Influence of subinhibitory levels of antibiotics on expression of Escherichia coli lipopolysaccharide and binding of anti-lipopolysaccharide monoclonal antibodies.

The expression of Escherichia coli lipopolysaccharide (LPS) and the binding capacity of anti-LPS monoclonal antibodies (MAbs) to E. coli grown in the presence or absence of subinhibitory concentrations of various antibiotics was studied. Four E. coli strains (three clinical blood-culture isolates and an isogenic, non-capsulate mutant of the O18:K1 parent) were grown in the presence of the beta-lactam antibiotic, ampicillin, the aminoglycoside gentamicin, the fluoroquinolone ciprofloxacin and chloramphenicol. The techniques of silver staining, immunoblotting, whole-cell ELISA and flow cytometry were all used to monitor the expression of LPS on the bacteria and the binding of the anti-LPS MAbs. Treatment with ampicillin, chloramphenicol and ciprofloxacin resulted in enhanced binding of anti-core reactive MAbs to most E. coli strains. Overall, treatment with gentamicin produced the least effect on MAb binding. The presence of chloramphenicol decreased the expression of high molecular mass O-antigen or increased the expression of low molecular mass substituted E. coli LPS or both. These results further illustrate that LPS core, especially the inner-core region, becomes more accessible to antibodies when bacteria are grown in the presence of certain antibiotics. Possible synergy between antibodies and antibiotics for treatment of septicaemia and septic shock remains an intriguing possibility.