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Review

Azo Dyes and the Microbial World: Synthesis, Breakdown, and Bioactivity

by
Valery M. Dembitsky
* and
Alexander O. Terent’ev
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect, 47, 119334 Moscow, Russia
*
Author to whom correspondence should be addressed.
Microbiol. Res. 2025, 16(5), 100; https://doi.org/10.3390/microbiolres16050100
Submission received: 16 April 2025 / Revised: 10 May 2025 / Accepted: 14 May 2025 / Published: 16 May 2025
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Abstract

:
This review discusses natural and synthetic azo compounds found in bacteria, fungal endophytes, fungi, plants, and invertebrates. More than 100 of these compounds have demonstrated significant pharmacological activity, including antitumor, antimicrobial, and antibacterial effects. Using mathematical algorithms and the PASS program, researchers predict new potential applications based on their structure–activity relationships. This review emphasizes the importance of natural azo compounds as promising drug prototypes and key players in drug discovery. It also explores the synthesis and degradation of azo dyes and their potential uses in medicine, food, cosmetics, and related fields. Additionally, the role of microorganisms in producing natural azo compounds and their synthetic counterparts is examined, showcasing their potential in drug development and human health advancements.

1. Introduction

Azo compounds, whether natural or synthetic, contain one or more azo groups (-N=N-), a functional group where two nitrogen atoms are joined by a double bond. Their general formula is R-N=N-R’, with R and R’ being either aryl or alkyl groups. The azo group plays a key role in determining their chemical properties, which are also influenced by other attached groups [1,2,3]. Azo ligands significantly affect the color of these compounds, creating bright and intense shades. These colors may change depending on pH, which may cause changes in the properties of the molecules, probably due to changes in the protonation state, which may lead to changes in electron density [4,5,6].
Azoxy metabolites, in contrast, feature an azoxy functional group, where two nitrogen atoms are linked by a double bond, with one also bonded to an oxygen atom. Their general formula is R-N(O)=N-R’, where R and R’ represent organic groups [7,8,9].
Some bacteria might be capable of producing azo or azoxy compounds, either as part of their metabolism or as secondary metabolites. If so, this ability would be specific to certain strains. Bacteria can also transform or break down synthetic compounds, including azoxy compounds, which may contribute to environmental cleanup processes like bioremediation [1,2,10,11,12,13].
Bacteria are often genetically engineered in biotechnology to produce useful chemical compounds. It is possible that bacteria could also be modified to synthesize azo or azoxy compounds for industrial purposes [14,15,16].
This review explores the structures and biological activities of natural azo and azoxy compounds, particularly those produced by microorganisms, as potential sources for drug discovery. It also discusses how microorganisms degrade azo dyes. The presented data allow us to understand the role of microorganisms in the biosynthesis and degradation of these unusual natural metabolites. The findings on azo compounds are particularly relevant to pharmacologists, chemists, and biologists.

