Three DT193 isolates (1434, 5317, and 752) had PF-6463922 a significant increase in invasion during early-log growth in the presence of 16 μg/ml tetracycline, and all three of these isolates have in common the presence of a single tetracycline resistance gene, tetA (Table 1). Tetracycline exposure did not enhance the invasion phenotype of the other DT193 isolates or the three DT104 isolates. Figure 2 Changes in S. Typhimurium invasiveness at early- and late-log growth after tetracycline

exposure. Invasion assays were performed on S. Typhimurium isolates grown to either early- or late-log phase and exposed to four different tetracycline concentrations (0, 1, 4, and 16 μg/ml) for 30 GS-9973 chemical structure minutes. Changes in invasion were normalized to the control dose (0 μg/ml) for each isolate at (A) early-log and (B) late-log growth phase. The “*” indicates a significant change based on the pre-normalized data. The numbers in parentheses indicate percent invasion at the control dose (0 μg/ml) for GF120918 each isolate. To determine if tetracycline exposure enhances Salmonella

invasiveness during late-log phase, isolates were grown to OD600 = 0.60 and exposed to 0, 1, 4, and 16 μg/ml of tetracycline for 30 minutes. Tetracycline did not increase the invasiveness of Salmonella during late-log growth in any of the isolates (Figure 2B; Additional file 1). However, the level of invasion induced by 16 μg/ml tetracycline during early-log phase in the three DT193 isolates was similar to the invasion levels of their respective controls (0 μg/ml) during late-log phase. These results demonstrate that when Salmonella is at its highest level of normal invasion (late-log), exposure to sub-inhibitory levels of tetracycline does not result in hyperinvasiveness; instead, tetracycline exposure triggers the invasive phenotype in specific isolates during a phase of growth that Salmonella is not otherwise fully

invasive (early-log). Gene expression changes due to tetracycline exposure The relative transcript levels of three genes associated with invasion regulation (hilA, prgH, and invF), as well as the tetracycline resistance genes in each isolate (tetA, B, C, D, and/or G), were determined many by real-time PCR. The hilA gene is essential for invasion as HilA activity regulates downstream invasion factors, which includes the prgH and invF genes [21, 22]. Together, these genes provide a direct and indirect measure of both the hilA transcript and HilA protein, respectively. During early-log phase, all three invasion genes were significantly up-regulated in seven of the eight isolates at 16 μg/ml compared to the 0 μg/ml control, while four isolates had one or more of the invasion genes significantly up-regulated at 4 μg/ml; no invasion gene expression changes occurred in any isolate at 1 μg/ml (Figure 3; Additional file 1).