Sub-inhibitory concentrations of cefotaxime treatment enhances biofilm formation of monophasic Salmonella Typhimurium variant strain SH16SP46.

Salmonella is one of the most important foodborne and zoonotic pathogens, and monophasic S. Typhimurium is ranked among the top-five Salmonella serovars causing animal and human infections worldwide. Resistance to the third- and higher-generation cephalosporins in Salmonella has attracted great attention. Bacteria are frequently exposed to sub-minimal inhibitory concentrations (sub-MICs) of antimicrobials that can trigger diverse adaptive responses such as biofilm formation. Biofilms can promote bacterial defense to external and internal harsh conditions. This study aimed to investigate the effect of sub-MICs of cefotaxime, one of the third-generation cephalosporins, on biofilm formation by non-clinical S. enterica strains. Crystal violet staining demonstrated that cefotaxime at 1/8 MIC enhanced biofilm formation by two monophasic S. Typhimurium strains. Confocal laser scanning microscopy and enzymatic treatment assay revealed that cellulose was the most dominant extracellular matrix component contributing to Salmonella biofilm formation. Scanning electron microscopy demonstrated that cefotaxime treatment led to bacterial incomplete cell division and filamentous morphology during the whole process of biofilm formation. Our study is the first to report the enhancement effect of cefotaxime on non-clinical, monophasic S. Typhimurium by affecting bacterial morphology. The results will contribute to conducting risk assessments of Salmonella in the pork production chain and guiding the rational use of antimicrobial agents to reduce the risk of biofilm formation.