Microorganisms are biotic, ubiquitous, diverse creatures broadly categorized into viruses, bacteria, archaea, fungi, and protists. Predominantly, bacteria and fungi are explored as potential sources of novel antimicrobial agents. For instance, cyclic peptides- mathiapeptide A, destotamide B, Marfomycins A, B, E; spirotetronates polyketides-abyssomycin C, Lobophorin F, H, as well as alkaloids and sesquiterpenes derivatives, caboxamyxin and mafuraquinocins A, D (
Table 1;
Figure 1) isolated from bacteria, have antimicrobial properties suicidal against clinically resistant bacteria, including
Staphylococcus aureus (
S. aureus), Methicillin-resistant
Staphylococcus aureus (
MRSA),
Micrococcus luteus (
M. luteus),
Bacillus subtilis (
B. subtilis), and
Enterococcus faecalis (
E. faecalis)
[12]. Similarly, ambuic acid analogs, the penicyclones classes; depsidone analogs, the spitomastixones groups; xanthones derivatives, emerixanthones, and engyodontiumsones from fungi, exhibit an anti-infective activity against Gram-negative bacteria,
Escherichia coli (
E. coli) and
Klebsiella pneumoniae (
K. pneumoniae), and several other Gram-positive pathogenic bacteria
[12]. Furthermore, in vivo and in vitro assays have also demonstrated the anti-infective potentials of other microbial products extracted from cyanobacteria
[13][14], microalgae
[14][15], and yeast
[16][17].
2. Bacterial Sources of Antimicrobials
Lactic acid bacteria (LAB) have the tendency to produce antimicrobial compounds (i.e., bacteriocin, organic acids, diacetyl, and hydrogen peroxide), which are effective against harmful bacteria
[19]. Bacteriocin production by
Lactobacillus pentosus (
L. pentosus) ST712BZ isolated from boza antagonizes the proliferation of
Lactobacillus casei (
L. casei),
E. coli,
Pseudomonas aeruginosa (
P. aeruginosa),
E. faecalis,
K. pneumoniae, and Lactobacillus curvatus (
L. curvatus)
[20]. Bacteriocins are low molecular weight polypeptides synthesized in ribosomes and comprise 20–60 amino acid residues
[19]. In 1925, Andre Gratias discovered bacteriocin when he realized that the growth of some
E. coli strains was being impeded by an antibacterial compound, which he named colicin V
[21]. Although there are different classes of bacteriocins produced by other Gram-positive and Gram-negative bacteria as well as archaea, those produced by LAB are the most studied due to their use as food preservatives as well as the frequent incidence of food-borne infectious diseases
[21]. According to Klaenhammer, four groups of bacteriocins exist based on their molecular mass, enzyme sensitivity, thermos-stability, presence of post-translationally modified amino acids, and mode of action
[22]. Class I is made up of lantibiotics and can further be grouped into Ia or Ib depending on the structure and charge of compound. Class II bacteriocins consist of heat-stable peptides with molecular masses less than 10 kDa and can also further be categorized into classes IIa, IIb, and two other types of IIc
[22]. The third class, which consists of high molecular weight (usually >30 kDa) thermo-labile peptides, are represented by Helveticin J and the last class IV, comprises a mixture of large peptides and carbohydrates or lipids
[23]. However, since there is no standard classification for bacteriocins, studies by Cotter et al.
[24], Drider et al.
[25], and others reveal contrasting theories about their classification. In modern times, classification of bacteriocins into three classes based on genetics and biochemical properties is most often used. These classes are class I (lantibiotics), class II (non-lantibiotics), and class III
[25]. Each class of bacteriocins has their own way of exhibiting antimicrobial activity based on their primary structure
[26] see
Table 2. Some bacteriocins attack energized membrane vesicles of target microbes by tampering with their proton motive force
[27], while others enter the cell and disrupt gene expression and protein synthesis
[26]. Lantibiotics fight bacteria in two ways. They alter the bacterial cell wall formation process by binding to lipid II, a hydrophobic carrier of peptidoglycan monomers from the cytoplasm to the cell wall, making the cell unsuitable for certain actions. Lipid II is responsible for membrane insertion and pore formation in the cell membrane of bacteria
[26][28][29].
Non-lantibiotics on the other hand, kill their target cells by binding to MptC and MptD subunits of mannose phosphotransferase permease (Man-PTS) causing an intra-membrane channel to open and ions to continuously diffuse through
[29][30]. Without requiring any receptor molecule circular bacteriocins owing to their high net positive charges are electrostatically attracted to the negatively charged bacteria membrane. This interaction leads to pore formation, efflux of ions, changes in membrane potential, and eventually cell death
[31].
