INTRAPHAGOCYTIC GROWTH INDUCES AN ANTIBIOTIC-RESISTANT PHENOTYPE OF LEGIONELLA-PNEUMOPHILA

The antimicrobial susceptibilities of Legionella pneumophila isolates grown either in U937 human monocytic cells or in Acanthamoeba polyphag a were studied after release from the host cells without further subcu lture, Time-survival studies showed that exposure of L. pneumophila ce lls, grown exclusively in vitro, to 5 mu g of rifampin per ml resulted in at least 99.9% killing after 6 h and no detectable survivors at 24 h. Similar rates of killing were observed for in vitro-grown cells te sted by exposure to ciprofloxacin, Conversely, time-survival studies r evealed that macrophage-grown and amoeba-grown cells were ca, 1,000-fo ld more resistant to the activities of both drugs, Macrophage-grown ce lls treated with 5 mu g of rifampin per mi showed 70 and 62% survival after 6 and 24 h, respectively, Intracellularly grown legionellae were also highly resistant to erythromycin (8 mu g/ml). After 24 h of expo sure to the drug, there was 70 and 60% survival for amoeba-grown and m acrophage-grown legionellae, respectively, whereas in vitro-grown cell s showed a 2-log(10) reduction in viable count, When intracellularly g rown L. pneumophila cells were subcultured in broth for 48 h, they rev erted to the phenotype characteristic of in vitro growth, Morphologica lly, the cells were larger than their intracellularly grown counterpar ts and resistance characteristics were lost, The susceptibilities of t he subcultured cells to all three drugs were similar to those of Legio nella cells grown exclusively in vitro, In view of these findings, the successful treatment of Legionnaires' disease may be related as much to the resistance phenotype induced by intramacrophage growth as to th e ability of the antibiotic to enter phagocytic cells.