How to kill your rivals: microbial warfare mediated by the Type VI secretion system of Serratia marcescens

The Type VI secretion system (T6SS) is a key weapon in the competitiveness and virulence of very many Gram-negative bacteria. Most T6SSs appear to be ‘anti-bacterial’, used to efficiently kill rival bacterial cells and provide a competitive advantage in a variety of polymicrobial niches. T6SSs deliver multiple, diverse toxins (‘effectors’) directly into target cells by a contraction-based firing mechanism. We have used the potent anti-bacterial T6SS of the opportunistic pathogen Serratia marcescens as a model to study the mechanisms and consequences of T6SS effector delivery into competitors. This has revealed a varied portfolio of effector toxins used for several forms of inter-microbial competition. In particular, we have characterised several new anti-bacterial effectors and also discovered that the S. marcescens T6SS not only targets bacterial competitors, but can also deploy anti-fungal effector proteins against microbial fungi.