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Science  04 Jan 2013:
Vol. 339, Issue 6115, pp. 12
DOI: 10.1126/science.339.6115.12-a

The bacteria Salmonella enterica is a major cause of food poisoning. Salmonella invades host cells by injecting these cells with virulence factors by means of a molecular machine called a type 3 secretion system, encoded by the Salmonella pathogenicity island 2 (T3SS-SPI-2); replicates, and then rapidly disperses systemically. It is important to check the dynamics and mechanisms of spread in intact animals because new vaccine candidates using strains mutated in T3SS-SPI-2 are being developed for use in humans. Grant et al. found that mutant S. enterica, in contrast to wild type, replicated to high numbers within individual spleen and liver cells and formed only a few new foci of infection. Further experiments showed that the mutant bacteria were trapped in the initial cell because they were held in check by intracellular oxidase activity that generates free radicals. Unexpectedly, the pathogen appeared to require T3SS-SPI-2 to exit cells and spread through an organ. This implies that a net bacterial cell count alone will not tell you whether a Salmonella infection has successfully established and disseminated.

PLoS Pathog. 8, e1003070 (2012).

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