Type VI-secretion system-mediated competition and virulence in clinical and environmental Stenotrophomonas maltophilia.

UNLABELLED: The escalating global threat of antimicrobial resistance highlights the urgency of understanding pathogens like Stenotrophomonas maltophilia, an intrinsically multidrug-resistant opportunistic bacterium. While increasingly isolated in nosocomial infections, S. maltophilia is also ubiquitous in diverse environmental settings. Crucially, in both clinical and environmental niches, this bacterium thrives within polymicrobial communities, making its interactions with other microorganisms particularly significant. However, the cooperative and competitive dynamics within these communities, which could be used in innovative therapeutic strategies, are largely unexplored. Our study investigated the prevalence, functional contribution, and effector diversity of the type VI secretion system (T6SS), a powerful bacterial molecular weapon, in a collection of clinical and environmental Brazilian S. maltophilia isolates. Through whole-genome sequencing, comparative genomics, and functional assays, we found that T6SS clusters, predominantly of the i4-clade, are present in a small but lineage-specific subset of isolates. Notably, one clinical strain also harbored an additional i3-clade T6SS. Interbacterial competition assays against Escherichia coli showed significant variations in killing efficacy, with a subset of isolates from the Sgn4 monophyletic lineage exhibiting robust antibacterial activity and increased pathogenicity in a Galleria mellonella infection model. Importantly, genetic dissection demonstrated that both phenotypes were T6SS-dependent. Further analysis of T6SS effector profiles identified diverse toxin-immunity pairs, including predicted pore-forming toxins and peptidoglycan hydrolases, correlating with observed phenotypes and suggesting possible anti-eukaryotic potential. Our results underscore the complex genomic variability and ecological strategies of the S. maltophilia complex (Smc), highlighting the T6SS's importance as a mediator of competition.

IMPORTANCE: Stenotrophomonas maltophilia is an emerging opportunistic pathogen associated with severe nosocomial infections with limited treatment options due to resistance to multiple antibiotics. Mechanisms of pathogenicity and fitness are relatively poorly understood in the species, which is ubiquitous in the environment and shows high genome plasticity. Type VI secretion systems (T6SS) are molecular machines that inject effector proteins into target cells, playing fundamental roles in bacterial interactions within polymicrobial communities and hosts. In this study, we identify and characterize T6SSs in a diverse set of clinical and environmental isolates of S. maltophilia from Brazil, revealing their role in bacterial antagonism and virulence. This study provides new insights into the mechanisms that contribute to S. maltophilia's ability to thrive in distinct environments, also contributing to the understanding of the complex genomic variability within the species.