Synergistic interaction of josamycin with human neutrophils bactericidal function in vitro

Josamycin, a 16-membered ring macrolide is concentrated up to 20-fold in phagocytic cells compared with serum. We have studied the in-vitro interaction of this drug with human neutrophils (PMN) bactericidal function by using two strains resistant to this antibiotic, Pseudomonas aeruginosa and Klebsiella pneumoniae, and a sensitive one, Staphylococcus aureus 209P. It was shown that josamycin-pretreated adherent PMN displayed an increased phagocytic activity (about 30 to 40%) for S. aureus or K. pneumoniae, mainly due to the recruitment of an additional phagocytizing subset of PMN. Furthermore, the bacterial killing was enhanced in josamycin-treated PMN in a dose-dependent manner for K. pneumoniae (60-130% increase in the range of concentration 0.1-25 mg/l) and independently of the dose for S. aureus (about 425-460% increase for josamycin 0.1-10 mg/l). P. aeruginosa killing by whole blood was also significantly increased in the presence of 10 and 1 mg/l of josamycin. Other PMN functions were not much altered by josamycin except an enhancement of the formyl-methionyl-leucyl-phenylalanine-induced oxidative response. Chemotaxis was only increased by the presence of a high concentration (100 mg/l) of josamycin. These data suggest that the bactericidal synergy between PMN and josamycin could be related, partly at least, to a direct enhancing effect of josamycin on some PMN functions such as phagocytosis, chemotaxis and FMLP-induced chemiluminescence. On the other hand, alterations of bacteria, either inside the phagolysosome or in the extracellular medium, could lead to an enhanced susceptibility to the phagocytes' microbial mechanisms.