POSTANTIBIOTIC EFFECT OF AZITHROMYCIN AND ERYTHROMYCIN ON STREPTOCOCCAL SUSCEPTIBILITY TO PHAGOCYTOSIS

The effect of azithromycin and erythromycin on growth, cell surface hy drophobicity and the susceptibility to the bactericidal activity of hu man polymorphonuclear leucocytes (PMNL) was examined in four Streptoco ccus species. Exposure to either 10 x MIC azithromycin or erythromycin induced a post-antibiotic effect (PAE) of between 2.4 and 4.3 h. Eryt hromycin caused a longer PAE for S. sanguis than azithromycin under th e same conditions. The cell surface charge (hydrophobic or hydrophilic ) of the streptococci was altered significantly during PAE; loss of hy drophobicity was induced by both macrolides, and this effect was varia ble amongst the species. The decrease in hydrophobicity was not relate d to inhibition of growth. The effect of each drug during PAE on the i nteraction of opsonised suspensions of the streptococci with human PMN L revealed that erythromycin, and to a lesser extent azithromycin, inc reased susceptibility to the bactericidal activity of human PMNL; this effect was abolished following PAE. The present study clearly showed that PAE should not only be considered as delayed bacterial growth, bu t also as modulation of bacterial susceptibility to phagocytosis which may influence the outcome of the host-parasite relationship.