2. Microorganisms as Producers of Azo and Azoxy Compounds

A well-known example of a natural azoxy antibiotic is valanimicin (1), which is derived from the fermentation broth of Streptomyces viridifaciens MG456-hF10. Recent research has identified the genes responsible for valanimicin biosynthesis. The VlmF protein, part of the major facilitator superfamily, provides resistance to valanimicin. Meanwhile, VlmD, VlmH, and VlmR convert L-valine into isobutyl-hydroxylamine, and VlmL facilitates the formation of L-seryl-tRNA from L-serine (see Figure 1) [17,18].
Valanimicin is effective against both Gram-positive and Gram-negative bacteria, particularly E. coli BE1121, a mutant strain of E. coli K12 that is deficient in DNA repair. It has also shown toxicity against L1210, P388/S (doxorubicin-sensitive) and P388/ADR (doxorubicin-resistant) murine leukemia cells, with IC50 values of 0.8, 2.7, and 1.4 pg/mL, respectively. In animal studies, valanimicin extended the survival of mice with Ehrlich carcinoma or L1210 leukemia cells [19].
Several valinomycin derivatives have been discovered from different sources. A valanimicin derivative (2, see structure in Figure 2) was found in the culture broth of Streptomyces viridifaciens MG456-hF10 during valanimicin biosynthesis [20], with its structure confirmed using a more stable ammonia adduct (3) [21]. Table 1 summarizes the biological activities of compounds (1–27). The antibiotic LL-BH872α (4), which contains an α,β-unsaturated azoxy group, was isolated from Streptomyces hinnulinis [22]. More recently, LL-BH872a, 2z-OH (5), produced by Actinomadura sp., was found in apricot (Prunus armeniaca) roots [21], along with LL-BH872a, 2z-OH, 4′z-OH (6), produced by Streptomyces misionensis FERM 10459 [23].
Several microorganisms produce antifungal antibiotics. For example, manivamycins A (7) and B (8), isolated from Streptomyces prasinopilosus, showed broad antifungal and antibacterial activity against multiple Candida and Trichophyton species, Cryptococcus neoformans, and Staphylococcus aureus [24]. Another antifungal agent, azoxybacillin (9), was isolated from Bacillus cereus NR2991 and was particularly effective against Aspergillus fumigatus and Trichophyton mentagrophytes [25,26]. Azoxyalkene (10), an unstable azoxy compound, was discovered in Actinomadura sp. (strain A7), which grows in apricot roots, though it only displayed weak antifungal activity against Rhodotorula sp. [21].
Elaiomycin (11), an azoxy antibiotic isolated from Streptomyces hepaticus, was found to strongly inhibit Mycobacterium tuberculosis growth [27,28,29,30]. Elaomycins D–G (12–15), produced by Streptomyces sp. HKI0708, exhibit antimicrobial, anti-mycobacterial, and cytotoxic activities [31,32]. Additionally, elaomycins K (16), L (17), and elaomycin K amide (18), azoxy-type antibiotics found in Streptomyces sp. Tü 6399, showed weak antibacterial effects against Bacillus subtilis, Staphylococcus lentus, and Xanthomonas campestris, a plant pathogen [33].
Antiparasitic antibiotics, particularly those targeting nematodes, are rare. Jietacins A (19) and B (20), isolated from Streptomyces sp. [34,35], showed ten-times the activity of avermectin Bla, a veterinary nematicidal agent, against the pine wood nematode Bursaphelenchus hylocomi [36,37]. Additionally, geralcin C (21), D, and E, isolated from Streptomyces sp. LMA-545, exhibited anticancer properties, with geralcin C showing an IC50 of 0.8 μM against KB and HCT116 cell lines. It also inhibited E. coli DnaG primase, a Gram-negative antimicrobial target, with an IC50 of 0.7 μM [38]. Lastly, propanosine (K-76, 22), identified in Micromonospora chalcea 671-AV2 extracts, inhibited Valsa ceratosperma growth [39].
Lyophylline (23) is an antibiotic obtained from the mushroom Lyophyllum shimeji or its fungal endophytes. It has been found to prevent forebrain blister formation in the cranial mesenchyme at a concentration of 50 μg/mL [40].
The antitumor antibiotics DC1881A (24) and DC1881B (25) are produced by Streptomyces sp. DO-118 [41]. Additionally, the azoxy compounds KA-7367A (26) and KA-7367B (27, activity see in Table 2), which have antifungal properties, were discovered in Streptomyces sp. (KC-7367, FERM BP-1277). KA-7367A (26) showed activity against Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Trichophyton mentagrophytes, and T. rubrum [42].
Calvacic acid (also called alvacic acid or calvatinic acid, 28) and its methyl derivatives (29, see structure in Figure 3, and Table 3) belong to a group of antitumor antibiotics with a diazene N-oxide structure. These compounds are produced by the fungi Calvatia craniformis (Agaricaceae) [43], Calvatia lilacina [44], and Lycoperdon pyriforme [45]. Another antibiotic, clavulanic acid, inhibits the growth of Gram-positive and Gram-negative bacteria at concentrations of 3–6 μg/mL [44]. It has cytotoxic effects, preventing the growth of Yoshida sarcoma cells [43], and has shown carcinostatic activity against hepatoma and leukemia K562 cells [46]. Clavulanic acid is also effective against Helicobacter pylori, a Gram-negative bacterium associated with gastric infections [47].
Amoxicillin–clavulanic acid is frequently used to treat many of the World Health Organization’s Priority Infectious Syndromes in adults and children. However, in some cases, a delayed antibiotic prescription or amoxicillin alone could be sufficient, reducing unnecessary use [48,49]. Additionally, clavulanic acid is being investigated for its potential in preventing and treating scopolamine-induced Alzheimer’s disease [50]. Some of its analogues (30 and 31) have been found to exhibit anti-microtubule activity [51].
Azoformamide (32), its (E)-form (33 and 35), and its azoxy derivatives (34 and 36) have been isolated from the puffball fungus Lycoperdon pyriforme [52,53] and have also been found in Calvatia craniformis [54]. A detection method for azoformamide in food additives has been developed [55].
Table 1. Biological activity of azo dyes produced by microorganisms (1–13) [13,24,34].
Table 1. Biological activity of azo dyes produced by microorganisms (1–13) [13,24,34].
No.Dominated ActivityRankAdditional ActivityRank
1AntineoplasticStrongAntibioticStrong
2AntineoplasticStrongHepatic disorders treatmentStrong
3AntineoplasticStrong
4AntineoplasticModerate
5Antiviral (Arbovirus)ModerateAntineoplasticModerate
6AntineoplasticModerateAntifungalWeak
7AntineoplasticModerateAntifungalModerate
8Mucositis treatmentModerateAntiviral (Arbovirus)Moderate
9AntifungalModerateAntiviral (Arbovirus)Moderate
10Antiviral (Arbovirus)ModerateAntifungalModerate
11VasodilatorModerateAntifungalModerate
12AntifungalModerateAntiviral (Arbovirus)Moderate
13AntifungalModerateNatural killer cell stimulantWeak
Extracts from the fungus Lycoperdon pyriforme yielded two compounds, 4-methoxy-benzene-1-azoformamide (33) and 4-methoxybenzene-1-OH N-azoxyformamide (34), both of which were found to be effective against the parasitic nematode Meloidogyne incognita. A related compound, 3,5-dichloro- 4-methoxybenzene-1-OH-azoxyformamide (37), showed weaker nematicidal effects but higher cytotoxicity than compounds (33) and (34) [56].
Azoxyformamides (34 and 36) and two azoformamide derivatives (38 and 39) were isolated from the fruiting bodies of Calvatia craniiformis and Lycoperdon hiemale, respectively. Compounds (34) and (39) inhibited rootlet growth in lettuce seedlings, suggesting that their azoxy structure contributes to this effect. Additionally, compounds (34, 36, and 39) also inhibited the growth of pearl millet seedlings [57,58].
Fungi and their endophytes are known to produce colored pigments that contain azo groups. One such pigment, the red minor compound deoxyrubroflavin (40), was isolated from the lump fungus Calvatia rubro-flava [59,60]. Another pigment, the orange-colored rubroflavin (41), was found in the dried fruiting bodies of Calvatia rubro-flava and Calvatia craniformis [60,61]. Additional pigments, including oxyrubroflavin (42), craniformin (43), and cranformin (44, activity see in Table 4), were also identified in Calvatia rubro-flava [60,61,62].
The insect-pathogenic fungus Entomophthora virulenta produces a mixture of 4,4′-azoxybenzenedicarboxylic acid (45) and 4,4′-hydroxymethylazoxybenzene carboxylic acid (46), both of which have insecticidal properties [63,64]. A formazan derivative (47) was isolated from Agaricus silvicola [64]. Another notable fungal compound, necatorin (48), was discovered in Lactarius necator. This mutagenic alkaloid contains an azo group within a polycyclic aromatic structure that includes a 1-benzopyran moiety and a ketone group at carbon C2 [65,66,67,68].
