The opportunistic pathogen Stenotrophomonas maltophilia utilizes a type IV secretion system for interbacterial killing

Bacterial type IV secretion systems (T4SS) are a highly diversified but evolutionarily related family of macromolecule transporters that can secrete proteins and DNA into the extracellular medium or into target cells. They have been long known to play a fundamental role in bacterial conjugation and virulence of several species. It was recently shown that a subtype of T4SS harboured by the plant pathogenic bacteriumXanthomonas citritransfers toxins into other bacteria cells resulting in cell death. In this study, we show that a similar T4SS from the multi-drug-resistant global opportunistic pathogenStenotrophomonas maltophiliais proficient in killing competitor bacterial species. T4SS-dependent duelling betweenS. maltophiliaandX. citriwas observed by time-lapse fluorescence microscopy. A bioinformatic search of theS. maltophiliaK279a genome for proteins containing a C-terminal domain (XVIPCD) conserved inX. citriT4SS effectors identified eleven putative effectors secreted by theS. maltophiliaT4SS. Six of these effectors have no recognizable domain except for the XVIPCD. We selected one of these new effectors (Smlt3024) and its cognate inhibitor (Smlt3025) for further characterization and confirmed that Smlt3024 is indeed secreted in a T4SS-dependent manner byS. maltophiliawhen in contact with a target bacterial species. Expression of Smlt3024 in the periplasm ofE. coliresulted in greatly reduced growth rate and cell size, which could be counteracted by co-expression with its cognate periplasmic inhibitor, Smlt3025. This work expands our current knowledge about the diverse function of T4SSs subtypes and increases the panel of effectors known to be involved in T4SS-mediated interbacterial competition. Further elucidation of the mechanism of these antibacterial proteins could lead to the discovery of new antibacterial targets. The study also adds information about the molecular mechanisms possibly contributing to the establishment ofS. maltophiliain different biotic and abiotic surfaces in both clinical and environmental settings.Author SummaryCompetition between microorganisms for nutrients and space determines which species will emerge and dominate or be eradicated from a specific habitat. Bacteria use a series of mechanisms to kill or prevent multiplication of competitor species. Recently, it was reported that a subtype of type IV secretion system (T4SS) works as a weapon to kill competitor bacterial species. In this study, we show that an important human opportunistic pathogen,Stenotrophomonas maltophilia, harbours a T4SS that promotes killing of competitor species. We also identified a series of new toxic proteins secreted byS. maltophiliavia its T4SS to poison competitor species. We showed that two different bacterial species that harbour a bacteria-killing T4SS can kill each other; most likely due to differences in effector-immunity protein pairs. This work expands our current knowledge about the bacterial arsenal used in competitions with other species and expands the repertoire of antibacterial ammunition fired by T4SSs. In addition, the work contributes with knowledge on the possible mechanisms used byS. maltophiliato establish communities in different biotic and abiotic surfaces in both clinical and environmental settings.