In Vivo-Selected Mutations in Methyl-Directed Mismatch Repair Suppress the Virulence Attenuation ofSalmonella damMutant Strains following Intraperitoneal, but Not Oral, Infection of Naïve Mice

Abstract

Salmonella enterica serovar Typhimurium that lacks the DNA adenine methylase (Dam) ectopically expresses multiple genes that are preferentially expressed during infection, is attenuated for virulence, and confers heightened immunity in vaccinated hosts. The safety of dam mutant Salmonella vaccines was evaluated by screening within infected mice for isolates that have an increased capacity to cause disease relative to the attenuated parental strain. Since dam mutant strains are sensitive to the DNA base analog 2-aminopurine (2-AP), we screened for 2-AP-resistant (2-AP r ) isolates in systemic tissues of mice infected with dam mutant Salmonella. Such 2-AP r derivatives were isolated following intraperitoneal but not oral administration and were shown to be competent for infectivity via intraperitoneal but not oral infection of naïve mice. These 2-AP r derivatives were deficient in methyl-directed mismatch repair and were resistant to nitric oxide, yet they retained the bile-sensitive phenotype of the parental dam mutant strain. Additionally, introduction of a mutH null mutation into dam mutant cells suppressed the inherent defects in intraperitoneal infectivity and nitric oxide resistance, as well as overexpression of SpvB, an actin cytotoxin required for Salmonella systemic survival. These data suggest that restoration of intraperitoneal virulence of dam mutant strains is associated with deficiencies in methyl-directed mismatch repair that correlate with the production of systemically related virulence functions.

Cohabitation within mice of Salmonella typhimurium seqA mutant increases its virulence

“…Similar results were recently demonstrated with Salmonella typhimurium dam mutant (Chatti et al 2008;Heithoff et al 2007). It was demonstrated that Salmonella typhimurium dam mutant increases its fitness after passage through mice.…”

“…Recently, it has been demonstrated that cohabitation of Salmonella typhimurium dam mutant within mice enhances its fitness and virulence after intraperitoneal administration (Heithoff et al 2007;Chatti et al 2008). Indeed, we are demonstrating that acid adaptation of Salmonella dam mutant enhances in vitro and in vivo virulence of these strains (Chatti and Landoulsi, in press).…”

Epigenetic Influence of Dam Methylation on Gene Expression and Attachment in Uropathogenic Escherichia coli

“…This was made evident by Badie et al ( 36 ) signifying amplified defects in the gene expression of virulence genes for flagellin synthesis, motility, and bile resistance in pathogenic Salmonella strain 14028 ( 34 ). To date, studies centered on the modulation of responses in the host immune system to attenuated Salmonella Dam-serovars have indicated that greater immunity is conferred to vaccinated hosts ( 36 ) and may be highly effective as live vaccines against murine typhoid fever in an increased attenuated state ( 33 , 45 ) taking in consideration the persistence of Dam mutants in infected animals ( 46 ).…”