Salmonella typhimurium mutants that downregulate phagocyte nitric oxide production.

To examine the potential and strategies of the facultative intracellular pathogen Salmonella typhimurium to increase its fitness in host cells, we applied a selection that enriches for mutants with increased bacterial growth yields in murine J774-A.1 macrophage-like cells. The selection, which was based on intracellular growth competition, rapidly yielded isolates that out-competed the wild-type strain during intracellular growth. J774-A.1 cells responded to challenge with S. typhimurium by mounting an inducible nitric oxide synthase (iNOS) mRNA and protein expression and a concomitant nitric oxide (NO) production. Inhibition of NO production with the use of the competitive inhibitor N-monomethyl-L-arginine (NMMA) resulted in a 20-fold increase in bacterial growth yield, suggesting that the NO response prevented bacterial intracellular growth. In accordance with this observation, five out of the nine growth advantage mutants isolated inhibited production of NO from J774-A.1 cells, despite an induction of iNOS mRNA and iNOS protein. Accompanying bacterial phenotypes included alterations in lipopolysaccharide structure and in the profiles of proteins secreted by invasion-competent bacteria. The results indicate that S. typhimurium has the ability to mutate in several different ways to increase its host fitness and that inhibition of iNOS activity may be a major adaptation.

Novel Salmonella typhimurium properties in host--parasite interactions.

Inflammatory bowel disease (IBD) comprises different diseases in the gastrointestinal tract in human, of which Crohn's disease (CD) and ulcerative colitis (UC) are the most prominent. A key factor in the etiology of IBD is the chronic inflammatory process, and a large body of evidence suggests that the transcription factor nuclear factor-kappa B (NF-kappaB) is the key regulator of responses determining the clinical inflammatory condition. Recent findings using antisense oligonucleotides provide direct evidence that the p65 subunit of NF-kappaB plays a central role in chronic intestinal inflammation. It has previously been shown that the Gram negative bacteria Yersinia pseudotubercolosis targets the eukaryotic signal transduction pathway(s) that lead to NF-kappaB activation (and thus avoid an anti-bacterial inflammatory response). In this paper, growth-based selected Salmonella typhimurium clones have been used to generate a clearer picture of the molecular mechanisms involved in host-parasite interactions. From the results presented here, S. typhimurium and Y. pseudotubercolosis may use the same mechanism to block NF-kappaB activation, following host cell infection. A new adaptational feature could also be shown, where a growth-based selected bacteria avoided the normally induced translocation of NF-kappaB in host cells.

Identification of Salmonellae with the 4-methylumbelliferyl caprilate fluorescence test.

We have tested 750 Salmonella strains and 130 strains of other species of the family Enterbacteriaceae with the 4-methylumbelliferyl caprilate reagent (MUCAP) test. The MUCAP test is a fluorescence test for rapid identification of Salmonella strains. The non-Salmonella strains were strains sent for identification as suspected Salmonella strains and thus have phenotypes similar to those of Salmonella strains. All 748 tested Salmonella strains of subgroups I, II, III, and IV were positive in the MUCAP test. Of the two tested rare Salmonella subgroup V strains, one was positive and the other was negative. In the selected material containing strains with phenotypes similar to those of Salmonella strains, only one Hafnia alvei strain of 130 Enterbacteriaceae bacteria tested was positive. The fluorescence of the H. alvei strain, the six tested Salmonella dublin strains, and the positive Salmonella subgroup V strain was weaker than that of the other salmonellae. The MUCAP assay is simple and is performed within 5 min. With an almost 100% sensitivity for Salmonella strains, apart from a single Salmonella subgroup V strain, we found the MUCAP test to be a convenient complement to traditional biochemical identification methods, especially for atypical and unusual Salmonella strains.