A mixed community of actinomycetes(Bacteria) produce multiple antibiotics for the fungus farming Acromyrmex octospinosus(ant) 
Attine ants live in an intensely studied tripartite mutualism with the fungus Leucoagaricus gongylophorus, which provides food to the ants, and with antibiotic-producing actinomycete bacteria. One hypothesis suggests that bacteria from the genus Pseudonocardia are the sole, co-evolved mutualists of attine ants and are transmitted vertically by the queens. A recent study identified a Pseudonocardia-produced antifungal, named dentigerumycin, associated with the lower attine Apterostigma dentigerum consistent with the idea that co-evolved Pseudonocardia make novel antibiotics. An alternative possibility is that attine ants sample actinomycete bacteria from the soil, selecting and maintaining those species that make useful antibiotics.Acromyrmex octospinosus was recently shown to produce the well-known antifungal candicidin. Candicidin production is widespread in environmental isolates of Streptomyce Actinomycetes from A. octospinosus garden worker ants and, in a single colony of ants, identified a Pseudonocardia and a Streptomyces species that produce antifungals in laboratory culture. The Streptomyces species, which is named S4, contains candicidin biosynthesis genes and produces candicidin, consistent with a report on antifungal-producing actinomycetes associated with A. octospinosus. The actinomycetes studied in this work were isolated from A. octospinosus ants from one region, whereas the previous study used A. octospinosus ants collected from the other. However, despite this geographic separation, the candicidin-producing Streptomyces strains identified in the two studies show 99% 16 S rDNA sequence identity suggesting that candicidin-producing Streptomycesare common mutualists of A. octospinosus. Candicidin-producing Streptomyces are widespread in the environment and attine ants most likely acquire them selectively from the soil.
The Pseudonocardia species P1, isolated from the same colony as Streptomyces S4, showed relatively weak antifungal activity that was only observed in cultures grown on solid growth medium. This made it difficult to purify enough of the compound for analysis and identification. Using a genome scanning approach we identified a biosynthetic gene cluster for a polyene antifungal in Pseudonocardia P1 and then isolated and identified this antifungal using LC-MS/MS. This combined chemical and genomic approach provides a powerful tool for identifying and isolating new antibiotics and confirmed that Pseudonocardia P1 produces a polyene antifungal that was tentatively named NYSTATIN P1. This compound is markedly different from the antifungal dentigerumycin produced by Pseudonocardia associated with the lower attine ant species A. dentigerum although it is notable that both Pseudonocardia strains are making previously unknown antifungals, consistent with the idea that the Pseudonocardia mutualists co-evolved with attine ants. There was no detection of any compounds in extracts from Pseudonocardia P1 agar plates and mycelium that matched the isotopic mass of dentigerumycin. However, since the biosynthetic gene cluster for this compound is not known, it wasn't possible exclude the possibility that this strain also has the ability to make dentigerumycin.
Taken together, this work provides the first direct evidence that individual leaf-cutting ant colonies have access to multiple antifungals via the diversity of hosted actinomycetes and increases the number of known antifungals used by attine ants to three. This work also provides evidence to support the two current possibilities for the identity and acquisition of mutualistic bacteria,Pseudonocardia co-evolution, and the environmental acquisition of useful actinomycetes. This strongly suggests that both possibilities apply, at least in the attine species A. octospinosus. Careful experimental work will be needed in order to demonstrate that multiple compounds are in fact produced and confer benefits in vivo. It is interesting that the only two antifungal compounds to be isolated and identified from A. octospinosus colonies so far are polyenes, which are active against dimorphic fungi, yeasts (Candida) and molds (Escovopsis), but which apparently do not kill the fungal cultivar. The isolation of a nystatin-like polyene from a leaf-cutting ant-associated Pseudonocardia species that some Pseudonocardia bacteria associated with attine ants have non-specific antibiotic properties that inhibit a range of fungi and are not targeted specifically at Escovopsis.