Table 2. Biological activity of azo dyes produced by microorganisms (14–27) [13,36,37,41].
Table 2. Biological activity of azo dyes produced by microorganisms (14–27) [13,36,37,41].
No.Dominated ActivityRankAdditional ActivityRank
14AntifungalModerateNatural killer cell stimulantWeak
15Vasodilator, peripheralWeakAntineoplasticWeak
16Antiviral (Arbovirus)StrongMucositis treatmentModerate
17Pre-neoplasticStrongMucositis treatmentModerate
18Mucositis treatmentModerateNatural killer cell stimulantWeak
19Hepatic disorders treatmentStrongAntifungalWeak
20Hepatic disorders treatmentStrongAntifungalWeak
21AntineoplasticStrongAntibioticModerate
22AntineoplasticStrongAntiviral (Picornavirus)Moderate
23AntineoplasticStrongAntiviral (Arbovirus)Weak
24AntineoplasticStrongAntibacterialModerate
25AntineoplasticStrongAntibacterialModerate
26Hepatic disorders treatmentModerateAntifungalModerate
27Hepatic disorders treatmentStrongAntibacterialModerate
The azo dyes 4,4′-dihydroxyazobenzene (49) and its methyl derivative (50) were detected in fresh sporophores of Agaricus xanthodermus and exhibited antimicrobial activity against Staphylococcus species [69,70,71]. Several fungi, including Beauveria bassiana, Beauveria brongniartii, Metarhizium anisopliae, and Verticillium lecanii, produce azo dye toxins (51–57) [72,73,74,75,76].
Additionally, three aromatic azoxy compounds—azoxymycins A (58), B (59), and C (60)—were discovered in Streptomyces chattanoogensis L10 and Streptomyces sp. strain P8-A2 [77,78,79]. Azoxymycin B demonstrated cytotoxicity against human leukemia HL-60 cells, with an IC50 value of 2.2 μM [78].
Asterionellins A (61), B (62), and C (63) are eight-membered compounds with an azoxy-like structure, discovered in the microalga Asterionella sp. [80,81,82]. In the fruiting bodies of Agaricus xanthoderma, researchers identified an unstable agaritine derivative (64) and a metabolite (65) [83]. Another compound, glutamylase phenol (62), was also found in Agaricus sp. [84]. Among these, compound (65) displayed strong antibiotic and anticancer properties [84,85].
Table 3. Biological activity of azo dyes produced by microorganisms (28–41) [13,44,47].
Table 3. Biological activity of azo dyes produced by microorganisms (28–41) [13,44,47].
No.Dominated ActivityRankAdditional ActivityRank
28Anti-Helicobacter pyloriStrong
29Anti-Helicobacter pyloriStrong
30Anti-Helicobacter pyloriStrongFibrinolyticWeak
31Anti-Helicobacter pyloriStrongPre-neoplastic conditions treatmentWeak
32Anti-inflammatoryStrongHemostaticWeak
33Anti-inflammatoryStrongHemostaticWeak
34Anti-Helicobacter pyloriStrong
35Anti-Helicobacter pyloriStrongPre-neoplastic conditions treatmentWeak
36Anti-Helicobacter pyloriStrong
37Anti-Helicobacter pyloriStrongPre-neoplastic conditions treatmentWeak
38Apoptosis agonistStrongAntineoplasticStrong
39Apoptosis agonistStrongAntineoplasticStrong
40Anti-inflammatoryStrongAntineoplasticStrong
41HemostaticStrongAnti-inflammatoryStrong
Azamerone, an anti-infective meroterpenoid obtained from marine Streptomyces sp. grown in salt culture, features a unique chloropyranphthalazinone core with a distinctive side chain. It showed mild cytotoxic effects on mouse T-cell splenocytes and macrophages in laboratory tests. Additionally, the biosynthetic precursor of its azo-containing derivative (66, 3D structure see in Figure 4) was also detected in the same Streptomyces sp. strain [86,87]. The Gram-positive bacterium Streptomyces sp. Ank75 produces azoxy antibiotics 67 and 68, which have antifungal properties against Candida albicans and Mucor miehei [88].
Researchers identified two vasodilators, WS-1228 A (triaxin C, 69) and WS-1228 B (triaxin D, 70), in the culture filtrate of Streptomyces aureofaciens [89,90]. A few years later, Omura and colleagues [91] discovered triaxins A (69) and B (70) in Streptomyces sp. SK-1894 and found that they function as acyl-CoA synthetase inhibitors. These compounds, along with WS-1228 A and B, share a unique N-hydroxytriazene structure and are known for their vasodilatory effects. Additionally, they inhibit acyl-CoA synthetase, with IC50 values of 5.5 μg/mL for triaxin A and 3.6 μg/mL for WS-1228 A. The triaxins (A, B, C, and D) differ in their inhibition levels, with triaxin C (71) being the most potent, followed by triaxin A, triaxin D (72), and triaxin B [92,93].
A rare compound, citreoazopyrone (73), was isolated from a hybrid strain of Penicillium citreo-viride and was found to inhibit lettuce seedling growth [94]. The fluviol groups of antibiotics (74–77) and pseudoiodine (78), produced by Pseudomonas fluorescens and Nostoc spongiaeforme, exhibit antimicrobial, antiviral, and antitumor effects [95,96,97,98]. The activities of compounds 78–98 are detailed in Table 5. Additionally, schizocommunin (79), isolated from the fungus Schizophyllum commune, showed strong cytotoxic effects against mouse lymphoma cells [99].
Table 4. Biological activity of azo dyes produced by microorganisms (42–60) [13,72,73,74,75,76,77,78].
Table 4. Biological activity of azo dyes produced by microorganisms (42–60) [13,72,73,74,75,76,77,78].
No.Dominated ActivityRankAdditional ActivityRank
42Anti-inflammatoryStrongHemostaticStrong
43Anti-inflammatoryStrongHemostaticStrong
44Anti-inflammatoryStrongHemostaticStrong
45Kidney function stimulantWeakPre-neoplastic conditionsWeak
46ImmunosuppressantWeakFibrinolyticWeak
47Pre-neoplastic conditionsWeakKidney function stimulantWeak
48Anti-mutagenicStrongSpasmolytic, urinaryWeak
49Anti-seborrheicStrong
50CarminativeStrongAnti-microbialModerate
51Phobic disorders treatmentStrongKidney function stimulant (0.786)Moderate
52FibrinolyticModeratePre-neoplastic conditionsModerate
53AcaricideStrongAnti-seborrheicStrong
54Alopecia treatmentModerateAnti-inflammatory, intestinalWeak
55Anti-seborrheicStrongAcaricideModerate
56Pre-neoplastic conditions treatmentWeakImmunosuppressantWeak
57FibrinolyticModerateAcaricideWeak
58Mucositis treatmentStrongImmunosuppressantWeak
59Mucositis treatmentModerateImmunosuppressantWeak
60Antiviral(Arbovirus)WeakImmunosuppressantWeak
Researchers identified 3- and 4-methylcinnolines (80 and 81) in the volatile components of Hibiscus esculentus pods [100], while azoxy compounds (82 and 83) were found in yeast extract [101]. Two cinnoline derivatives, compound 84 and 4849F (85), were isolated from Streptomyces sp. 4849.171. Compound 84 functions as an IL-4 receptor inhibitor, while 4849F (85) shows antibacterial properties [102].
Pyridazomycin (86), an antifungal antibiotic from Streptomyces violaceoniger sp. griseofuscus, inhibits Mucor hiemalis [103,104]. Both pyridazomycin (86) and its chloride salt analogue (87) exhibit antimicrobial activity [105]. The compound 5-amino-1,4-dihydro-7H-1,2,3-triazolo [4,5-d]-pyrimidin-7-one (88 or 89), also called 8-azaguanine, is produced from guanine by Streptomyces albus [106]. Its cytotoxic effects on various cancers, including carcinoma and sarcoma, were first reported over 70 years ago [107].
Another form of 8-azaguanine, known as pathocidin (89), is an antifungal antibiotic obtained from Actinomycetes [108,109,110], which effectively inhibits fungal growth, including Penicillium chrysogenum. Two related natural metabolites, 8-azaguanine-3N-β-D-ribofuranosyl (90) and its 5′-phosphate version (91, see structure in Figure 5), have demonstrated anticancer effects against lymphoid leukemia and adenocarcinoma [111].
The red dinoflagellate Gymnodinium breve (syn. Ptychodiscus brevis) produces the antibiotic 6-azidotetrazolo [5,1-a]phthalazine (92) [112]. Additionally, the pathogenic Gram-negative Burkholderia species secretes extracellular enzymes involved in various biological processes, as well as toxins, antibiotics, and siderophores [113]. A rare dimeric siderophore, malleobactin D (93, see structure in Figure 6, and activity shown in Table 6), was identified in Burkholderia pseudomallei (formerly Pseudomonas pseudomallei) [114].
Amatoxins are toxic bicyclic octapeptides found in several fungi from the Agaricales family, including Amanita phalloides, A. ocreata, A. verna, A. bisporigera, Conocybe filaris, and Galerina marginata (syn. G. autumnalis), among others. These toxins are known for their harmful effects, including heat-induced toxicity, gastrointestinal issues, and the ability to inhibit RNA polymerase II [115,116,117,118]. Azoamanitins (94–97), which are semi-synthetic versions of amatoxins, have been studied for their potential in antiviral, antimicrobial, and anticancer treatments [119,120,121].

