Chemical basis of the synergism and antagonism in microbial communities in the nests of leaf-cutting ants

Abstract

Leaf-cutting ants cultivate the fungus Leucoagaricus gongylophorus, which serves as a major food source. This symbiosis is threatened by microbial pathogens that can severely infect L. gongylophorus. Microbial symbionts of leaf-cutting ants, mainly Pseudonocardia and Streptomyces, support the ants in defending their fungus gardens against infections by supplying antimicrobial and antifungal compounds. The ecological role of microorganisms in the nests of leaf-cutting ants can only be addressed in detail if their secondary metabolites are known. Here, we use an approach for the rapid identification of established bioactive compounds from microorganisms in ecological contexts by combining phylogenetic data, database searches, and liquid chromatography electrospray ionisation high resolution mass spectrometry (LC-ESI-HR-MS) screening. Antimycins A 1 -A 4 , valinomycins, and actinomycins were identified in this manner from Streptomyces symbionts of leaf-cutting ants. Matrix-assisted laser desorption ionization (MALDI) imaging revealed the distribution of valinomycin directly on the integument of Acromyrmex echinatior workers. Valinomycins and actinomycins were also directly identified in samples from the waste of A. echinatior and A. niger leaf-cutting ants, suggesting that the compounds exert their antimicrobial and antifungal potential in the nests of leaf-cutting ants. Strong synergistic effects of the secondary metabolites produced by ant-associated Streptomyces were observed in the agar diffusion assay against Escovopsis weberi. Actinomycins strongly inhibit soil bacteria as well as other Streptomyces and Pseudonocardia symbionts. The antifungal antimycins are not only active against pathogenic fungi but also the garden fungus L. gongylophorus itself. In conclusion, secondary metabolites of microbial symbionts of leaf-cutting ants contribute to shaping the microbial communities within the nests of leaf-cutting ants.

The semiochemically mediated interactions between bacteria and insects

“…There is an increasing body of examples for symbiotic interactions between insects and bacteria with the associated microorganisms providing nutrients or defensive compounds for their hosts (e.g. Oliver et al 2003Oliver et al , 2005Oliver et al , 2008Oliver et al , 2010Scarborough et al 2005;Douglas 2006;Nakabachi et al 2006;Thao and Baumann 2004;Baumann et al 2002;Kaltenpoth 2009;Schoenian et al 2011;Oh et al 2009aOh et al , b, 2011Brachmann et al 2006;Piel et al 2004;Barke et al 2010;Haeder et al 2009;Scott et al 2008;Leroy et al 2011;Sabri et al 2010) and several examples illustrate pathogenic interactions (e.g. Grenier et al 2006;Ffrench-Constant et al 2007;Herbert and Goodrich-Blair 2007;Lecadet et al 1999;Schnepf et al 1998;de Maagd et al 2003) while the semiochemically mediated interactions are more rarely described.…”

Refining the Roots of the Beewolf-Streptomyces Symbiosis: Antennal Symbionts in the Rare Genus Philanthinus (Hymenoptera, Crabronidae)

“…Several such symbioses involve actinobacteria that are well known for the production of a large number of antibiotics, many of which are of outstanding clinical relevance (13). Notably, actinobacteria are an important first line of defense against pathogenic or commensal fungi in the fungus gardens of leaf-cutting ants (7,11,22,25), as well as in the fungal galleries of Southern pine beetles (26).…”

Specialized Fungal Parasites and Opportunistic Fungi in Gardens of Attine Ants

“…Thus, rather than a one-to-one symbiosis between the ants and their fungi, recent work suggests that rather the ants rely on a consortium of microbes and their compounds to defend themselves against Escovopsis sp. parasites [26].…”

“…The most important strategies applied by attine ants in order to preserve their nests from harmful microbes consist of mechanical and chemical barriers including (i) careful cleaning of the leaf fragments used as substrate for the fungal cultivar in order to put away spores and microorganisms [18,19]; (ii) massive inoculation of the mutualistic fungus mycelium onto the clean plant fragment increasing the colonization of this substrate by the cultivar [2]; (iii) the use of antimicrobial glandular secretions [20][21][22][23][24] and faecal droplets [25,26]; (iv) weeding and grooming of infected parts of the garden when an undesired microorganism is detected [18]; (v) antagonistic activity of the mutualistic fungus against alien microorganisms [14,19]; (vi) unspecific microbial interrelationship between microorganisms which benefit the whole nest [26]; (vii) control of humidity in disposal chambers [27]; (viii) association with antibioticproducing bacteria [28][29][30][31][32][33][34][35][36]. Despite such mechanisms to suppress the development of alien microbes, a plethora of bacteria, filamentous fungi and yeasts are still found in ant gardens [35,[37][38][39][40][41].…”