Sub-MICs of cefuroxime and ciprofloxacin influence interaction of complement and immunoglobulins with Klebsiella pneumoniae.

Growth of encapsulated (K+) and nonencapsulated (K-) Klebsiella pneumoniae strains in media containing sub-MICs of either cefuroxime or ciprofloxacin resulted in cell elongation but had little effect on the outer membrane protein or lipopolysaccharide profiles. Exposure to serum complement increased the surface hydrophobicity of a K- strain but failed to interact or to increase the surface hydrophobicity of the K+ strains. However, after growth of the K+ strains in sub-MICs of the antibiotics, complement increased their surface hydrophobicity and complement C3 was detected bound to their surface. Antisera raised against a K-O- strain agglutinated the K+ strains grown in the presence but not in the absence of cefuroxime or ciprofloxacin. These findings suggest that the filamentous morphology induced by these antibiotics influences the distribution or amount of capsular polysaccharide such that cell envelope components previously masked by the capsule become accessible to complement and immunoglobulins.