration [104]. For instance, the skin and dermis of carp (Cyprinus carpio) show progressive ulcers [7], while green-spotted flathead (Thamnaconus septentrionalis) and Schlegeli’s sea perch (Sebastes schlegeli) frequently present with swollen mouths accompanied by base-of-fin ulceration [105]. Moreover, systemic symptoms can also include swimming disorders, reduced food intake, shedding of scales, edema of internal organs, hemorrhagic lesions, etc. [106,107,108]. These combined clinical manifestations are significantly different from the traditional pathological features of furunculosis.

4.2. Others

The emergence of thermophilic strains of A. salmonicida has led to an increase in cases of infection in humans and mammals, highlighting the potential risk of cross-species transmission. As of now, there have been no reports of specific pathological changes in the clinical symptoms of this bacterial infection in mammals. Secondary infection cases are the main source of most research results. For instance, there has been a succession of multiple reports of human clinical infection with this bacterium in India. A 34-year-old female patient’s blood [109], a 67-year-old male with weakened immunity’s skin infection [110], and a 55-year-old female’s right eye after recovery from cataract surgery [111] were all found to be infected with this bacterium. It is worth noting that in 2019, Vincent’s team reported two cases of human infection. The researchers confirmed through a mouse model that the isolates could cause necrotizing fasciitis and liver damage, which was the first time that their pathogenicity to mammals was confirmed in animal experiments [112]. In terms of animal-source infections, research has found that the strains from different host sources all demonstrate pathogenic potential. For instance, Vincent et al. isolated strains carrying key virulence genes from the liver and kidney tissues of the dead recurvirostra avosetta that suffered from sepsis [113], and Deng et al. reported that an atypical isolate from pigs could cause infection in mice [114]. In their study on goat acute interstitial pneumonia, Wang et al. [115] isolated A. salmonicida from lung tissue and subsequently infected mice with this pathogen. The infected mice exhibited pathological features similar to those observed in the original goat host, further confirming the bacterium’s remarkable cross-host adaptability. These findings fundamentally challenge the historical view of A. salmonicida as a cold-restricted pathogen. Its demonstrated thermotolerance at mammalian body temperature (37 °C), combined with cross-species transmission capacity, raise significant concerns regarding zoonotic risks and food safety implications.

5. Prevention and Treatment of A. salmonicida

At present, the clinical treatment of bacterial diseases still relies primarily on traditional antimicrobial drugs. For A. salmonicida infections, timely antibiotic intervention can yield significant therapeutic efficacy. Commonly used antibiotics and their sensitivities are shown in

Table 2. Antimicrobial resistance and susceptibility of A. salmonicida.

Antibiotics	Classification	Sensitivity	References
Oxytetracycline	Tetracyclines	I	[116]
Tetracycline		S	[117]
Vancomycin	Glycopeptides	R	[118]
Florfenicol	Amides	S	[119]
Kanamycin	Aminoglycosides	S	[118]
Gentamicin		S	[120]
Cefoxitin	Cephalosporins	S	[121]
Ceftriaxone		S
Cefepime		S
Cefpodoxime		S
Cephalothin		S	[120]
Rifamycin		R	[117]
Erythromycin	Macrolides	I	[116]
Trimethoprim	Sulfonamides	R	[122]
Sulfadiazine		R
Enrofloxacin	Quinolones	S	[118]
Norfloxacin		S	[116]
Ciprofloxacin		S	[117]

Note: R—resistant; I—intermediately sensitive; S—sensitive.

.

This treatment strategy originated from the groundbreaking discovery by Snieszko’s team in 1949 [123] regarding the bacteriostatic effects of sulfadiazine, subsequently leading to the development of a series of effective drugs, including sulfonamides, tetracyclines (oxytetracycline), oxolinic acid, and amoxicillin [124]. However, clinical monitoring data reveal that prolonged antibiotic overuse has resulted in the widespread dissemination of resistant plasmids and a marked increase in the frequency of drug resistance gene mutations in A. salmonicida, severely compromising treatment outcomes. This has made the standardized use of antibiotics a pivotal issue restricting the sustainable development of aquaculture.

