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Abstract

Insects develop efficient antimicrobial strategies to flourish in a bacterial world. It has long been proposed
that native gut microbiota is an important
component of host defense; however, the responsible
species have rarely been isolated to elucidate
the mechanism of action. Here we show that the
dominant symbiotic bacterium Enterococcus mundtii
associated with the generalist herbivore Spodoptera
littoralis actively secretes a stable class IIa
bacteriocin (mundticin KS) against invading bacteria,
but not against other gut residents, facilitating the
normal development of host gutmicrobiota. A mundticin-
defective strain lost inhibitory activity. Furthermore,
purified mundticin cures infected larvae.
Thus, the constitutively produced antimicrobials by
native extracellular symbionts create a significant
chemical barrier inside limiting invader expansion.
This unique property also benefits E. mundtii itself
by providing a competitive advantage, contributing
to its dominance within complex microbial settings
and its prevalence across Lepidoptera, and probably promotes the long-term cooperative symbiosis between both parties.