The advantage to the ants of deploying two antifungals is not clear. Polyene antifungals are thought to work by interacting hydrophobically with ergosterol in the fungal cell membrane and forming channels that increase membrane permeability, but this may not be their only mechanism of action, and there may therefore be some advantage to the ants in using more than one. However, as fungi do not develop resistance to polyene antifungals (at least in a clinical setting), it is unlikely that resistance is the basis for any such advantage. Nevertheless, as candicidin and nystatin are not antibacterial, neither of these compounds is likely to be involved in competition amongst the bacteria for host resources. Thus, the identities of these two antifungal compounds are consistent with the longstanding hypothesis that these actinomycete associates of leaf-cutting ants can be mutualists of the ant and the attine fungus, provided that the compounds are applied correctly by the ant. So the conclusion is:-
The Pseudonocardia species P1, isolated from the same colony as Streptomyces S4, showed relatively weak antifungal activity that was only observed in cultures grown on solid growth medium. This made it difficult to purify enough of the compound for analysis and identification. Using a genome scanning approach we identified a biosynthetic gene cluster for a polyene antifungal in Pseudonocardia P1 and then isolated and identified this antifungal using LC-MS/MS. This combined chemical and genomic approach provides a powerful tool for identifying and isolating new antibiotics and confirmed that Pseudonocardia P1 produces a polyene antifungal that was tentatively named NYSTATIN P1. This compound is markedly different from the antifungal dentigerumycin produced by Pseudonocardia associated with the lower attine ant species A. dentigerum although it is notable that both Pseudonocardia strains are making previously unknown antifungals, consistent with the idea that the Pseudonocardia mutualists co-evolved with attine ants. There was no detection of any compounds in extracts from Pseudonocardia P1 agar plates and mycelium that matched the isotopic mass of dentigerumycin. However, since the biosynthetic gene cluster for this compound is not known, it wasn't possible exclude the possibility that this strain also has the ability to make dentigerumycin.
Taken together, this work provides the first direct evidence that individual leaf-cutting ant colonies have access to multiple antifungals via the diversity of hosted actinomycetes and increases the number of known antifungals used by attine ants to three. This work also provides evidence to support the two current possibilities for the identity and acquisition of mutualistic bacteria,Pseudonocardia co-evolution, and the environmental acquisition of useful actinomycetes. This strongly suggests that both possibilities apply, at least in the attine species A. octospinosus. Careful experimental work will be needed in order to demonstrate that multiple compounds are in fact produced and confer benefits in vivo. It is interesting that the only two antifungal compounds to be isolated and identified from A. octospinosus colonies so far are polyenes, which are active against dimorphic fungi, yeasts (Candida) and molds (Escovopsis), but which apparently do not kill the fungal cultivar. The isolation of a nystatin-like polyene from a leaf-cutting ant-associated Pseudonocardia species that some Pseudonocardia bacteria associated with attine ants have non-specific antibiotic properties that inhibit a range of fungi and are not targeted specifically at Escovopsis.
The advantage to the ants of deploying two antifungals is not clear. Polyene antifungals are thought to work by interacting hydrophobically with ergosterol in the fungal cell membrane and forming channels that increase membrane permeability, but this may not be their only mechanism of action, and there may therefore be some advantage to the ants in using more than one. However, as fungi do not develop resistance to polyene antifungals (at least in a clinical setting), it is unlikely that resistance is the basis for any such advantage. Nevertheless, as candicidin and nystatin are not antibacterial, neither of these compounds is likely to be involved in competition amongst the bacteria for host resources. Thus, the identities of these two antifungal compounds are consistent with the longstanding hypothesis that these actinomycete associates of leaf-cutting ants can be mutualists of the ant and the attine fungus, provided that the compounds are applied correctly by the ant. So the conclusion is:-
A combined genomic and chemical approach that has proven useful for the identification of a new antifungal associated with Acromyrmex ants, this time produced by their Pseudonocardiamutualist. This approach should stimulate further chemical ecology studies of insect fungiculture systems, which are widespread in nature and which are likely to use symbiotic antibiotic-producing bacteria to protect their fungal partners.There is also a evidence that supports both of the possibilities proposed to explain the mutualism between actinomycetes and attine ants-co-evolution of Pseudonocardia with attine ants and environmental sampling by the ants of useful antibiotic-producing bacteria. Thus it is proposed that these possibilities are not mutually exclusive and that both are likely to apply to both attine ants and other systems of insect fungiculture.
A combined genomic and chemical approach that has proven useful for the identification of a new antifungal associated with Acromyrmex ants, this time produced by their Pseudonocardiamutualist. This approach should stimulate further chemical ecology studies of insect fungiculture systems, which are widespread in nature and which are likely to use symbiotic antibiotic-producing bacteria to protect their fungal partners.There is also a evidence that supports both of the possibilities proposed to explain the mutualism between actinomycetes and attine ants-co-evolution of Pseudonocardia with attine ants and environmental sampling by the ants of useful antibiotic-producing bacteria. Thus it is proposed that these possibilities are not mutually exclusive and that both are likely to apply to both attine ants and other systems of insect fungiculture.
No comments:
Post a Comment