3. Azo Dyes Derived from Terrestrial and Marine Sources

Azo dyes have been found in both terrestrial and marine environments, but it remains unclear whether bacteria or fungal endophytes played a role in their production, as these microorganisms often act as symbionts in plants and invertebrates [1,2,3,4,8,13,79]. Some key compounds isolated from terrestrial and marine sources include the following:
Geleganidin D (98, see structure in Figure 7 and activity in Table 7): A dimeric monoterpenoid indole alkaloid from Gelsemium elegans with moderate cytotoxic activity against MCF-7 and PC-12 cells [122]. G. elegans is used in clinical applications due to its indole alkaloids, which have antitumor, anti-inflammatory, analgesic, and immune-modulatory effects, though its therapeutic dose is close to toxic levels [122].
Brachystemidin G (99): An alkaloid from Brachystemma calycinum with potent immunosuppressant properties, inhibiting murine T- and B-lymphocyte proliferation (IC50 = 5.6 µg/mL) [123].
1,2,4-Triazine derivative (100): Extracted from Butea monosperma (family Caryophyllaceae) [124]. This plant’s odor repels mosquitoes, its flowers are used as a dye, and its gum, known as kamarkas in Hindi, is used in traditional cooking [125].
bis-(2,4-Dimethyl-6-nitrophenyl)-diazene (101): Isolated from the leaves of Aconitum sungpanense [126]. Azoxyglycosides, which contain the aglycone methylazoxymethanol (MAM), have been found in Cycadaceae plants, with all known glycosides having β-glycosidic linkages [127]. Metabolites, such as methylazoxymethane (102), MAM (103), methylazoxymethanol acetate (104), and cycasins (105,106), have been extracted from Cycadaceae, Stangeriaceae, and Zamiaceae, which are coniferous trees found in tropical and subtropical regions [127,128,129,130,131,132]. MAM (103) has been linked to liver and kidney carcinomas [133], while cycasins (105,106) and macrosamine (107) are highly toxic azoxyglycosides. Various insects, including caterpillars and weevils, feed on cycads’ roots, stems, leaves, and reproductive parts [127,132].
Azoxyglycosides are believed to function as natural herbivore deterrents [134,135,136]. Cycasin, the primary azoxyglycoside found in cycads, has been shown to cause effects similar to those seen in carcinogenicity, mutagenicity, and neurotoxicity tests. The first isolation of neocycasin A (108) was recorded in 1959 [137], and, more recently, neocycasins B–J (109–117) have been identified from different plant species [138,139,140,141,142,143,144,145,146].
The first cytotoxic bis-3-alkylpyridine alkaloid with an azoxy moiety, pyrinadine A (118), was discovered in the Okinawan marine sponge Cribrochalina sp. [147,148]. Further cytotoxic bis-3-alkylpyridine alkaloids, pyrinadines B–H (118–125), were later extracted from the same sponge, demonstrating cytotoxicity against P388 murine leukemia cells [147,148,149]. The activities of these compounds (118–125) are summarized in Table 7.
Table 7. Biological activity of azo dyes produced by microorganisms (98–125) [13,122,137,144].
Table 7. Biological activity of azo dyes produced by microorganisms (98–125) [13,122,137,144].
No.Dominated ActivityRankAdditional ActivityRank
98AntineoplasticModerateAntiprotozoal (Plasmodium)Weak
99β-1,3-Galactosyl-O-glycosyl-glycoproteinStrongβ-1,6-N-acetylglucosaminyl
transferase inhibitor		Strong
100Neurodegenerative diseases treatmentStrong
101AntiparkinsonianStrongAnxiolyticStrong
102AcaricideModerateAntiviral (Arbovirus)Weak
103AntineoplasticStrong
104AntineoplasticModerate
105AntineoplasticModerate
106Anti-infertility, femaleStrongAntineoplasticStrong
107CarcinogenicStrongEmbryotoxicStrong
108CarcinogenicStrongEmbryotoxicStrong
109AntineoplasticStrong
110CarcinogenicStrongEmbryotoxicStrong
111AntineoplasticStrong
112AntineoplasticStrongAnti-infectiveStrong
113AntineoplasticStrongAnti-infectiveStrong
114AntineoplasticStrongAnti-infectiveStrong
115AntineoplasticStrongAnti-infectiveStrong
116AntineoplasticStrongAnti-infectiveStrong
117AntineoplasticStrongAnti-infectiveStrong
118CytotoxicModerateAnti-inflammatoryWeak
119CytotoxicModerateAnti-inflammatoryWeak
120CytotoxicModerateAnti-inflammatoryWeak
121CytotoxicModerateAnti-inflammatoryWeak
122CytotoxicModerateAnti-inflammatoryWeak
123CytotoxicModerateAnti-inflammatoryWeak
124CytotoxicModerateAnti-inflammatoryWeak
125CytotoxicModerateAnti-inflammatoryWeak

