Synergistic killing of Gram-negative bacilli by cefotaxime, its desacetyl metabolite and human polymorphonuclear neutrophils

Using an in-vitro model the effects of sub-MIC cefotaxime and its desacetyl metabolite singly and in combination on killing of E. coli by PMNs were studied. Our purpose was to determine if the parent compound and its metabolite had a synergistic effect on killing of E. coli by PMNs. Thymidine-labelled serum resistant Escherichia coli 018:K1:H7 were incubated during log phase growth with varying sub-MICs (1/2, 1/8, 1/32) of cefotaxime, its desacetyl metabolite and both agents together, or phosphate buffered saline (PBS) as a control for 90 min at 37 degrees C. The bacteria were then washed and a series of opsonization experiments was performed using intact and sonicated PMNs. Killing of bacteria was determined at 3, 10 and 20 min. Uptake and killing of bacteria by PMNs were measured using standard techniques. Pre-treatment of E. coli with cefotaxime alone and desacetyl cefotaxime and cefotaxime together resulted in significantly enhanced bacterial killing by PMNs at all three exposure times to PMNs. Pre-treatment of the bacteria with sub-MICs of desacetyl cefotaxime alone showed enhanced killing only after exposure for three minutes. In all cases, any increased killing was independent of ingestion by the phagocytes. The opsonization experiments demonstrated that contact between bacteria and PMNs was necessary for optimal killing to occur. The enhanced killing of the sub-MIC antibiotic pre-treated bacteria was seen even when sonicated PMNs were used. The extent of bacterial killing, however, was less than that seen with intact PMNs.