In recent years, with the rapid development of the Chinese medicine industry, researchers have begun to explore the feasibility of using Chinese herbal preparations as alternatives to antibiotics, and have made some progress in this regard. For instance, Gu et al. [125] found that the organic acid extracts of honeysuckle and dandelion had significant inhibitory effects on A. salmonicida; and Li et al. [126] discovered that the compound Chinese herbal medicine preparation composed of Punica granatum peel, galla chinensis, and paeoniae radix alba (in a ratio of 1:1:1) showed significant antibacterial and bactericidal activities against A. salmonicida. These studies provide theoretical support for the use of Chinese herbal medicine in disease prevention and treatment. However, its practical application still faces numerous challenges: the aquaculture industry relies heavily on economic efficiency, yet Chinese herbal medicines currently cannot match traditional antibiotics in terms of production costs or speed of efficacy; moreover, related research remains largely confined to laboratory exploration, lacking mature promotion strategies, which results in low acceptance among farmers.

In the aquaculture industry, vaccination remains a critical measure for preventing and controlling epidemic outbreaks. However, concerning the prevention and treatment of A. salmonicida, China has yet to achieve the commercial application of vaccines, with current research still primarily confined to laboratory-stage achievements.

Phages can efficiently recognize and lyse bacteria, stimulating the non-specific immune systems of fish, making them potential alternatives to antibiotics [127]. Numerous studies have demonstrated their efficacy against A. salmonicida: The phage isolated by Xu et al. [119] exhibited potent antibacterial activity both in vitro and in vivo, significantly reducing mortality rates in turbot (Scophthalmus maximus). Nikapitiya et al. [128] identified a novel myoviridae phage that not only inhibited pathogen growth but also remained effective against multidrug-resistant strains, with notable environmental stability (temperature, pH, salinity). Notably, Hosseini et al. [129] demonstrated that a cocktail of 2–5 phages exhibits superior antibacterial efficacy compared to single-phage application. Meanwhile, Zhou Yan et al. [130] identified the phage Asfd-1, which displays strict host specificity without perturbing gut probiotic flora. These studies provide scientific evidence for using bacteriophages as antibiotic alternatives in aquaculture [131]. However, phage-based biologics (being viral agents) still pose safety concerns. Research demonstrates that some phage genes may integrate into the host genome or acquire bacterial virulence genes during replication [132]. More critically, their mediation of horizontal antibiotic resistance gene transfer could exacerbate pathogen drug resistance. These genetic interactions underscore the necessity for cautious evaluation before employing phage therapies.

6. Summary and Prospects

The transmission risks of A. salmonicida have been increased by the diversification of aquaculture species in recent years. A. salmonicida was initially thought to only affect salmonid fish, but it has now been proven to infect a broad range of fish species. In addition, the emergence of mesophilic strains in recent years, as well as the adaptation of psychrophilic strains to warmer conditions, has increased its host diversity. Despite the rarity of infections in mammals, the potential health risks still warrant attention.

Antibiotics continue to be the primary approach for prevention and control, but their long-term use has caused severe issues such as multidrug resistance and environmental pollution. Although vaccines, probiotics, and phage therapy have promising potential, most of them remain confined to laboratory-stage research. Compounding these challenges, the rapid evolution of pathogens driven by environmental pressures has made disease management increasingly complex.

The following measures are recommended to address these challenges: first, conducting precise taxonomic studies to clarify its classification, and then elucidating key virulence factors and pathogenic mechanisms. To provide scientific support for the prevention and treatment of A. salmonicida, it is necessary to advance systematic research, including on aquatic ecological monitoring, microbial community dynamics analysis, and rapid detection technologies.