A previously unknown protein, designated SvpA (urface irulence-associated rotein) and implicated in the virulence of the intracellular pathogen , was identified. This 64 kDa protein, encoded by , is both secreted in culture supernatants and surface-exposed, as shown by immunogold labelling of whole bacteria with an anti-SvpA antibody. Analysis of the peptide sequence revealed that SvpA contains a leader peptide, a predicted C-terminal transmembrane region and a positively charged tail resembling that of the surface protein ActA, suggesting that SvpA might partially reassociate with the bacterial surface by its C-terminal membrane anchor. An allelic mutant was constructed by disrupting in the wild-type strain LO28. The virulence of this mutant was strongly attenuated in the mouse, with a 2 log decrease in the LD and restricted bacterial growth in organs as compared to the wild-type strain. This reduced virulence was not related either to a loss of adherence or to a lower expression of known virulence factors, which remained unaffected in the mutant. It was caused by a restriction of intracellular growth of mutant bacteria. By following the intracellular behaviour of bacteria within bone-marrow-derived macrophages by confocal and electron microscopy studies, it was found that most mutant bacteria remained confined within phagosomes, in contrast to wild-type bacteria which rapidly escaped to the cytoplasm. The regulation of was independent of PrfA, the transcriptional activator of virulence genes in . In fact, SvpA was down-regulated by MecA, ClpC and ClpP, which are highly homologous to proteins of forming a regulatory complex controlling the competence state of this saprophyte. The results indicate that: (i) SvpA is a novel factor involved in the virulence of . , promoting bacterial escape from phagosomes of macrophages; (ii) SvpA is, at least partially, associated with the surface of bacteria; and (iii) SvpA is PrfA-independent and controlled by a MecA-dependent regulatory network.


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