Comparative assessment of virulence traits in Legionella spp.

Legionella pneumophila is a facultative intracellular pathogen that accounts for the majority of cases of Legionnaires' disease in the USA and Europe, but other Legionella spp. have been shown to cause disease. In contrast, Legionella longbeachae is the leading cause of Legionnaires' disease in Australia. The hallmark of Legionnaires' disease caused by L. pneumophila is the intracellular replication within phagocytes in the alveolar spaces, and the Dot/Icm type IV secretion system is essential for intracellular replication. Although it has been presumed that intracellular replication within phagocytes is the hallmark of other virulent legionellae, the virulence traits of Legionella spp. apart from L. pneumophila are not well defined. In this study, 27 strains of Legionella spp. belonging to 16 species that have been isolated from humans or from the environment were examined for five virulence traits exhibited by L. pneumophila: cytopathogenicity, intracellular replication within macrophages, induction of apoptosis/DNA fragmentation, pore-formation-mediated cytolysis of the host cell, and the presence of the dot/icm loci. The strains were divided into two broad groups (low and high cytopathogenic groups) based on cytopathogenicity assays using U937 human-derived macrophages. The other four virulence traits were evaluated in the low and high cytopathogenic groups of Legionella species. Most L. pneumophila serogroup 1 strains were highly cytopathogenic after 72 h, manifested high levels of intracellular growth, induced apoptosis/DNA fragmentation, and exhibited pore-forming activity. The majority of the other species were the low cytopathogenic group that did not induce apoptosis, neither did they exhibit pore-forming activity. All the species of legionellae tested have all the dot/icm loci, when examined by DNA hybridization. No correlation was found between cytopathogenicity and the other four pathogenic traits.