Intracellular multiplication of Legionnaires' disease bacteria (Legionella pneumophila) in human monocytes is reversibly inhibited by erythromycin and rifampin

Abstract

We have previously reported that virulent egg yolk-grown Legionella pneumophila, Philadelphia 1 strain, multiplies intracellularly in human blood monocytes and only intracellularly under tissue culture conditions. In this paper, we have investigated the effect of erythromycin and rifampin on L. pneumophila-monocyte interaction in vitro; erythromycin and rifampin are currently the drugs of choice for the treatment of Legionnaire's disease. The intracellular multiplication of L. pneumophila is inhibited by erythromycin and rifampin, as measured by colony-forming units, whether the antibiotics are added just before or just after infection of monocytes with L. pneumophila, or 2 d after infection when L. pneumophila is in the logarithmic phase of growth in monocytes. Intracellular multiplication of L. pneumophila is inhibited by 1.25 μg/ml but not ≤0.125 μg/ml erythromycin and 0.01 μg/ml but not ≤0.001 μg/ml rifampin. These concentrations of antibiotics are compared to those that inhibit extracellular multiplication of L. pneumophila under cell-free cnoditions in artificial medium; the minimal inhibitory concentration is 0.37 μg/ml for erythromycin and 0.002 μg/ml for rifampin. Multiplication of L. pneumophila in the logarithmic phase of growth in monocytes is inhibited within 1 h of the addition of antibiotics. Intracellular bacteria inhibited from multiplying by antibiotics are not killed. By electron microscopy, the bacteria appear intact within membrane-bound vacuoles, studded with ribosomelike structures. L. pneumophila multiplying extracellularly on artificial medium is killed readily by relatively low concentrations of erythromycin and rifampicin; the minimal bactericidal concentration is 1 μg/ml for erythromycin and 0.009 μg/l for rifampin. In contrast, L. pneumophila multiplying intracellularly is resistant to killing by these concentrations of erythromycin and rifampicin or by concentrations equal to greater than peak serum levels in humans. Extracellular L. pneumophila in stationary phase is also resistant to killing by erythromycin and rifampin. These findings, taken together with our previous work, indicate that; in vitro, L. pneumophila is resistant to killing by erythromycin and rifampin. Inhibition of L. pneumophila multiplication in monocytes by antibiotics is reversible; when the antibiotics are removed from infected monocyte cultures after 2 d, L. pneumophila resumes multiplication. This study indicates that patients with Legionnaires' disease under treatment with erythromycin and rifampin require host defenses to eliminate L. pneumophila, and that inadequate host defenses may result in relapse after cessation of therapy.