Virulence factors of the family Legionellaceae

Whereas bacteria in the genus Legionella have emerged as relatively frequent causes of pneumonia, the mechanisms underlying their pathogenicity are obscure. The legionellae are facultative intracellular pathogens which multiply within the phagosome of mononuclear phagocytes and are not killed efficiently by polymorphonuclear leukocytes. The functional defects that might permit the intracellular survival of the legionellae have remained an enigma until recently. Phagosome-lysosome fusion is inhibited by a single strain (Philadelphia 1) of Legionella pneumophila serogroup 1, but not by other strains of L. pneumophila or other species. It has been found that following the ingestion of Legionella organisms, the subsequent activation of neutrophils and monocytes in response to both soluble and particulate stimuli is profoundly impaired and the bactericidal activity of these cells is attenuated, suggesting that Legionella bacterial cell-associated factors have an inhibitory effect on phagocyte activation. Two factors elaborated by the legionellae which inhibit phagocyte activation have been described. First, the Legionella (cyto)toxin blocks neutrophil oxidative metabolism in response to various agonists by an unknown mechanism. Second, L. micdadei bacterial cells contain a phosphatase which blocks superoxide anion production by stimulated neutrophils. The Legionella phosphatase disrupts the formation of critical intracellular second messengers in neutrophils. In addition to the toxin and phosphatase, several other moieties that may serve as virulence factors by promoting cell invasion or intracellular survival and multiplication are elaborated by the legionellae. Molecular biological studies show that a cell surface protein named Mip is necessary for the efficient invasion of monocytes. A possible role for a Legionella phospholipase C as a virulence factor is still largely theoretical. L. micdadei contains an unusual protein kinase which catalyzes the phosphorylation of eukaryotic substrates, including phosphatidylinositol and tubulin. Since the phosphorylation of either phosphatidylinositol or tubulin might compromise phagocyte activation and bactericidal functions, this enzyme may well be a virulence factor. Administration of the L. pneumophila exoprotease induces lesions resembling those of Legionella pneumonia and kills guinea pigs, suggesting that this protein plays a role in the pathogenesis of legionellosis. However, recent work with a genetically engineered strain has convincingly shown that the protease is not necessary for intracellular survival or virulence. As might be expected with a complex process like intracellular parasitism, it appears that the capability of Legionella strains to invade and multiply in host phagocytes is multifactorial and that no single moiety which is responsible for the virulence phenotype will be found.