4. Microbial Degradation of Textile Azo Dyes

The textile industry relies heavily on both natural and synthetic azo dyes [150,151,152]. Natural dyes have been used since ancient times and vary in color and chemical structure. Among synthetic azo dyes, commonly used examples include Orange II (126, see 3D structure in Figure 8 and activity in Table 8), Methyl Orange (127), Congo Red (128), Evans Blue (129), Bismarck Brown (130), Xylidine Ponceau (131), and Eriochrome Black (132). These dyes are widely applied in the textile and leather industries but contribute to environmental pollution, particularly through wastewater discharge. This contamination affects ecosystems, microbial communities in the soil, and the natural balance of certain species.
Azo dyes are aromatic compounds containing azo (-N=N-) and sulfonic (-SO₃-) groups. They are produced through the diazotization of aromatic amines, followed by a coupling reaction, creating a range of colors from yellow to red and orange [150,151,152].
Every year, inefficient dyeing processes result in the release of hundreds of thousands of tons of textile dyes into wastewater. This improper disposal has severe environmental consequences and poses health risks to humans and animals. Many azo dyes are known to be carcinogenic and mutagenic [153,154,155,156]. While various wastewater treatment methods exist, one promising approach is the use of microorganisms capable of degrading azo dyes naturally [157,158,159,160].
Extensive research has been conducted in this field, with numerous studies published annually. However, in this discussion, we will focus on how Actinomyces and other microbes contribute to azo dye degradation [161,162,163]. Several species of Streptomyces have been found to completely break down azo pigments (128–133) (see structures in Figure 9). These include S. chromofuscus, S. coelicolor, S. viridosporus, S. rochei, S. psammoticus, S. diastaticus, S. ipomoeae, S. sviceus, S. ardesiacus, and S. cyaneus [164,165,166,167]. In addition, a mixed culture of two Pseudomonas strains has shown effective degradation of azo dyes. Microbial enzymes known as azoreductases play a crucial role in this process by cleaving azo bonds (-N=N-) under mild conditions (see Figure 10). These enzymes are valuable for developing environmentally friendly dye removal methods [168,169].
Table 8. Biological activity of textile azo dyes [164,165,166,167,168,169].
Table 8. Biological activity of textile azo dyes [164,165,166,167,168,169].
No.Dominated ActivityRankAdditional ActivityRank
126EmbryotoxicStrong
127CarcinogenicStrongMutagenicStrong
128CarcinogenicStrongMutagenicStrong
129AntineoplasticStrong
130AntiviralStrong
131AntibacterialModerateAntifungalModerate
132AntimicrobialModerateAntibacterialModerate

