Bacillus Thuringiensis Predominated in the Zombie Fungus Ophiocordyceps Unilateralis S. L.-Infected Ant Cadavers and Investigation of its Biological Properties

Abstract

Animal hosts infected and killed by parasitoid fungi become a nutrient-rich cadaver for saprophytes. Bacteria adapted to colonization of parasitoid fungi can be selected and can predominate in the cadaver, actions that consequently impact the fitness of the parasitoid fungi. In Taiwan, the zombie fungus, Ophiocordyceps unilateralis sensu lato (Clavicipitaceae: Hypocreales), was found to parasitize eight ant species, with preference for a principal host, Polyrhachis moesta. In this study, ant cadavers grew a fungal stroma that was predominated by Bacillus thuringiensis. The bacterial diversity in the principal ant host was found to be lower than the bacterial diversity in alternative hosts, a situation that might enhance the impact of B. thuringiensis on the sympatric fungus. The B. thuringiensis isolates from fungal stroma displayed higher resistance to a specific naphthoquinone (plumbagin) than B. thuringiensis from the environment. Naphthoquinones are known to be produced by O. unilateralis s. l., and hence the resistance displayed by B. thuringiensis isolates to these compounds suggests an advantage to B. thuringiensis to grow in the ant cadaver. Bacteria proliferating in the ant cadaver inevitably compete for resources with the fungus. However, the B. thuringiensis isolates displayed in vitro capabilities of hemolysis, production of hydrolytic enzymes, and antagonistic effects to co-cultured nematodes and entomopathogenic fungi. Thus, co-infection with B. thuringiensis offers potential benefits to the zombie fungus in killing the host under favorable conditions for reproduction, digesting the host tissue, and protecting the cadaver from being taken over by other consumers. With these potential benefits, B. thuringiensis is not entirely a competitor but could also be a synergist in O. unilateralis s. l. infections under certain conditions.