Invasiveness in chickens, stress resistance and RpoS status of wild-type Salmonella enterica subsp enterica serovar Typhimurium definitive type 104 and serovar Enteritidis phage type 4 strains

The heat and acid resistance and the ability to survive airdrying on common ly used kitchen surfaces were assessed for clinical and environmental strai ns of Salmonella enterica subsp. enterica serovar Typhimurium, definitive t ype (DT) 104, Three out of thirty-eight strains of DT 104 were found to be more sensitive in stationary phase to the stresses examined than the other strains. This compares to a previous study by the authors which showed that seven out of forty serovar Enteritidis phage type (PT) 4 strains were more sensitive. RpoS activity was examined indirectly in selected strains of DT 104 and PT 4. In those with normal stress resistance a 100-fold induction of an RpoS-dependent spvR/A':: luxCDABE fusion was observed upon entry into stationary phase. The sensitive strains examined showed either no inductio n or a reduced level of spvR/A':: luxCDABE expression. The rpoS gene was se quenced from these strains and three were found to harbour mutations includ ing one deletion, one base-pair substitution resulting in a nonsense codon, and one insertion causing a frameshift resulting in an early stop codon. S trains with negligible or reduced spvR/A':: luxCDABE expression had low str ess resistance. All strains of DT 104 could be recovered from liver and spl een tissues of infected hens 14 d post-infection, but one with no induction of spvR/A':: luxCDABE expression was significantly less likely to be recov ered from chicken reproductive tissues, liver or spleen than the majority o f other strains, including one with reduced spvR/A':: luxCDABE expression. This work has demonstrated that clinical and environmental strains of DT 10 4 and PT 4 not infrequently harbour mutations in the rpoS allele. It is pos sible that the rpoS mutations may have occurred during the initial isolatio n of the strains. The ability of a strain to cause infection, however, also depends on factors such as host susceptibility and dose.