Subinhibitory concentrations of tetracycline affect virulence gene expression in a multi-resistant Salmonella enterica subsp. enterica serovar Typhimurium DT104.

Treatment of salmonellosis with antibiotics is controversial and may prolong carriage and shedding. Therefore, this study sought to investigate if exposure to antimicrobials influences the expression of factors involved in virulence and host colonization. The effect of subinhibitory tetracycline treatment (16 microg/ml, 30 min) on a multi-drug resistant Salmonella Typhimurium DT104 strain was investigated using a targeted microarray. Real-time reverse transcriptase PCR was used to confirm and further assess transcription of 10 selected genes. An in vitro cell invasion assay was performed to assess the invasiveness of the tetracycline-treated isolate. Out of 323 genes, 11 were significantly up-regulated and four were down-regulated in the microarray assays. The hilD and hilA genes, both regulators of Salmonella Pathogenicity Island 1, were up-regulated. Other up-regulated genes included the fliC, fliD, motA and motB genes, involved in motility, the fur gene, an important regulator of iron acquisition systems and of acid tolerance. The drug-exposed replicates showed a 2.5-fold increase in intracellular bacteria over the non-exposed control in cell cultures. These findings suggest a drug-induced expression profile consistent with the early stages of Salmonella infection and invasion concomitant with an increased ability to invade epithelial cells in vitro.