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
Legionnaires' 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 microgram/ml but not less than or equal to 0.125 microgram/ml
erythromycin and 0.01 microgram/ml but not less than or equal to 0.001 microgram/ml
rifampin. These concentrations of
antibiotics are comparable to those that inhibit extracellular multiplication of L. pneumophila under cell-free conditions in artificial medium; the minimal inhibitory concentration is 0.37 microgram/ml for
erythromycin and 0.002 microgram/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
rifampin; the minimal bactericidal concentration is 1 microgram/ml for
erythromycin and 0.009 microgram/ml for
rifampin. In contrast, L. pneumophila multiplying intracellularly is resistant to killing by these concentrations of
erythromycin and
rifampin or by concentrations equal to or 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 vivo, 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.