Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2014
Publication Date: 1/30/2015
Citation: Brunelle, B.W., Bearson, B.L., Bearson, S.M. 2015. Chloramphenicol and tetracycline decrease motility and increase invasion and attachment gene expression in specific isolates of multidrug-resistant Salmonella enterica serovar Typhimurium. Frontiers in Microbiology. 5(801). Available: http://dx.doi.org/10.3389/fmicb.2014.00801.
Interpretive Summary: Salmonella bacteria cause an estimated one million cases in humans and a projected annual health care cost of over two billion dollars every year in the United States. Salmonella enterica serovar Typhimurium (S. Typhimurium) is one the most common types of Salmonella in humans and livestock, and over 27 percent of these isolates are resistant to three or more types of antibiotics. It has also been reported that antibiotic-resistant Salmonella are associated with enhanced virulence in humans and livestock. To better understand the cause of this increased virulence, we investigated the effect four different antibiotics (ampicillin, chloramphenicol, streptomycin, and tetracycline) had on isolates of multidrug-resistant S. Typhimurium. Laboratory tests demonstrate that invasion is enhanced in multidrug-resistant S. Typhimurium due to chloramphenicol and tetracycline exposure, but this response occurred only in a subset of the isolates tested. It was also found that chloramphenicol and tetracycline could induce invasion within 30 minutes; invasion typically takes over eight hours to occur. Identifying factors associated with enhanced invasion is necessary to understand and prevent the increased clinical disease associated with multidrug-resistant Salmonella.
Technical Abstract: Salmonella enterica serovar Typhimurium (S. Typhimurium) is one of the most common serovars isolated from humans and livestock, and over 35 percent of these isolates are resistant to three or more antibiotics. Multidrug-resistant (MDR) Salmonella is a public health concern as it is associated with increased morbidity in patients compared to antibiotic sensitive strains, though it is unknown how the antibiotic resistant isolates lead to a more severe infection. Cellular invasion is temporally regulated in Salmonella and normally occurs during late-log and stationary growth. However, our previous work determined that a 30 minute exposure to a sub-inhibitory concentration of tetracycline can induce the full invasion phenotype during early-log growth in certain MDR S. Typhimurium isolates. The current study examined whether sub-inhibitory concentrations of other antibiotics could also induce the invasiveness in the same set of isolates. Ampicillin and streptomycin had no effect on invasion, but certain concentrations of chloramphenicol were found to induce invasion in a subset of isolates. Two of the isolates induced by chloramphenicol were also inducible by tetracycline. Ribonucleic acid-sequence analyses demonstrated that chloramphenicol and tetracycline both down-regulated motility gene expression, while up-regulating genes associated with attachment, invasion, and intracellular survival. Eleven fimbrial operons were up-regulated, which is notable as only three fimbrial operons were thought to be inducible in culture; six of these up-regulated operons have been reported to play a role in Salmonella persistence in mice. Overall, these data show that the normal progression of the genetic pathways that regulate invasion can be expedited to occur within 30 minutes due to antibiotic exposure. This altered invasion process due to antibiotics may play a role in the increased intensity and duration of infection observed in patients with MDR Salmonella.