EFFECTS OF SUB-MINIMAL INHIBITORY CONCENTRATIONS OF ANTIMICROBIAL AGENTS ON THE CELL-SURFACE OF KLEBSIELLA-PNEUMONIAE AND PHAGOCYTIC KILLING ACTIVITY

Changes in the phagocytic killing activity, capsule structure, and phy sicochemical properties such as the hydrophobicity and charge of the c ell surface were studied in Klebsiella pneumoniae treated with sub-min imal inhibitory concentrations (MICs) of various antimicrobial agents. The phagocytic killing activity of macrophages was enhanced by penici llins, cephems, and monobactam in the absence of antibodies specific t o the capsule or complement. No enhancement was observed with new quin olones, aminoglycosides, macrolide, or carbapenem. The thickness of th e capsule structure was considerably reduced after the treatment with penicillins, cephems, and monobactam compared with the untreated contr ol, and it was slightly reduced by new quinolones. No changes were obs erved in the capsule structure with aminoglycosides, macrolide, and ca rbapenem. The hydrophobicity on the cell surface of the bacteria was c onsiderably increased after the treatment with penicillins, cephems, a nd monobactam compared with the control, slightly increased with new q uinolones and carbapenem, and not changed with aminoglycosides and mac rolide, The negative charge of the cell surface of the bacteria was re duced by penicillins, cephems, and monobactam compared with the contro l. It was slightly reduced by new quinolones and carbapenem but was no t reduced by aminoglycosides and macrolide. These findings suggest cha t sub-MIG beta-lactam drugs such as penicillins, cephems, and monobact ams cause thinning of the capsule of K. pneumoniae with increases in t he hydrophobicity and decreases in the negative charge of the cell sur face, which reduces the physical repulsion between the K. pneumoniae a nd phagocytes and enhances the sensitivity of the bacteria to phagocyt ic killing activity.