Evaluation of invasion-conferring genotypes and antibiotic-induced hyperinvasive phenotypes in multiple antibiotic resistant Salmonella typhimurium DT104.

Antibiotic resistance in pathogenic bacteria is a problem in both industrialized and developing countries. This is especially evident in Salmonella typhimurium, a foodborne pathogen that causes gastrointestinal and systemic disease throughout the world. S. typhimurium DT104 further poses a major health concern due to its apparent enhanced ability to acquire multiple antibiotic resistance genes and its putative hypervirulent phenotype. Recently, we demonstrated that multiresistant S. typhimurium do not appear to be more invasive than non-resistant cohorts. In the present study, we evaluated the presence of Salmonella pathogenicity island 1 (SPI1) flanking and internal sequences in over 400 isolates of multiresistant S. typhimurium. With these same isolates, we also used a tissue culture invasion assay to evaluate a potential relationship between antibiotic exposure and a hyperinvasive phenotype. Our studies revealed that SPI1 flanking sequences are similar in multiresistant and non-resistant S. typhimurium. Furthermore, we failed to identify any isolates that were hyperinvasive in the presence of any of the 14 antibiotics evaluated. These results further indicate that the putative hypervirulence of multiresistant S. typhimurium is not likely to occur at the level of invasion.