The role of the polysaccharide capsule as a novel virulence-associated trait of Stenotrophomonas maltophilia complex bacteria.

The polysaccharide capsule is recognized as a major virulence factor of many bacterial pathogens, playing a significant role in evading host immunity. However, it has not yet been characterized for the bacteria within the emerging multidrug-resistant opportunistic pathogens of the Stenotrophomonas maltophilia complex. In this study, we aimed to evaluate whether bacteria of the S. maltophilia complex form a bona fide polysaccharide capsule and determine its virulence-related functions. We demonstrated that a part of clinical and environmental isolates of the S. maltophilia complex forms a polysaccharide capsule and identified a distinct group IV-like, Wzx/Wzy-dependent capsule-associated locus architecture in these bacteria, with capsule-associated genes organized within one or two loci. Comparison of capsulated clinical isolates and their capsule-deficient mutants showed that the polysaccharide capsule protects bacteria of the S. maltophilia complex from blood plasma-mediated killing, phagocytosis, and desiccation. However, formation of the polysaccharide capsule can also limit biofilm formation and adhesion to epithelial cells in some isolates, suggesting a potential trade-off for the bacteria. Altogether, our results pinpoint the polysaccharide capsule as one of the previously undescribed virulence-associated factors of the S. maltophilia complex, enhancing its ability to escape host innate immunity mechanisms and contributing to pathogenicity.IMPORTANCEThe emerging pathogens employ a variety of diverse virulence strategies, which must be thoroughly investigated to uncover effective ways of preventing and combating their infections. In this work, emerging multidrug-resistant bacterial pathogens belonging to the Stenotrophomonas maltophilia complex were analyzed, with the goal of identifying their mechanisms responsible for evasion of the host immune system. A polysaccharide capsule, a previously uncharacterized virulence-associated trait of S. maltophilia complex bacteria, was identified and characterized in this work, revealing both the genetic factors required for its synthesis and its biological importance to the bacteria. This work expands our understanding of the survival strategies of emerging multidrug-resistant bacteria.