Accumulation of dead cells from contact killing facilitates coexistence in bacterial biofilms

Abstract

Bacterial communities are governed by a wide variety of social interactions, some of which are antagonistic with potential significance for bacterial warfare. Several antagonistic mechanisms, such as killing via the type VI secretion system (T6SS), require killer cells to directly contact target cells. The T6SS is hypothesized to be a highly potent weapon, capable of facilitating the invasion and defence of bacterial populations. However, we find that the efficacy of contact killing is severely limited by the material consequences of cell death. Through experiments with Vibrio cholerae strains that kill via the T6SS, we show that dead cell debris quickly accumulates at the interface that forms between competing strains, preventing physical contact and thus preventing killing. While previous experiments have shown that T6SS killing can reduce a population of target cells by as much as 10 6 -fold, we find that, as a result of the formation of dead cell debris barriers, the impact of contact killing depends sensitively on the initial concentration of killer cells. Killer cells are incapable of invading or eliminating competitors on a community level. Instead, bacterial warfare itself can facilitate coexistence between nominally antagonistic strains. While a variety of defensive strategies against microbial warfare exist, the material consequences of cell death provide target cells with their first line of defence.

Modular Molecular Weaponry Plays a Key Role in Competition Within an Environmental Vibrio cholerae Population

“…This is especially relevant when considering that in natural situations, two strains would rarely meet in equal concentrations, diminishing the effect a potentially superior E-I module composition could have. Similar interactions have been observed among co-occurring bacteria in Streptomyces , where the survival of potentially conflicting groups is posited to allow diversification of metabolic pathways and substrates utilized in a complex microbial community ( Wright and Vetsigian, 2016 ; McNally et al, 2017 ; Steinbach et al, 2020 ). The T6SS has previously been demonstrated to maintain distinct bacterial groups through spatial segregation, with antagonism occurring at the edge of incompatible groups ( Wong et al, 2016 ; McNally et al, 2017 ; Yanni et al, 2019 ; Steinbach et al, 2020 ).…”

High throughput identification of genes conferring resistance or sensitivity to toxic effectors delivered by the type VI secretion system

“…DgcM is best known for its role in regulating the expression of curli, an amyloid fiber protein produced as a component of biofilms (40). We and others have demonstrated that biofilm components can have a drastic influence on surviving T6SS attacks, including colanic acid capsules that appear to deflect incoming T6SS spears (7,11,12,14,15). Likely, deletion of dgcM leads to an altered cell envelope that is more resistant to T6SS attacks.…”

“…While immunity genes remain the gold standard of defence against T6SS effectors, numerous non-immunity defences have also been discovered (5,6): These include stress responses that help cells survive damage caused by effectors (7)(8)(9)(10), physical separation mechanisms (11)(12)(13), exopolysaccharides that act as armour to deflect incoming T6SS attacks (7,14,15), cell wall modifying enzymes that render the target molecules resistant to effectors (16), and proteins that play a role in survival by unclear mechanisms such as the periplasmic chaperone, Spy, the periplasmic protease inhibitor, Ecotin, and the outer membrane maltose porin, LamB, amongst others (7,17,18). Prey cell proteins can also play a role in activating incoming effectors: The ClpAP protease complex was shown to enhance susceptibility to the A. tumefaciens T6SS (19), and the Serratia marcescens effectors, Ssp2 and Ssp4, get activated in prey cells when their DsbA protein generates an intramolecular disulfide bridge (20).…”

“…Notably, periplasmic disulfide bridges formed by DsbA are also important for the functionality of numerous immunity genes (21)(22)(23)(24)(25). The metabolic state of the prey cell can also have a profound influence on survival: Stationary phase prey can be more susceptible to attacks due to their decreased ability to form microcolonies isolated from attackers, replace damaged components or produce sufficient immunity proteins for protection (7,11,12,(26)(27)(28). Finally, catabolite repression is another mechanism recently shown to influence E. coli survival against a T6SS attack, where growth with glucose or using mutant prey lacking the cAMP receptor protein (CRP) showed improved survival against the T6SS of Vibrio cholerae strain C6706 (29).…”

A New Contact Killing Toxin Permeabilizes Cells and Belongs to a Broadly Distributed Protein Family

“…Cells with an active T6SS (“killer cells” here) translocate toxic protein effectors into adjacent target cells. However, the outcome of T6SS-mediated aggression is influenced by the presence of immunity proteins in target cells, the external environment, and target cell stress responses ( 4 – 7 ). The harpoon-like proteinaceous apparatus is anchored to the membrane of killer cells by the membrane complex, which spans the inner membrane and periplasm ( 8 – 10 ).…”