A PhoP-regulated type II secretion system drives bacterial competition in Serratia marcescens

ABSTRACT Serratia marcescens is an enteric bacterium that can act as an opportunistic pathogen, with increasing incidence in clinical settings. This is largely due to its ability to express a broad repertoire of virulence factors and acquire antibiotic resistance mechanisms. Consequently, S. marcescens has been designated by the World Health Organization (WHO) as a research priority for the development of alternative antimicrobial strategies. In this study, we identify a previously unrecognized role of the type II secretion system (T2SS) in mediating bacterial competition in the clinical strain S. marcescens RM66262. Phylogenomic analysis reveals that the T2SS is conserved among clinical isolates but absent from most environmental strains, suggesting it confers a selective advantage in host-associated or highly competitive environments. Functional assays demonstrate that the T2SS contributes to both inter- and intraspecies antagonism and acts in concert with the contact-dependent type VI secretion system (T6SS), with both systems contributing additively to bacterial killing through distinct mechanisms. We further show that T2SS expression is positively regulated by the PhoP/PhoQ two-component system through direct binding to its promoter region and is induced in response to competitive cues such as attack by Acinetobacter nosocomialis . Altogether, our findings expand the functional scope of the T2SS beyond classical extracellular virulence, establishing it as a PhoP-regulated system involved in bacterial competition and a promising target for novel anti-virulence strategies. IMPORTANCE This work uncovers a previously uncharacterized role for the type II secretion system (T2SS) in Serratia marcescens , demonstrating its involvement in both inter- and intraspecies bacterial competition. The finding that the T2SS is conserved among clinical isolates but largely absent in environmental strains suggests that this system confers a selective advantage in host-associated or high-competition settings. Furthermore, we show that T2SS expression is regulated by the PhoP/PhoQ system and induced by interbacterial contact, underscoring its role in adaptive antagonistic responses. By expanding the known functional scope of the T2SS, this study provides new insights into the ecological strategies of opportunistic pathogens and highlights a promising target for anti-virulence therapeutic development.