No fitness cost entailed by type VI secretion system synthesis, assembly, contraction, or disassembly in enteroaggregative Escherichia coli

Abstract

ABSTRACT Polymicrobial communities are shaped by beneficial and antagonistic interactions between the different species. One of the key players for competition is the type VI secretion system, a large multiprotein complex that assembles a contractile tail-like structure spanning the entire cytoplasm. This contractile tail is comprised of an inner tube tipped by a puncturing complex and wrapped by a sheath that polymerizes under an extended conformation. Contraction of the sheath propels the needle-like tube complex toward the target cell where it delivers effectors. The ClpV AAA + ATPase is then recruited to the contracted sheath and uses the energy of ATP hydrolysis for its disassembly. Due to the size of the apparatus and its mechanism of disassembly, it is thought that the T6SS mechanism of action is costly for the cell, and hence can significantly impact its fitness. By using enteroaggregative Escherichia coli strains in which the T6SS is arrested at different stages of its mechanism of action, we show here that there is no significant cost associated with T6SS synthesis, assembly, contraction, or disassembly in vitro . IMPORTANCE Bacteria use weapons to deliver effectors into target cells. One of these weapons, the type VI secretion system (T6SS), assembles a contractile tail acting as a spring to propel a toxin-loaded needle. Due to its size and mechanism of action, the T6SS was intuitively thought to be energetically costly. Here, using a combination of mutants and growth measurements in liquid medium, on plates, and in competition experiments, we show that the T6SS does not entail a growth cost to enteroaggregative Escherichia coli .