Variable sensitivity of fungi and bacteria to compounds produced by the metapleural glands of leaf-cutting ants

Ants are the only group of insects that have metapleural glands. Secretions of these exocrine glands are known to have antibiotic properties and have been hypothesised to function as a general defence against microbial and fungal infections. Such defences are likely to be particularly important in leaf-cutting ants that need to protect both themselves and their clonal mutualistic fungus against pathogens. The metapleural gland of the leaf-cutting ant Acromyrmex octospinosus produces an array of organic compounds (Ortius-Lechner et al., 2000), suggesting that different compounds may be effective against different kinds of infections. Here we provide a detailed analysis of the sensitivity of two species of bacteria and seven species of fungi (including the mutualistic fungus) to these metapleural gland compounds, grouped in seven classes: acetic acid, short chain acids, medium chain acids, long chain acids, indoleacetic acid, gamma-lactones and gamma-ketoacids. All classes of compounds inhibited the growth of at least some of the tested micro-organisms. Cluster analysis produced four groups of micro-organisms differing in their overall sensitivity. Among-cluster differences explained a major part of the total variation in sensitivity (MANOVA), although differences between micro-organisms within clusters were also significant. Fungal hyphae and fungal spores never clustered together, indicating that defence mechanisms against these fungal life stages are fundamentally different. The mutualistic fungus was sensitive to all classes of compounds, which suggests that defence via metapleural gland secretion is under constraint when the protection of the fungus garden is concerned.