Bacteriolysins enhance cell wall hydrolysis causing the cell to gradually break down
[32][33]. Non-bacteriolysins disrupt glucose uptake in target cells, consequently starving them to death
[34][35][36]. Interactions between antimicrobial compounds and their susceptible microbes can be synergistic or antagonistic
[37].
In veterinary medicine, bacteriocins, such as nisin, have been clinically used to prevent dentobacterial plaque and gingivitis in dogs
[38][39], as a result of its brutal action against strains of
E. faecalis and other canine periodontal disease-causing bacteria
[26].
Rhamnolipid are popular anionic biosurfactants, generally produced by some species of
Pseudomonas and
Burkhloderia [40][41] These compounds have shown a broad spectrum of biological activities, including activities against microorganisms, biofilm, tumors, and oxidation
[42][43][44]. Of great interest is their activity against
Herpes simplex virus 1 and 2 (HSV-1 and HSV-2) and bovine coronaviruses, via interactions with viral lipid membranes and thereby altering viral membrane glycoproteins
[45][46]. Rhamnolipids (M15RL) produced by the Antarctic bacterium,
Pseudomanas gessardii (
P.gessardii) M15, has recently been reported to exert high antiviral activity against Coronaviridae and Herpesviridae families, especially against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)
[47].
Table 2. Examples of bacteriocins, organisms that produce them and microbes that are susceptible to them.
Bacteriocin |
Producer of Bacteriocin |
Susceptible Microorganisms |
Reference(s) |
Nisin A |
Lactococcus lactic subsp. lactis |
E. faecalis ssp. Liquefaciens, Streptococcus equinus, Staphylococcus epidermidis (S. epidermidis), S. aureus, Streptococcus uberis (S. uberis), Streptococcus dysgalactiae (S. dysgalactiae), Streptococcus agalactiae (S. agalactiae), Streptococcus suis (S. suis) Mycobacterium avium subsp. Paratuberculosis |
[48][49][50] |
Nisin ANisin V |
L. lactis NZ9700L. lactis NZ9800nisA:M21V |
Listeria monocytogenes |
[51] |
Pediocin A |
Pediococcus pentosaceus FBB61 |
Clostridium perfringens |
[52] |
Enterocin M |
Enterococcus faecium AL41 |
Campylobacter spp., Clostridium spp. |
[53] |
Enterocin CLE34 |
Enterococcus faecium CLE34 |
Salmonella pullorum |
[26][54] |
Enterocin E-760 |
Enterococcus durans, Enterococcus faecium, Enterococcus hirae |
Salmonella enterica serovar Enteritidis, S. enterica serovar Choleraesuis, S. enterica serovar Typhimurium, S. enterica serovar Gallinarum, E. coli O157:H7, Yersinia enterocolitica, S. aureus, Campylobacter jejuni |
[55] |
Lacticin 3147 |
Lactococcus lactis DPC3147. |
S. dysgalactiae, S. agalactiae, S. aureus, S. uberis, Mycobacterium avium subsp. paratuberculosis |
[50][56] |
Macedocin ST91KM |
Streptococcus gallolyticus subsp.macedonicus ST91KM |
S. agalactiae, S. dysgalactiae, S. uberis, S. aureus |
[57] |
3. Bacterial Sources of Antifungal Compounds
Red pigmented pradimicins A, B, and C are products of the bacteria
Actinomadura hibisca (
A. hibisca)
[58]. These pradimicins exhibit antifungal properties against
Candida and
Aspergillus species as well as other fungi
[59] see
Table 3. Spectral analysis and chemical degradation reveals pradimicins structurally to be a benzo[α]napthacenequinone carrying D alanine and sugars
[58]. Pradimicins use specific binding recognition to bind to terminal D mannosides of the cell wall of susceptible microbes to form a D-mannoside, pradimicin, and calcium complex that destroys fungal cell membrane
[59].
Table 3. Bacterial sources of antifungal compounds.