5. Microbial Degradation of Food Azo Dyes

Azo dyes are widely used in the food industry as color additives in products, such as soft drinks, jams, candies, and pickles (azo dye coloring in Figure 11) [170]. Their popularity comes from their ability to produce bright, vibrant colors while being inexpensive, stable, and free from unwanted tastes or odors.
However, while azo dyes make food visually appealing, they may also pose health risks. Consuming them within the acceptable daily intake is generally considered safe, but higher doses—especially in children—have been linked to various health issues. These include allergies, ADHD, asthma, anxiety, and even potential cytotoxic and genotoxic effects, which may contribute to cancer development. Additionally, azo dyes are highly stable and do not break down easily under aerobic conditions [171,172,173].
Several synthetic azo dyes, such as Sunset Yellow (133, see structure in Figure 12, and activity see in Table 9), Carmoisine (134), Amaranth (135), Ponceau 4R (136), Brown HT (137), and Allura Red (138), are commonly added to food [170]. They enhance color but offer no nutritional benefits, preservation properties, or health advantages. Because they are cost-effective, chemically stable, and provide intense coloration without altering taste, the food industry often prefers them over natural dyes.
Although food dyes are regularly tested for safety, their health effects remain controversial. Some studies suggest that the breakdown of azo dyes can produce harmful byproducts, raising concerns about their long-term safety [174,175].
Food azo dyes may have potential health risks, primarily due to their breakdown by intestinal bacteria after ingestion. When these dyes are reduced by gut microflora, their azo bonds are cleaved, producing aromatic amines. These amines can then undergo further chemical modifications, such as N-hydroxylation or N-acetylation. In mammals, azo dye reduction mainly takes place in the anaerobic environment of the lower gastrointestinal tract, where bacterial azoreductase enzymes are responsible for the process. While mammalian enzymes in the liver and kidney also contribute to azo dye metabolism, their exact role is not yet fully understood [176,177].
Many anaerobic bacteria found in the cecal contents of both humans and experimental animals have been shown to break down azo dyes into aromatic amines. This reaction is facilitated by azoreductases, which require flavins for optimal activity and are highly sensitive to oxygen [172,173,174,175,176,177].
Aromatic amines and their derivatives are a major class of chemicals formed when azo dyes are broken down by azoreductases, which cleave the azo bond. Different microorganisms produce azoreductases with varying kinetic properties and substrate specificities. Research shows that flavin-containing azoreductases follow a Ping-Pong bi-bi reaction mechanism, where the enzyme’s activity depends on the substrate concentration. Many azoreductases show a higher affinity for NADPH over NADH, although they can utilize either as an electron source, with or without flavin [178,179,180].
Not all bacteria produce azoreductases, but several known species do. These include strains from both animals and humans, such as Aquiflexum sp., Bacillus sp., Clostridium perfringens, Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, Rhodobacter sphaeroides, Shewanella oneidensis, Staphylococcus aureus, and Xenophilus azovorans KF46F [178,181]. Several azo dyes are commonly used in food production:
Sunset Yellow (133) is an artificial dye used to enhance food appearance and shelf life. It has antioxidant properties, which increase with concentration, but also exhibits phytotoxicity [182,183,184].
Carmoisine (134) is a red azo dye used in foods that undergo heat treatment after fermentation. While azo dyes have been linked to carcinogenicity, especially bladder cancer, carmoisine itself has not been proven carcinogenic. Interestingly, its copper complex has demonstrated anticancer activity against colon cancer cells (HT-29), making it a potential candidate for future treatments [185,186,187].
Amaranth (135) is a red food dye used worldwide, although its use is banned in India due to concerns about its safety. Despite this, illegal production continues, with India contributing 2% of global output. Some studies suggest that amaranth may be carcinogenic under certain conditions [188,189].
Ponceau 4R (136) is widely used in pharmaceuticals and food products. It is stable in light, heat, and acidic conditions but breaks down when exposed to ascorbic acid. While approved in Europe, Asia, and Australia, the US FDA has not approved it due to concerns about its potential to form non-sulfone aromatic amines, which may be carcinogenic [190,191].
Brown HT (137), also called “food brown 3” or “chocolate brown HT”, is a pigment with antioxidant properties. It is found in small amounts in coal tar [192,193,194].
Allura Red (138) is a widely used food dye, commonly found in sweets, beverages, and processed foods. It dissolves in water but poorly in 50% ethanol. Long-term consumption may cause liver and kidney toxicity [195,196,197]. Degradation of azo dyes and the cleavage of the (-N=N-) bond are shown in Figure 13.

6. Conclusions

Natural and/or synthetic azo compounds represent a valuable class of bioactive molecules with diverse pharmacological applications. Their confirmed antitumor, antimicrobial, and antibacterial properties, along with predicted novel activities, highlight their potential as drug prototypes and leads in pharmaceutical development. The role of microorganisms in the biosynthesis of natural azo compounds and their synthetic analogs further expands opportunities for drug discovery and therapeutic advancements. Additionally, the synthesis and degradation pathways of azo dyes present promising applications in medicine, food, cosmetics, and related industries. Continued research into the structure–activity relationships and biological mechanisms of azo compounds will be essential for unlocking their full potential in improving human health.

