Comparative analysis of Legionella pneumophila and Legionella micdadei virulence traits

While the majority of Legionnaire's disease has been attributed to Legionel la pneumophila, Legionella micdadei can cause a similar infection in immuno compromised people. Consistent with its epidemiological profile, the growth of L. micdadei in cultured macrophages is less robust than that oft. pneum ophila. To identify those features of the Legionella spp, which are correla ted, to efficient growth in macrophages, two approaches were taken. First, a phenotypic analysis compared four clinical isolates of L. micdadei to one well-characterized strain of L. pneumophila, Seven traits previously corre lated with the virulence of L. pneumophila were evaluated: infection and re plication in cultured macrophages, evasion of phagosome-lysosome fusion, co ntact-dependent cytotoxicity, sodium sensitivity, osmotic resistance, and c onjugal DNA transfer. By nearly every measure, L, micdadei appeared less vi rulent than L. pneumophila. The surprising exception was L. micdadei 31B, w hich evaded lysosomes and replicated in macrophages as efficiently as L. pn eumophila, despite lacking both contact-dependent cytopathicity and regulat ed sodium sensitivity. Second, in an attempt to identify virulence factors genetically, an L. pneumophila genomic library was screened for clones whic h conferred robust intracellular growth on L. micdadei, No such loci were i solated, consistent with the multiple phenotypic differences observed for t he two species. Apparently, L. pneumophila and L. micdadei use distinct str ategies to colonize alveolar macrophages, causing Legionnaire's disease.