Microorganism |
Compound(s) |
Susceptible Organism(s) |
Reference |
A. hibisca |
Pradimicins A, B, C |
Candida spp. and Aspergillus spp. |
[58] |
Actinoplanes spp. |
Purpuromycin |
T. mentagrophytes |
[60] |
Micromonospora species ATCC 53803 |
Spartanamycin B |
C. albicans, A. cladosporium, and Cryptococcus spp. |
[61] |
M. neiheumicin |
Neihumicin |
S. cerevisae |
[61] |
Micromonospora species SCC 1792 |
Sch 37137 |
Dermatophytes and Candida spp. |
[62] |
B. subtilis |
Iturin A and related peptides |
Phytopathogens |
[60][63] |
Micromonospora species SF-1917 |
Dapiramicins A and B |
R. solania |
[64] |
B. cereus |
Azoxybacilin, Bacereutin, Cispentacin, and Mycocerein |
Aspergillus spp., Saccharomyces spp, and C. albicans |
[60] |
B. lichenformis |
Fungicin M-4 |
Microsporum canis, Mucor spp., and Sporothrix spp. |
[65][66] |
Actinoplanes species also produce antifungal metabolites. An example is
Actinoplanes ianthinogenes (
A. ianthinogenes), which produces purpuromycin, a compound that has activity against
Trycophyton mentagrophytes (
T. mentagrophytes)
[60]. Another species is known as
Octamycini produces octamycin
[60]
Soil-occurring
Micromonospora species have been identified with the production of antifungal compounds
[60].
Micromonospora species ATCC 53803, through metabolism, produces spartanamycin B as a secondary metabolite, which has activity against
Candida albicans (
C. albicans),
Aspergillus cladosporium (
A. Cladosporium), and
Cryptococcus spp.
Micromonospora neiheumicin (
M. neiheumicin) produces neihumicin, which is active against
Saccharomyces cerevisae (
S. cerevisiae) activity
[61]. Sch 37137, a dipeptide formed by
Micromonospora species SCC 1792, also fights against dermatophytes and
Candida species [62]. Lastly, Nishizawa et al. reported that
Micromonospora species SF-1917 produces nucleoside antibiotics, dapiramicins A and B. Dapiramicins B inhibits growth of
Rhizoctonia solania (
R. solania) of rice plants in a greenhouse test
[64].
Aerobic Gram-positive branching bacilli,
Streptomyces species, yield some antifungal compounds. These compounds include nystatin, phoslatomycins
[67], UK-2A, B, C, D
[68], phthoxazolin A
[69], faeriefungin
[70], butyrolactols A and B
[71], sultriecin
[72], polyoxin
[73]), and dunaimycins
[74].
Some bacilli species are also known to be the source of several antifungal compounds.
Bacillus subtilis produces iturin and other closely related peptides, including bacillomycin D, F, and L, mycosubtilin, and mojavensin. These agents have been shown to be active against phytopathogens and hence, have been commercialized as biological control agents against fungal plant pathogens. Notably, there has not been any reported resistance against fungi for these compounds. These agents act by creating pores in the membrane of susceptible fungi, thereby causing leakage of cell contents and subsequent cell death
[60][63].
According to Kerr, the compounds; azoxybacilin, bacereutin, cispentacin, and mycocerein can be isolated from the products of
Bacillus cereus (
B. cereus) and are active against
Aspergillus species,
Saccharomyces spp.,
Candida albicans, and other fungi
[60]. Another
Bacilli species,
B. licheniformis, produces fungicin M-4 and peptide A12-C
[65][66].
The compound, pyrrolnitrin, has been reported by Chernin et al. to be the factor responsible for the antimicrobial action of
Enterobacter agglomerans (
E. agglomerans) on the
Candida species,
Aspergillus niger (
A. niger), dermatophytes and phytopathogenic fungi.
E. agglomerans again produces herbicolins A and B. which are active against yeasts and filamentous fungi
[75][76][77]. CB-25-1, a water soluble dipeptide, produced by
Serratia plymuthica (
S. plymuthica) is known to inhibit growth of
C. albicans [78].
P. aeruginosa present in the gut of healthy subjects has been identified as a source of three novel antifungal compounds, namely dihydroaeruginoic acid
[79], pyocyanin, and 1-hydroxyphenazine
[80]. Other antimicrobial compounds produced by pseudomonas include 2,4-diacetophluoroglucinol
[81], peptide pseudomycin family
[82], caryoynencins
[83], and cyclic hydroxamic acid, G1549
[84].