Author Contributions

Conceptualization, V.M.D.; methodology, V.M.D.; software, A.O.T.; investigation, V.M.D.; resources, V.M.D.; writing—original draft preparation, A.O.T. and V.M.D.; writing—review and editing, A.O.T. and V.M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no funding.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Common biosynthetic pathway for valanimycin by Streptomyces species, showing structures of the primary precursors l-valine and l-serine and structures of known intermediates. The pathway proceeds from L-valine and L-serine through the intermediacy of isobutylhydroxylamine.
Figure 1. Common biosynthetic pathway for valanimycin by Streptomyces species, showing structures of the primary precursors l-valine and l-serine and structures of known intermediates. The pathway proceeds from L-valine and L-serine through the intermediacy of isobutylhydroxylamine.
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Figure 2. Bioactive azo and azoxy compounds derived from microorganisms. The presented natural metabolites demonstrate strong or moderate antineoplastic, antiviral, or antifungal activities.
Figure 2. Bioactive azo and azoxy compounds derived from microorganisms. The presented natural metabolites demonstrate strong or moderate antineoplastic, antiviral, or antifungal activities.
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Figure 3. Bioactive azo compounds are produced by fungi and fungal endophytes. The presented natural metabolites demonstrate strong or moderate antineoplastic, anti-inflammatory, anti-Helicobacter pylori or antifungal activities.
Figure 3. Bioactive azo compounds are produced by fungi and fungal endophytes. The presented natural metabolites demonstrate strong or moderate antineoplastic, anti-inflammatory, anti-Helicobacter pylori or antifungal activities.
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Figure 4. 3D Chemical structure of azo-containing derivative (66) produced by Streptomyces sp. The (-N=N-) bond is shown in blue, oxygen is shown in red, and chlorine atoms are shown in green. The chemical formula is shown from different directions to better view the structure.
Figure 4. 3D Chemical structure of azo-containing derivative (66) produced by Streptomyces sp. The (-N=N-) bond is shown in blue, oxygen is shown in red, and chlorine atoms are shown in green. The chemical formula is shown from different directions to better view the structure.
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Figure 5. Bioactive azo compounds are produced by bacteria, fungi, and fungal endophytes. The azo compounds shown exhibit strong or moderate antineurotic, anti-allergic, antifungal, antineurotic or analgesic activity, and are also DNA synthesis inhibitor, and Lysase stimulant.
Figure 5. Bioactive azo compounds are produced by bacteria, fungi, and fungal endophytes. The azo compounds shown exhibit strong or moderate antineurotic, anti-allergic, antifungal, antineurotic or analgesic activity, and are also DNA synthesis inhibitor, and Lysase stimulant.
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Figure 6. Bioactive azo dyes produced by the bacterium Pseudomonas pseudomallei. The presented natural metabolites demonstrate strong or moderate antiviral, antineoplastic, anti-ischemic, anxiolytic or anti-Helicobacter activities, and also demonstrate properties for the prevention of guanyl-specific ribonuclease T1 inhibitor, hepatic disorders treatment, gout treatment.
Figure 6. Bioactive azo dyes produced by the bacterium Pseudomonas pseudomallei. The presented natural metabolites demonstrate strong or moderate antiviral, antineoplastic, anti-ischemic, anxiolytic or anti-Helicobacter activities, and also demonstrate properties for the prevention of guanyl-specific ribonuclease T1 inhibitor, hepatic disorders treatment, gout treatment.
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Figure 7. Bioactive azo compounds isolated from various sources. The presented natural metabolites demonstrate moderate or strong cytotoxic, antineoplastic, anti-parkinsonian activity, and also demonstrate strong properties for neurodegenerative diseases treatment.
Figure 7. Bioactive azo compounds isolated from various sources. The presented natural metabolites demonstrate moderate or strong cytotoxic, antineoplastic, anti-parkinsonian activity, and also demonstrate strong properties for neurodegenerative diseases treatment.
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Figure 8. Azo dyes used in the textile industry. The presented synthesized compounds demonstrate strong carcinogenic activity and are dangerous to human health. Degradation using various microorganisms is important. Details are described in the text.
Figure 8. Azo dyes used in the textile industry. The presented synthesized compounds demonstrate strong carcinogenic activity and are dangerous to human health. Degradation using various microorganisms is important. Details are described in the text.
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Figure 9. 3D structure of bioactive synthetic azo dye Orange II (126). Orange II or other azo dyes from the food industry are ingested via digestion and are metabolized to aromatic amines by intestinal microorganisms. Reductive enzymes in the liver can also catalyze the reductive cleavage of the azo bond to produce aromatic amines. However, evidence suggests that intestinal microbial azo reductase may be more important than liver enzymes in azo reduction. Azoreductase activity in various intestinal preparations was dependent on various dietary factors such as cellulose, proteins, fibers, antibiotics, or the addition of live Lactobacilli cultures.
Figure 9. 3D structure of bioactive synthetic azo dye Orange II (126). Orange II or other azo dyes from the food industry are ingested via digestion and are metabolized to aromatic amines by intestinal microorganisms. Reductive enzymes in the liver can also catalyze the reductive cleavage of the azo bond to produce aromatic amines. However, evidence suggests that intestinal microbial azo reductase may be more important than liver enzymes in azo reduction. Azoreductase activity in various intestinal preparations was dependent on various dietary factors such as cellulose, proteins, fibers, antibiotics, or the addition of live Lactobacilli cultures.
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Figure 10. Summarizing the articles describing the degradation of azo dyes by various bacteria Pseudomonas strains or Streptomyces strains, we can conclude that bacteria adsorb azo dyes and with the help of azoreductase enzyme the (-N=N-) bond is cleaved. Aromatic hydrolysis products undergo oxidative coupling with subsequent opening of the aromatic ring. And at the final stage, the oxidation products of the aromatic ring are included in the citric acid cycle. The figure shows images Pseudomonas and Streptomyces strains that may be involved in the degradation of azo dyes and the cleavage of the azo bond.
Figure 10. Summarizing the articles describing the degradation of azo dyes by various bacteria Pseudomonas strains or Streptomyces strains, we can conclude that bacteria adsorb azo dyes and with the help of azoreductase enzyme the (-N=N-) bond is cleaved. Aromatic hydrolysis products undergo oxidative coupling with subsequent opening of the aromatic ring. And at the final stage, the oxidation products of the aromatic ring are included in the citric acid cycle. The figure shows images Pseudomonas and Streptomyces strains that may be involved in the degradation of azo dyes and the cleavage of the azo bond.
