Antibiotics and human defence mechanisms: the effects of ampicillin and cefotaxime on intra- and extracellular killing of Salmonella enteritidis

In-vitro reaction mixtures were set up containing bacteria, human serum, human neutrophils and ampicillin or cefotaxime at the MIC, MIC/2 or MIC/4. Serum was untreated, heat-inactivated, absorbed or both. Five indices measuring intra- and extracellular killing were calculated and the effects of heat-labile and absorbable serum factors and of the various concentrations of antibiotics on these indices were calculated. Both heat-labile and absorbable factors enhanced intra- and extracellular killing; antibiotics had a similar effect except that phagocytosis was reduced. Antibiotics at MIC/4 exerted as great an effect as did higher concentrations and this was especially so for cefotaxime. Both heat-labile and absorbable serum factors had a greater effect on all five indices the lower the concentration of antibiotic with advantage for cefotaxime. In the presence of immunological serum factors the overall effect against Salmonella enteritidis is maintained as the concentration of antibiotic falls below the MIC and this is especially seen in the presence of cefotaxime.