A critical concentration of neutrophils is required for effective bacterial killing in suspension

We have examined the effect of neutrophil concentration on killing of a clinical isolate of Staphylococcus epidermidis. Human neutrophils at concentrations varying from 10(5) to 10(7) per ml were mixed in suspension with S. epidermidis at concentrations varying from 10(3) to 10(8) colony-forming units/ml, and the concentration of viable bacteria was assayed after various times at 37°C. The rate of bacterial killing depended on the concentration of neutrophils and not on the ratio of neutrophils to bacteria. Below a critical concentration of neutrophils, bacteria growth was greater than neutrophil killing of bacteria even when the ratio of neutrophils to bacteria was 100:1. We fitted the time course of bacterial concentration and its dependence on neutrophil concentration with an exponential function, the exponent of which is (−kp + g)t, where k is the second-order rate constant for bacterial killing, p is the neutrophil concentration, g is the first-order rate constant for bacterial growth, and t is time. We found that k ≈ 2 × 10(−8) ml per neutrophil per min, and g ≈ 8 × 10(−3)/min. Only when p is greater than g/k, which we call the critical neutrophil concentration, does the bacterial concentration fall. Under optimal assay conditions, the critical neutrophil concentration was 3–4 × 10(5) per ml, a value very close to that (≤5 × 10(5) per ml) known to predispose humans to bacterial and fungal infections.