Microbiolres 16 00100 g010
Microbiolres 16 00100 g011
Figure 11. Food azo dyes coloring: Sunset Yellow (1), Carmoisine (2), Amaranth (3), Ponceau 4R (4), Brown HT (5), and Allura Red (6). The azo dyes mentioned above are added to foods to impart color, but they have no nutritional, food preservation, or health benefits. Because of their availability, accessibility, stability, and low cost, and because they provide intense color to the product without introducing undesirable flavors, the food industry often prefers to use synthetic azo dyes instead of natural dyes.
Figure 11. Food azo dyes coloring: Sunset Yellow (1), Carmoisine (2), Amaranth (3), Ponceau 4R (4), Brown HT (5), and Allura Red (6). The azo dyes mentioned above are added to foods to impart color, but they have no nutritional, food preservation, or health benefits. Because of their availability, accessibility, stability, and low cost, and because they provide intense color to the product without introducing undesirable flavors, the food industry often prefers to use synthetic azo dyes instead of natural dyes.
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Microbiolres 16 00100 g012
Figure 12. Azo dyes, as an important part of food additives, can not only improve the sensory properties of food products, but also increase appetite. They cannot provide the human body with any nutrients and even have toxicity, carcinogenic or mutagenic effects, which can cause serious harm to the health of consumers.
Figure 12. Azo dyes, as an important part of food additives, can not only improve the sensory properties of food products, but also increase appetite. They cannot provide the human body with any nutrients and even have toxicity, carcinogenic or mutagenic effects, which can cause serious harm to the health of consumers.
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Microbiolres 16 00100 g013
Figure 13. General scheme degradation of azo dyes which occurs in several stages when using microorganisms. In the first stage, azoreductases cleave the azo bond using NADH or NADPH, forming an intermediate product, hydrazo intermediate, which, when cleaved, forms aromatic amines. Microorganisms producing azoreductases participate in this process. In the second stage, aromatic amines are converted into the corresponding phenols and their derivatives, under the action of deaminase. Dioxygenase breaks down aromatic phenols into the corresponding dicarboxylic acids and their derivatives. And in the last stage, the enzymes decarboxylase and then hydrolase convert dicarboxylic acids into pyruvic acid and acetaldehyde, and the latter metabolites can be converted into water and carbon dioxide. The figure shows images of various bacteria that may be involved in the degradation and utilization of azo dyes.
Figure 13. General scheme degradation of azo dyes which occurs in several stages when using microorganisms. In the first stage, azoreductases cleave the azo bond using NADH or NADPH, forming an intermediate product, hydrazo intermediate, which, when cleaved, forms aromatic amines. Microorganisms producing azoreductases participate in this process. In the second stage, aromatic amines are converted into the corresponding phenols and their derivatives, under the action of deaminase. Dioxygenase breaks down aromatic phenols into the corresponding dicarboxylic acids and their derivatives. And in the last stage, the enzymes decarboxylase and then hydrolase convert dicarboxylic acids into pyruvic acid and acetaldehyde, and the latter metabolites can be converted into water and carbon dioxide. The figure shows images of various bacteria that may be involved in the degradation and utilization of azo dyes.
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Table 5. Biological activity of azo dyes produced by microorganisms (61–78) [13,89,90,95].
Table 5. Biological activity of azo dyes produced by microorganisms (61–78) [13,89,90,95].
No.Dominated ActivityRankAdditional ActivityRank
61Genital warts treatmentModerateAntineoplasticWeak
62Genital warts treatmentModerateAntineoplasticWeak
63Genital warts treatmentModerateAntineoplasticWeak
64Mucositis treatmentStrongAntiviral (Arbovirus)Moderate
65Alopecia treatmentWeakVascular (peripheral) disease treatmentWeak
66AntineoplasticStrongAntibacterialWeak
67Hepatic disorders treatmentStrongHepatoprotectantStrong
68Hepatic disorders treatmentStrongAntineoplasticModerate
69VasodilatorStrongVasodilator, peripheralModerate
70VasodilatorStrongVasodilator, peripheralModerate
71VasodilatorStrongSpasmolyticModerate
72VasodilatorStrongSpasmolyticModerate
73Anti-ischemic, cerebralModerateAntimicrobialModerate
74Antineoplastic (solid tumors)ModerateAntineoplastic (renal cancer)Moderate
75Gout treatmentStrong
76Guanyl-specific ribonuclease T1 inhibitorModerate
77AnxiolyticStrongPsychotropicModerate
78Atherosclerosis treatmentStrong
Table 6. Biological activity of azo dyes produced by microorganisms (79–97) [13,102,105].
Table 6. Biological activity of azo dyes produced by microorganisms (79–97) [13,102,105].
No.Dominated ActivityRankAdditional ActivityRank
79Endothelial growth factor antagonistStrong
80Phobic disorders treatmentModerateAntineuroticModerate
81AntineuroticModeratePhobic disorders treatmentWeak
82Lysase stimulantStrong
83Lysase stimulantStrong
84Interleukin 4 antagonistModerate
85AntineuroticStrongPhobic disorders treatmentModerate
86AntifungalStrongAntimicrobialModerate
87AntifungalModerateAntimicrobialModerate
88Pterin deaminase inhibitorStrong
89Anti-allergicModerateCytostaticModerate
90DNA synthesis inhibitorStrongCytostaticModerate
91DNA synthesis inhibitorStrongNeuroprotectorStrong
92Antineoplastic (sarcoma)Strong
93AntineoplasticStrongAntibacterialModerate
94AnalgesicModerateGlycopeptide-like antibioticWeak
95Glycopeptide-like antibioticModerateAnalgesicWeak
96Glycopeptide-like antibioticModerateAnalgesicWeak
97Glycopeptide-like antibioticModerateAnalgesicWeak
Table 9. Biological activity of food azo dyes.
Table 9. Biological activity of food azo dyes.
No.Dominated ActivityRankAdditional ActivityRank
133EmbryotoxicStrong
134CarcinogenicStrongMutagenicStrong
135CarcinogenicStrongMutagenicStrong
136AntineoplasticStrong
137AntiviralStrong
138AntibacterialModerateAntifungalModerate
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Dembitsky, V.M.; Terent’ev, A.O. Azo Dyes and the Microbial World: Synthesis, Breakdown, and Bioactivity. Microbiol. Res. 2025, 16, 100. https://doi.org/10.3390/microbiolres16050100

AMA Style

Dembitsky VM, Terent’ev AO. Azo Dyes and the Microbial World: Synthesis, Breakdown, and Bioactivity. Microbiology Research. 2025; 16(5):100. https://doi.org/10.3390/microbiolres16050100

Chicago/Turabian Style

Dembitsky, Valery M., and Alexander O. Terent’ev. 2025. "Azo Dyes and the Microbial World: Synthesis, Breakdown, and Bioactivity" Microbiology Research 16, no. 5: 100. https://doi.org/10.3390/microbiolres16050100

APA Style

Dembitsky, V. M., & Terent’ev, A. O. (2025). Azo Dyes and the Microbial World: Synthesis, Breakdown, and Bioactivity. Microbiology Research, 16(5), 100. https://doi.org/10.3390/microbiolres16050100

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