Penicillin-binding protein 1b encoded by mrcB gene mediates the enhancement of biofilm formation by subinhibitory concentrations of cefotaxime in monophasic Salmonella Typhimurium strain SH16SP46.

Development of cefotaxime-resistance and biofilm formation increase the difficulty to prevent and control the infection and contamination of Salmonella, one of the most important foodborne and zoonotic bacterial pathogen. Our previous study observed that 1/8 minimum inhibitory concentration (MIC) of cefotaxime induced the enhancement of biofilm formation and filamentous morphological change by a monophasic Salmonella Typhimurium strain SH16SP46. This study was designed to explore the role of three penicillin-binding proteins (PBPs) in mediating the induction effect of cefotaxime. Three deletion mutants of the genes mrcA, mrcB, and ftsI, encoding the proteins PBP1a, PBP1b, and PBP3, respectively, were constructed using the parental Salmonella strain SH16SP46. Gram staining and scanning electron microscopy showed that these mutants showed normal morphology comparable to the parental strain without cefotaxime treatment. However, under the stress of 1/8 MIC of cefotaxime, the strains WT, ΔmrcA, and ΔftsI, rather than ΔmrcB, exhibited filamentous morphological change. Moreover, cefotaxime treatment significantly enhanced biofilm formation by the strains WT, ΔmrcA, and ΔftsI, but not by the ΔmrcB strain. The complement of mrcB gene in the ΔmrcB strain recovered the enhanced biofilm formation and filamentous morphological change induced by cefotaxime. Our results suggest that PBP1b encoded by mrcB gene may be a binding target of cefotaxime for initiating the effect on Salmonella morphology and biofilm formation. The study will contribute to further understanding of the regulatory mechanism of cefotaxime on Salmonella biofilm formation.