The evolution of tit-for-tat in bacteria via the type VI secretion system

Abstract

Tit-for-tat is a familiar principle from animal behavior: individuals respond in kind to being helped or harmed by others. Remarkably some bacteria appear to display tit-for-tat behavior, but how this evolved is not understood. Here we combine evolutionary game theory with agent-based modelling of bacterial tit-for-tat, whereby cells stab rivals with poisoned needles (the type VI secretion system) after being stabbed themselves. Our modelling shows tit-for-tat retaliation is a surprisingly poor evolutionary strategy, because tit-for-tat cells lack the first-strike advantage of preemptive attackers. However, if cells retaliate strongly and fire back multiple times, we find that reciprocation is highly effective. We test our predictions by competing Pseudomonas aeruginosa (a tit-for-tat species) with Vibrio cholerae (random-firing), revealing that P. aeruginosa does indeed fire multiple times per incoming attack. Our work suggests bacterial competition has led to a particular form of reciprocation, where the principle is that of strong retaliation, or ‘tits-for-tat’.

The evolution of strategy in bacterial warfare via the regulation of bacteriocins and antibiotics

“…Upon detecting an incoming attack, a cell will activate its own T6SS in response ( Basler et al, 2013 ). Consistent with our findings, recent work suggests that a key benefit to reciprocation via the T6SS is the ability to save energy and only attack when necessary, alongside a benefit that comes from improved aiming which is specific to this mode of attack ( Smith et al, 2020 ). Finally, there is evidence that bacteria may also detect and respond to incoming attacks via proxies such as the detection of lysate produced when surrounding cells are killed ( LeRoux et al, 2015a ), or molecules that are made by an attacker alongside a toxin ( Cornforth and Foster, 2013 ; LeRoux et al, 2015b ).…”

The cost of bacterial predation via type VI secretion system leads to predator extinction under environmental stress

“…The cost of T6SS-mediated predation under specific stress can further have evolutionary implications. A recent study suggests that a random firing of T6SS machinery can have a more selective advantage to win the microbial competition than a “tit-for-tat” mechanism, by which the T6SS-harboring bacterium counterattacks only if it is under attack by similar T6SS apparatus ( Smith et al., 2020 ). However, our findings indicate that a random firing of T6SS, if it takes place, can be “costlier” under bile salt stress since this may increase the chance of bile salt influx into the cell.…”

Human commensal gut Proteobacteria withstand type VI secretion attacks through immunity protein-independent mechanisms

“…The cause of the superiority of the T6SSs in the present study are not known. They could result from the strains' effector repertoire 12 , the assembly and/or firing rate of the T6SS machinery, the precise targeting of prey 38,46 , the sheath length-dependent force generation, or a combination of all of these features. Apart from promoting their own survival and growth, T6SS-active commensals such as the two commensal Enterobacter strains described above, could therefore play a major role in the colonization resistance in human adults toward pathogens.…”