Symbiotic bacteria on the cuticle of the leaf-cutting ant Acromyrmex subterraneus subterraneus protect workers from attack by entomopathogenic fungi

Abstract

Although only discovered in 1999, the symbiotic filamentous actinobacteria present on the integument of certain species of leaf-cutting ants have been the subject of intense research. These bacteria have been shown to specifically suppress fungal garden parasites by secretion of antibiotics. However, more recently, a wider role for these bacteria has been suggested from research revealing their generalist anti-fungal activity. Here we show, for the first time, evidence for a role of these bacteria in the defence of young worker ants against a fungal entomopathogen. Experimental removal of the bacterial bio-film using an antibiotic resulted in a significant increase in susceptibility of worker ants to infection by the entomopathogenic fungus Metarhizium anisopliae. This is the first direct evidence for the advantage of maintaining a bacterial bio-film on the cuticle as a defensive strategy of the ants themselves and not exclusively for protection of the fungus garden.

Figure 1.

Survival curves of A. subterraneus subterraneus workers that had been pre-treated with gentamicin (GENT) or sterile distilled water (SDW) before exposure to the entomopathogenic fungus Metarhizium anisopliae (MET). Controls were pre-treated with GENT or SDW and then exposed to Tween (TW). The results are the mean survival of each group of five ants per Petri dish, with a total of 90 ants used per treatment. Error bars: s.e.m. Filled circles, GENT + MET; filled squares, SDW + MET; open circles, GENT + TWEEN; open squares, SDW + TWEEN.

Figure 2.

Effects of antibiotic and water treatment on bacterial bio-films: (a) A. subterraneus subterraneus major caste workers with extensive bio-film before antibiotic treatment; (b) antibiotic treatment; (c) treatment with water. Scale bar, 2 mm.