Mig‐14 is an inner membrane‐associated protein that promotes Salmonella typhimurium resistance to CRAMP, survival within activated macrophages and persistent infection

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Abstract
Salmonella enterica serovar Typhimurium (S. typhimurium) infects a wide variety of mammalian hosts and in rodents causes a typhoid‐like systemic disease involving replication of bacteria inside macrophages within reticuloendothelial tissues. Previous studies demonstrated that the mig‐14 and virK genes of Salmonella enterica are important in bacterial resistance to anti‐microbial peptides and are necessary for continued replication of S. typhimurium in the liver and spleen of susceptible mice after orogastric inoculation. In this work we report that inflammatory signalling via interferon‐gamma (IFN‐γ) is crucial to controlling replication of mig‐14 mutant bacteria within the liver and spleen of mice after oral infection. Using a Salmonella persistence model recently developed in our laboratory, we further demonstrate that mig‐14 contributes to long‐term persistence of Salmonella in the spleen and mesenteric lymph nodes of chronically infected mice. Both mig‐14 and virK contribute to the survival of Salmonella in macrophages treated with IFN‐γ and are necessary for resistance to cathelin‐related anti‐microbial peptide (CRAMP), an anti‐microbial peptide expressed at high levels in activated mouse macrophages. We also show that both Mig‐14 and VirK inhibit the binding of CRAMP to Salmonella, and demonstrate that Mig‐14 is an inner membrane‐associated protein. We further demonstrate by transmission electron microscopy that the primary locus of CRAMP activity appears to be intracytoplasmic, rather than at the outer membrane, suggesting that Mig‐14 may prevent the penetration of the inner membrane by CRAMP. Together, these data indicate an important role for mig‐14 in anti‐microbial peptide resistance in vivo, and show that this resistance is important to the survival of Salmonella in systemic sites during both acute and persistent infection.