Characterization of the RpoS Status of Clinical Isolates of Salmonella enterica

Abstract

The stationary-phase-inducible sigma factor, S (RpoS), is the master regulator of the general stress response in Salmonella and is required for virulence in mice. rpoS mutants can frequently be isolated from highly passaged laboratory strains of Salmonella. We examined the rpoS status of 116 human clinical isolates of Salmonella, including 41 Salmonella enterica serotype Typhi strains isolated from blood, 38 S. enterica serotype Typhimurium strains isolated from blood, and 37 Salmonella serotype Typhimurium strains isolated from feces. We examined the abilities of these strains to produce the S protein, to express RpoS-dependent catalase activity, and to resist to oxidative stress in the stationary phase of growth. We also carried out complementation experiments with a cloned wild-type rpoS gene. Our results showed that 15 of the 41 Salmonella serotype Typhi isolates were defective in RpoS. We sequenced the rpoS allele of 12 strains. This led to identification of small insertions, deletions, and point mutations resulting in premature stop codons or affecting regions 1 and 2 of S , showing that the rpoS mutations are not clonal. Thus, mutant rpoS alleles can be found in freshly isolated clinical strains of Salmonella serotype Typhi, and they may affect virulence properties. Interestingly however, no rpoS mutants were found among the 75 Salmonella serotype Typhimurium isolates. Strains that differed in catalase activity and resistance to hydrogen peroxide were found, but the differences were not linked to the rpoS status. This suggests that Salmonella serotype Typhimurium rpoS mutants are counterselected because rpoS plays a role in the pathogenesis of Salmonella serotype Typhimurium in humans or in the transmission cycle of the disease.

Regulation of Vi Capsular Polysaccharide Synthesis in Salmonella enterica Serotype Typhi

“…However, it is not clear whether this mutation was present in the original isolate or is a result of laboratory passage. In one recent study, 36% of fresh human S. Typhi isolates were found to be rpoS mutants [53]. We confirmed that the rpoS allelic state is in fact responsible for these observations by constructing an rpoS deletion in ISP1820 [51].…”

Evolutionary loss of the rdar morphotype in Salmonella as a result of high mutation rates during laboratory passage

“…The competition between the drive for protection against stress via a functioning s 38 , and improved scavenging via increased s 70 leads to varied mutations in rpoS (Notley-McRobb et al, 2002;Chen et al, 2004;King et al, 2004;Ferenci, 2005;Mandel and Silhavy, 2005;Maharjan et al, 2006). rpoS mutations have been identified in clinical isolates of S. enterica serovar Typhi (Robbe-Saule et al, 2003), though are rare, and are not demonstrated in serovar Typhimurium, thus it is unclear whether these mutants truly exist in clinical settings. This study emphasizes how easy it is to generate rpoS mutations in the laboratory and care must be taken in clinical studies to ensure that rpoS À mutants are not selected by accident from an rpoS þ ancestor.…”

Comparative Genetics of the rdar Morphotype inSalmonella

“…The other isolates, Sarc4 and Sarc16, were identified as being RpoS deficient. Mutations in rpoS have been detected in other Salmonella isolates and are thought to reflect either natural variation or laboratory-induced changes (34,47,60).…”