Burkholderia species are another bacterial source of antimicrobial compounds. Cepacidine A, which antagonizes plant and animal fungi growth, can be generated by
B. cepacia [85].
B. cepacia also produces cepalycin
[86], xylocandins
[87], and heptylmethyl-quinolinone
[88]. Another group of antibacterial compounds is enacyloxcins, known to originate from the Burkholderia species
[89]. Enacyloxcins consists of eight closely related antibacterial compounds (86–87). Maltophilin is the active compound responsible for the antifungal action of the Rhizosphere strain of
Stenotrophomonas maltophilia. Polyenic antibiotics produced by the genus
Gluconobacter have also been reported to possess some antifungal activity against the fungus
Neurospora crassa but not against yeast
[90]
4. Fungal Sources of Antimicrobials
The discovery of penicillin G in 1928 from fungal species has led to the exploration of these organisms
[91]. Their ability to produce a plethora of active secondary metabolites that can serve as lead compounds for the synthesis of antimicrobials is significant.
Hormonema species that yielded enfumafungin, a triterpenoid, was discovered over a decade ago and was shown to be highly effective against
Candida spp. and
Aspergillus spp. It is still being studied in order to produce a number of developmental compounds
[92]. Enfumafungin yielded a semisynthetic derivative, SCY-078 that is in phase II clinical trial. The biosynthetic encoding genes for this peculiar triterpenoid were only recently discovered, but have shown a lineage of hopene-type cyclases, including ERG7, which is necessary for the biosynthesis of fungal ergosterol
[93], see
Table 4.
Testing of metabolites in the strobilurins, known as antifungal agents in agriculture, has not been explored since it was identified in 1999 as being harmful to humans
[94]. In recent times however, favolon, produced by
Favolaschia calocera (
F. calocera), a metabolite of strobilurins has been identified and shown to be less toxic, and with potent antifungal activity against human pathogens
[95].
Fungal metabolites, by their ability to interfere with quorum sensing, inhibits the formation of biofilms. Coprinuslactone, derived from
Coprinus comatus (
C. comatus), acts on
P. aeruginosa biofilms
[96]. Microporenic acid A from a Kenyan basidiomycetes also inhibit
S. aureus and
C. albicans biofilms and has an additional advantage of destroying pre-formed biofilms
[97]. Biofilm inhibitors are promising adjuncts to antibiotics.
Mutulins and its derivative, retapamulin from the basidiomycetes
Clitopilus passeckerianus, represents a source of antimicrobials. They have shown to have potent antibacterial activity, and more derivatives are undergoing clinical trials. The drawback with them is the difficulty in reaching a large scale since they grow slowly and generate low yields
[96].
A novel rubrolide, rubrolide S, discovered from the marine fungus
Aspergillus terreus (
A. terreus) OUCMDZ-1925, has shown to significantly inhibit the activity of
Influenza A virus (H1N1)
[98]. A novel hybrid polyketide, Cladosin C, isolated from
Cladosporium sphaerospermum 2005-01-E3, has demonstrated an activity against
Influenza A H1N1
[99].
Penicillium chrysogenum PJX-17 represents a source of two antiviral sorbicillinoids, named sorbicatechols A and B, with significant activity against the H1N1
[100].
Trypilepyrazinol and β-hydroxyergosta-8,14,24 (28)-trien-7-one isolated from extracts of the fungus
Penicillium sp. IMB17-046 exhibited a broad spectrum antiviral activity against different types of viruses, including human immunodeficiency virus (HIV) and hepatitis C virus (HCV)
[101].
Aspergillus niger SCSIO Jcsw6F30 produces aspernigrin C and malformin C, which exhibited significant antiviral activity against HIV-1
[102]. Antimycin A, an isolate from
Streptomyces kaviengensis (F7E2f), shown a strong antiviral activity against Western equine encephalitis virus (WEEV) via the interruption of mitochondrial electron transport and pyrimidine biosynthesis
[103].
Table 4. Antimicrobial activity of chemical compounds from fungi.
Microorganism |
Compounds |
Antimicrobial Activity |
Reference(s) |
Hormonema spp. |
Enfumafungin |
Candida spp. and Aspergillus spp. |
[92] |
F. calocera |
Favolon |
Candida tenuis and Mucor plumbeus |
[95] |
C. comatus |
Coprinuslactone |
P. aeruginosa |
[96] |
Sanghuangporus spp. |
Microporenic acid A |
S. aureus and C. albicans |
[97] |
Aspergillus terreus |
Rubrolide S |
Influenza A virus (H1N1) |
[98] |
Cladosporium sphaerospermum 2005-01-E3 |
Cladosin C |
Influenza A H1N1 |
[99] |
Penicillium sp. IMB17-046 |
Trypilepyrazinol and β-hydroxyergosta-8,14,24 (28)-trien-7-one |
HIV and HCV |
[101] |