CONICET | Buscador de Institutos y Recursos Humanos

Serratia marcescens is an opportunistic human pathogen that poses an emerging challenge to public health, especially among hospitalized or immunocompromised patients. Despite its clinical significance, the factors and mechanisms contributing to Serratia pathogenesis remain unclear. The ability of S. marcescens to thrive in challenging and dynamic environments is closely linked to its capacity to secrete a wide range of enzymes, including hemolysin, chitinases, phospholipases, nucleases, and proteases. The type II secretion system (T2SS) is a complex multiprotein secretion mechanism found in various organisms and frequently associated with virulence. In our clinical RM66262 strain, we have identified the presence of a T2SS encoded within the chromosome, a feature common in most clinical isolates but absent in many non-clinical strains, including the entomopathogenic reference strain S. marcescens Db11. However, the specific substrates of the T2SS of Serratia, as well as the environmental cues and regulatory factors governing its expression, remain unknown. This study aims to elucidate the role of T2SS in S. marcescens RM66262. The regulation of T2SS was assessed using a green fluorescent protein-containing reporter plasmid PT2SS-gfp. Our findings indicate that T2SS expression is induced during the stationary growth phase. Additionally, we observed heightened transcription levels of PT2SS-gfp under iron-depleted conditions. Conversely, high osmolarity conditions led to the repression of T2SS expression. Performing killing assays between S. marcescens RM66262 and E. coli, P. aeruginosa or S. marcescens Db11, we have determined that T2SS contributes to inter-species and intra-species elimination of microbial competitors, similarly to the T6SS. To investigate this further, we generated a mutant strain lacking both T2SS and T6SS secretion systems. Our results demonstrated that of both systems are required in competitive scenarios. Furthermore, our investigations revealed that T2SS expression increases significantly, up to five-fold, when S. marcescens RM66262 is challenged in competitive assays with Acinetobacter nosocomialis as the aggressor species. Additionally, by chromosomally labeling the protein with a 3xFLAG tag, we identified the GbpA chitin-binding protein as a substrate for T2SS. We are presently constructing a complemented T2SS strain to ascertain whether the observed phenotypes can be reversed. In conclusion, our findings collectively suggest that regulated T2SS expression in S. marcescens serves as a survival strategy during bacterial competition, thereby enhancing its proliferation across diverse ecological niches.

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