The influence of subminimal inhibitory concentrations of netilmicin and ceftriaxone on the interaction of Escherichia coli with host defences.

The effect of sub-MICs of netilmicin and ceftriaxone on the interaction between encapsulated and unencapsulated strains of Escherichia coli and certain host defence mechanisms, i.e. complement activation, opsonization, phagocytosis by human polymorphonuclear leucocytes (PMN), and serum bactericidal activity have been studied. Experiments were carried out testing antibiotics either alone or in combination. Non-capsulated strains of E. coli activated complement rapidly and were easily phagocytosed and killed after opsonization in human pooled serum. Pretreatment of these strains with sub-MICs of antibiotics did not change the rate of opsonization or the degree of uptake by PMN, but did enhance serum sensitivity. Capsulated strains of E. coli were both poorly opsonized and resistant to serum bactericidal activity. Treatment of these strains with sub-MICs of antibiotics enhanced complement consumption as well as phagocytosis by PMN, but did not affect serum-resistance.