Multiplication of Legionella pneumophila Philadelphia-1 in cultured peritoneal macrophages and its correlation to susceptibility of animals.

Intracellular growth of Legionella pneumophila Philadelphia-1 strain in peritoneal macrophages (PMP) from various rodents was measured and its correlation to the level of susceptibility of the animal was examined. In guinea pig PMP, the organism grew well and the guinea pig was very susceptible to it (50% lethal dose, LD50 = 7.6 X 10(4)). On the other hand, the bacteria hardly multiplied in mouse PMP and the animal was resistant to infection (LD50 = 6.7 X 10(7)). Intracellular growth rate correlated well with susceptibility in these animals. In golden hamsters, a discrepancy between intracellular growth and susceptibility was found. The organism grew intracellularly as rapid as in guinea pig PMP, but the golden hamster was very resistant to infection (LD50 = 2.2 X 10(8)). In rat PMP, the organism did not grow intracellularly during a 24-h period of infection, but started to grow after that and the growth rate thereafter was as rapid as in guinea pig PMP. WKA rats were resistant and the LD50 in the animal was 1.9 X 10(7). In vivo natural resistance of rats and golden hamsters to the organism was considered to be a result of other factors than macrophages.