Submitted to: BMC Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/2/2013
Publication Date: 9/11/2013
Publication URL: http://handle.nal.usda.gov/10113/57942
Citation: Brunelle, B.W., Bearson, S.M., Bearson, B.L. 2013. Tetracycline accelerates the temporally-regulated invasion response in specific isolates of multidrug-resistant Salmonella enterica serovar Typhimurium. BMC Microbiology. DOI: 10.1186/1471-2180-13-202. Interpretive Summary: Salmonella is a highly prevalent foodborne disease in the United States with an estimated 1 million cases in humans per year and a projected annual health care cost of over $2 billion. Salmonella enterica serovar Typhimurium is one the most common salmonellae in humans and livestock, and over 27 per cent of the isolates are resistant to three or more types of antibiotics, including tetracycline. Since antibiotic-resistant bacteria have been associated with enhanced virulence in humans and livestock, and because tetracycline is given to both humans and animals, we investigated the effect of tetracycline on isolates of multidrug resistant S. Typhimurium. Laboratory tests demonstrate that invasion is enhanced in multidrug-resistant S. Typhimurium due to tetracycline exposure, but this response occurred only in a subset of the isolates tested. Identifying the factors needed to establish enhanced invasion is necessary to determine what is causing the increased virulence associated with multidrug-resistant Salmonella.
Technical Abstract: Background Multidrug-resistant (MDR) Salmonella is associated with increased morbidity compared to antibiotic-sensitive strains and is an important health and safety concern in both humans and animals. Salmonella enterica serovar Typhimurium is a prevalent cause of foodborne disease, and a considerable number of S. Typhimurium isolates from humans and livestock are resistant to three or more antibiotics. The majority of these MDR S. Typhimurium isolates are resistant to tetracycline, a commonly used and clinically and agriculturally relevant antibiotic. Because exposure of drug-resistant bacteria to antibiotics can affect cellular processes associated with virulence, such as invasion, we investigated the effect tetracycline had on the invasiveness of tetracycline-resistant MDR S. Typhimurium isolates. Results The isolates selected and tested were from two common definitive phage types of S. Typhimurium, DT104 and DT193, and were resistant to tetracycline and at least three other antibiotics. Although Salmonella invasiveness is temporally regulated and normally occurs during late-log growth phase, tetracycline exposure induced the full invasive phenotype in a cell culture assay during early-log growth in several DT193 isolates. No changes in invasiveness due to tetracycline exposure occurred in the DT104 isolates during early-log growth or in any of the isolates during late-log growth. Real-time PCR was used to test expression of the virulence genes hilA, prgH, and invF, and these genes were significantly up-regulated during early-log growth in most isolates due to tetracycline exposure; however, increased virulence gene expression did not always correspond with increased invasion, and therefore was not an accurate indicator of elevated invasiveness. The induction of the invasion response during early-log was observed only in a subset of the DT193 isolates and was dependent on tetracycline concentration. Conclusion In this report we demonstrate that the invasiveness of MDR S. Typhimurium can be modulated in the presence of tetracycline, and this effect is dependent on growth phase, antibiotic concentration, and strain background. Identifying the conditions necessary to establish an invasive phenotype is important to elucidate the underlying factors associated with increased virulence of MDR Salmonella.