A genetic screen for suppressors of bile sensitivity in
DNA adenine methylase (dam) mutants of Salmonella enterica serovar Typhimurium yielded insertions in an uncharacterized locus homologous to the Escherichia coli
asmA gene. Disruption of
asmA suppressed bile sensitivity also in phoP and wec mutants of S. enterica and increased the MIC of
sodium deoxycholate for the parental strain ATCC 14028. Increased levels of marA
mRNA were found in
asmA,
asmA dam,
asmA phoP, and
asmA wec strains of S. enterica, suggesting that lack of
AsmA activates expression of the marRAB operon. Hence,
asmA mutations may enhance bile resistance by inducing gene expression changes in the marRAB-controlled Mar regulon. In silico analysis of
AsmA structure predicted the existence of one transmembrane domain. Biochemical analysis of subcellular fractions revealed that the
asmA gene of S. enterica encodes a
protein of approximately 70 kDa located in the outer membrane. Because
AsmA is unrelated to known transport and/or efflux systems, we propose that activation of marRAB in
asmA mutants may be a consequence of envelope reorganization. Competitive
infection of BALB/c mice with
asmA(+) and
asmA isogenic strains indicated that lack of
AsmA attenuates Salmonella virulence by the oral route but not by the intraperitoneal route. Furthermore,
asmA mutants showed a reduced ability to invade epithelial cells in vitro.