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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., (© The Author(s) 2023. Published by Oxford University Press on behalf of FEMS.)

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., (© The Author(s) 2022. Published by Oxford University Press on behalf of FEMS.)

The antimicrobial resistance of Salmonella strains is rapidly increasing worldwide, which poses significant threats to animal and public health. In this study, a total of 249 porcine Salmonella isolates collected in China during 2008-2015 were examined, including 155 clinical isolates from diseased pigs and 94 nonclinical isolates from healthy pigs. Based on the minimum inhibitory concentration of seven antimicrobial agents, 96.4% of the isolates were resistant to at least one of the tested antibiotics and 81.0% of them showed multidrug resistance. The highest antimicrobial resistance was observed for tetracycline (85.9%), and the lowest was found for cefotaxime (13.3%). The isolates from diseased pigs exhibited significantly higher levels of antimicrobial resistance than those from healthy pigs. Twenty-two isolates from healthy pigs were resistant to ciprofloxacin, which may inhibit the curative effectiveness of fluoroquinolones on bacterial food-borne poisoning and infections in humans caused by contaminated food. Moreover, cefotaxime resistance of the strains isolated from diseased pigs during 2013-2015 was significantly higher compared with the strains isolated during 2008-2010. Further study showed that the correlation between phenotypic and genotypic resistance varied among the isolates from different sources, and in many cases, the presence of resistance genes was not consistent with the resistance to the corresponding antimicrobials. These results are very significant for veterinary practice and public health. [ABSTRACT FROM AUTHOR]