Comparison of virulence of Legionella longbeachae strains in guinea pigs and U937 macrophage-like cells

A guinea pig model of experimental legionellosis was established for assess ment of virulence of isolates of Legionella longbeachae. The results showed that there were distinct virulence groupings of L. longbeachae serogroup 1 strains based on the severity of disease produced in this model. Statistic al analysis of the animal model data suggests that Australian isolates of L . longbeachae may be inherently more virulent than non-Australian strains. Infection studies performed with U937 cells were consistent with the animal model studies and showed that isolates of this species were capable of mul tiplying within these phagocytic cells. Electron microscopy studies of infe cted lung tissue were also undertaken to determine the intracellular nature of L. longbeachae serogroup 1 infection. The data showed that phagosomes c ontaining virulent L. longbeachae serogroup 1 appeared bloated, contained c ellular debris and had an apparent rim of ribosomes while those containing avirulent L. longbeachae serogroup 1 were compact, clear and smooth.