Mechanism of enhancement of bactericidal activity of phagocytes against Klebsiella pneumoniae treated with subminimal inhibitory concentrations of cefodizime.

The effects of a sub-MIC of cefodizime on the morphology of the capsular structures and on the surface physicochemical properties, such as hydrophobicity and charge, of encapsulated Klebsiella pneumoniae were studied. The enhancement of bactericidal activity of macrophages against bacteria treated with sub-MICs of antibiotics was evaluated as the killing index. Cefodizime treatment gave the highest value of 32. Electron microscope observations revealed that the capsular material layer of cefodizime-treated K. pneumoniae was markedly thinner (32 nm) than that of untreated bacteria (160 nm) or bacteria treated with other antibiotics (75-90 nm). Contact angle measurement revealed that the surface of cefodizime-treated K. pneumoniae was more hydrophobic than that of untreated bacteria or bacteria treated with other antibiotics. Furthermore, the negative charge of the surface of K. pneumoniae decreased significantly with cefodizime treatment compared with the surface of untreated bacteria. These findings suggest that the treatment of K. pneumoniae with a sub-MIC of cefodizime reduced the thickness of the capsular material layer and that these changes increased the surface hydrophobicity of the bacteria and decreased the negative charge of the bacterial surface to render K. pneumoniae more susceptible to phagocytic activity by reducing the physical repulsion between the bacteria and phagocytes.