Role of the mar locus in virulence of Salmonella enterica serovar Typhimurium DT104 in chickens

The virulence of a Salmonella enterica serovar Typhimurium DT014 strain in which marA was insertionally inactivated was compared to its isogenic paren t in vitro and in vivo. In vitro, the numbers of the marA mutant phagocytos ed by porcine lung macrophages were significantly increased, while survival at 24 h inside macrophages and adherence to human gut cells were significa ntly reduced in comparison with the parent strain. In vivo, the marA inacti vated strain, in competition with its parent strain, persisted for a shorte r period in chickens, was present in the caeca at significantly lower level s and invaded the deeper organs to a significantly lesser extent. Therapeut ic antibiotic treatment of one group of chickens with oxytetracycline favou red the persistence of both the parent strain and, to a lesser extent, the marA inactivated strain; but interestingly, increased tetracycline resistan ce of Salmonella isolates after treatment of birds with antibiotic was seen only for the parent strain. Further work is needed to elucidate how mar is involved in virulence and if its inactivation can minimise the ability of bacteria to become antibiotic-resistant in vivo.