Submitted to: mSphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2017
Publication Date: 11/1/2017
Citation: Brunelle, B.W., Bearson, B.L., Bearson, S.M., Casey, T. 2017. Multidrug-resistant Salmonella enterica serovar Typhimurium isolates are resistant to antibiotics that influence their swimming and swarming motility. mSphere. 2:e00306-17. https://doi.10.1128/mSphere.00306-17.
Interpretive Summary: Multidrug-resistant (MDR) Salmonella are resistant to three or more antibiotics and are associated with enhanced virulence in humans and livestock. The ability for bacteria to move and find a specific biological niche is associated with virulence, and the goal of our study was to assess how different antibiotics influenced MDR Salmonella enterica serovar Typhimurium motility. We tested four different antibiotics (chloramphenicol, kanamycin, streptomycin, and tetracycline) at several different concentrations using various motility assays. Our results demonstrated that the combination of many variables influence how antibiotics can impact motility in MDR S. Typhimurium, including antibiotic used, antibiotic concentration, antibiotic-resistance gene, and isolate-specific factors. Most interestingly was the result that kanamycin actually enhanced swarming motility, which is important as there is a high level of kanamycin resistant in S. Typhimurium isolates from turkeys (34 percent), cattle (24 percent), pigs (10 percent), chicken (8 percent), and humans (6 percent). Identifying these isolate-specific factors and how they interact will be important to better understand how antibiotics can influence Salmonella motility and virulence.
Technical Abstract: Motile bacteria utilize one or more strategies for movement, such as darting, gliding, sliding, swarming, swimming, and twitching. The ability to move is considered a virulence factor in many pathogenic bacteria, including Salmonella. Multidrug-resistant (MDR) Salmonella encodes acquired factors that provide resistance to specific antibiotics, and the goal of our study was to determine how antibiotic exposure influenced swimming and swarming in resistant Salmonella isolates. Utilizing six MDR Salmonella enterica serovar Typhimurium isolates, swimming and swarming-promoting media containing chloramphenicol, kanamycin, streptomycin, or tetracycline were used to test the effect each antibiotic at various sub-inhibitory concentrations had on motility. Chloramphenicol and tetracycline reduced both swimming and swarming, though the effect was more pronounced for swimming compared to swarming at the same antibiotic concentration. Swimming was limited by kanamycin and streptomycin, but these antibiotics had little-to-no influence on decreasing swarming. Moreover, kanamycin significantly increased swarming in one of the isolates. Removal of the kanamycin-resistance gene and complementation with different genes revealed that the aphA1 kanamycin-resistance gene, along with some unknown Salmonella genetic factor, was required for the kanamycin-enhanced swarming phenotype. Screening of 25 additional kanamycin-resistant isolates identified two that had significantly increased swarming motility in the presence of kanamycin. This study demonstrated that the combination of many variables influence how antibiotics can impact swimming and swarming motility in MDR S. Typhimurium, including antibiotic, antibiotic concentration, antibiotic-resistance gene, and isolate-specific factors. Identifying these isolate-specific factors and how they interact will be important to better understand how antibiotics can influence Salmonella motility.