Project description:Bacteria must contend with near constant environmental stresses, and to tolerate these cellular assaults, have evolved numerous stress response pathways and programs. One such strategy is the adoption of a dormant state, whereby cells restrict their growth and await a reversion to favourable conditions. A notable example of this is the formation of persister cells, which are a quiescent subpopulation that display increased tolerance to antibiotic killing. The genetics of persistence remain poorly understood, however bacterial genes called toxin-antitoxin (TA) systems have long been implicated in the persistence phenotype. TA systems are abundant genetic elements in bacteria that can function as growth toggling molecular switches, yet their role in persister formation is highly controversial. To address this, we constructed a pan-TA deletion strain (∆7TA) of the bacterial pathogen Legionella pneumophila to examine how a strain devoid of these genes tolerates cellular stress. We identified a single putative TA system (Lpg1604-05) that is activated specifically in response to genotoxic stress, however this system leads to cell death during stress rather than promoting survival. In the absence of Lpg1604-05, cells do not die during stress exposure and instead adopt a viable but non-culturable state. Strikingly, while persister survival during DNA stress is dramatically improved when Lpg1604-05 is deleted, this enhanced survival can be conferred to wild-type cells in a contact-dependent manner during co-culture with the deletion strain. This is dependent, however, on a sufficient proportion of the mixed population being deletion mutants. Despite having homology only to other TA systems, Lpg1604-05 displays non-canonical activity and we hypothesize that it has undergone exaptation, leading to integration into the cell’s stress response pathway and conservation within L. pneumophila’s core genome. Overall, our work demonstrates the stress-specific activation of a putative TA system and its involvement in facilitating cell death, rather than survival, during stressful conditions. These findings reveal both a new physiological function for TA systems in bacteria, as well as a heretofore undescribed phenomenon of contact-dependent survival within persister cells. This expands the functional scope of an omnipresent class of bacterial genes and suggests that domestication of mobilome genes may be an overlooked source of de novo genetic circuitry.

Dataset Information

Molecular mechanism of toxin neutralization in the HipBST toxin-antitoxin system of Legionella pneumophila.

Publications

Molecular mechanism of toxin neutralization in the HipBST toxin-antitoxin system of Legionella pneumophila.

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