Effect of Subinhibitory Concentrations of Amikacin and Ciprofloxacin on the K1 Antigen Expression and Phagocytosis of Escherichia coli Strains

Background. Escherichia coli strains with capsular K1 antigen are the major cause of neonatal meningitis and are also responsible for a large number of cases of bacteraemia and urinary tract infections in children and adults. The polysaccharide K1 antigen of E. coli is an important virulence factor that increases resistance to phagocytosis. Although the subinhibitory concentrations (sub-MICs) of antibiotics do not kill microorganisms, they are able to modulate their characteristics, e.g., the morphology of cells, the bacterial surface components. As a result a reduction of bacterial pathogenicity is observed. This may also be associated with changes in the susceptibility of pathogens to the host immune response. Objectives. The aim of this study was to evaluate the influence of sub-MICs of amikacin and ciprofloxacin on the presence of cell-surface K1 antigen of uropathogenic E. coli strains and their phagocytosis by neutrophils. Material and Methods. Fifteen E. coli strains expressing K1 antigen were selected from among the one hundred and twenty samples isolated from the urine of children with urinary tract infections. The bacteriophage K1A was used to detect the presence of the capsular antigen K1. The MICs of amikacin and ciprofloxacin for each strain incubated in Mueller-Hinton broth were obtained by microdilution method. The phagocytic index was determined by microscopic examination of slides prepared with May-Grunwald stain. Results. Exposure of E. coli K1 strains to 1/2, 1/4 and 1/8 of MICs of amikacin and ciprofloxacin reduced the number of clones sensitive to K1A phage, however the effect of amikacin was minute. Moreover, changes in the susceptibility to phagocytosis of E. coli strains incubated in the presence of antibiotics were observed. Conclusions. The treatment of encapsulated bacteria with sub-MICs of amikacin or ciprofloxacin inhibits the K1 capsule expression and enhances phagocytosis by human neutrophils (Adv Clin Exp Med 2010, 19, 4, 429-436).