Temperature effects on Legionella pneumophila killing by and multiplication in phagocytes of guinea pigs

We examined the effects of temperature on the interaction between Legionella pneumophila and phagocytes of guinea pigs. The body temperatures of guinea pigs infected with a sublethal dose (1.2 x 10(4) CFU) or a lethal dose (1.0 x 10(5) CFU) of L. pneumophila elevated from 38.4 +/- 0.15 C to 40.2 +/- 0.42 C or 40.3 +/- 0.62 C, respectively. The intracellular bacterial killing by and bacterial proliferation in the phagocytes were examined at 33, 37, 40, and 42 C, using in vitro culture systems of peritoneal macrophages or polymorphonuclear leukocytes (PMN) of guinea pigs. In all the macrophages incubated at different temperatures, significant intracellular bacterial killings were observed at 4 hr after in vitro phagocytosis. After 24 hr of incubation, there was about a 100-fold increase of CFU and the number reached a maximum after 48 hr of incubation in the macrophages incubated at 42 C as well as 37 and 40 C, suggesting that macrophages support the intracellular bacterial growth in hyperthermia. In the PMN, L. pneumophila CFU 4 hr or 12 hr after the infection were significantly lower at 42 C than those at 37 C (P < 0.05), indicating that the bactericidal capacity of PMN was enhanced at 42 C compared to 37 C. However, in all the PMN incubated at different temperatures, there were about 10-fold increases of CFU 24 hr after the infection, suggesting that PMN as well as macrophages support intracellular bacterial growth in hyperthermia. The extracellular bacterial growth was examined at 33, 37, 40, and 42 C in buffered yeast extract (BYE) broth or RPMI 1640 medium containing 50% guinea pig serum as a permissive or non-permissive liquid medium for the bacterial growth, respectively. Inhibition of bacterial growth in BYE broth at 42 C, and a decrease of CFU in RPMI 1640 medium containing 50% guinea pig serum at 42 C were observed. In conclusion, hyperthermia may be beneficial by restricting extracellular bacterial survival, but it exerts no beneficial effect on the restriction of intracellular bacterial growth in phagocytes, though PMN showed enhanced initial killing at 42 C. These results suggest that fever, or hyperthermia itself, may not largely contribute as a nonspecific host defense early in the course of legionellosis.