Effect of polymorphonuclear neutrophils on serum bactericidal activity against Streptococcus pneumoniae after amoxicillin administration

The effect of phagocytic killing on serum bactericidal activity against Streptococcus pneumoniae was investigated 0, 1.5, 8 and 12 h after a single 875 mg oral dose of amoxicillin in healthy adults. Killing curves were determined with polymorphonuclear neutrophils (PMN), serum or PMN plus serum. Global killing (i.e. intracellular and extracellular killing) over 3 h of incubation was expressed as the area under the killing curve (AUKC; log cfu x h/ml). Amoxicillin did not affect the activity of PMN alone. For serum alone, the AUKC of post-administration samples (with supra-inhibitory amoxicillin concentrations) was significantly lower than in baseline samples. For serum plus PMN, significant bactericidal activity of serum was still found in samples after antibiotic concentrations had reached sub-inhibitory levels.