ABSTRACT It was previously demonstrated that the mig - 14 gene of Salmonella enterica serovar Typhimurium is necessary for bacterial proliferation in the liver and spleen of mice following intragastric inoculation and that mig - 14 expression, which is induced within macrophages, is under the control of the global regulator PhoP. Here we demonstrate that the mig - 14 promoter is induced by growth in minimal medium containing low magnesium or acidic pH, consistent with regulation by PhoP. In addition, mig - 14 is strongly induced by polymyxin B, protamine, and the mammalian antimicrobial peptide protegrin-1. While phoP is necessary for the induction of mig - 14 in response to protamine and protegrin, mig - 14 is still induced by polymyxin B in a phoP background. We also demonstrate that mig - 14 is necessary for resistance of S. enterica serovar Typhimurium to both polymyxin B and protegrin-1. Gram-negative resistance to a variety of antimicrobial peptides has been correlated with modifications of lipopolysaccharide structure. However, we show that mig - 14 is not required for one of these modifications, the addition of 4-aminoarabinose to lipid A. Additionally, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of wild-type and mig - 14 lipopolysaccharide also shows no detectable differences between the two strains. Therefore, mig - 14 contributes to Salmonella resistance to antimicrobial peptides by a mechanism that is not yet fully understood.