Gene expression changes in S. Typhimurium at early- and late-log growth after tetracycline exposure. Real-time gene expression 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 minutes. Virulence genes (hilA, prgH, and invF) and tetracycline resistance genes (tetA, B, C, D, and G) were profiled. Compared to the control for each gene (0 μg/ml), black indicates no gene expression change, green indicates an increase in gene expression, and red indicates a decrease in gene expression; the brighter the green or red, the greater the change. The white “*” denotes a significant change in expression compared to the control.