The influence of extracellular and phagolysosomal pH changes on the bactericidal activity of bovine neutrophils against Staphylococcus aureus.

The influence of the pH of suspending medium on bovine neutrophil (PMN) function was assessed in tests of phagocytosis and killing of Staphylococcus aureus. Intracellular killing was markedly inhibited by moderate extracellular acidification whereas phagocytosis was little affected, except at the lowest pH level (pH 5.0). The killing of S. aureus by extracts of isolated PMN lysosomal granules showed a similar pH dependence and was optimal at pH levels above neutrality. Survival of S. aureus within PMN from different cows varied significantly and the relative differences in PMN bactericidal efficiency were maintained at all pH levels. The acidification of extracellular medium during incubation which resulted from metabolic activity of the PMN themselves, increased with increasing ratios of bacteria:PMN and varied significantly among cows. Addition of methylamine (10 mM) to elevate phagolysosomal pH inhibited phagocytosis and had no effect on intracellular survival of S. aureus. However, a lower concentration (1.5 mM) did not affect phagocytosis, but reduced bacterial survival without altering the relative differences in efficiency of PMN from different cows. It is suggested that the acidity of the extracellular medium may both reflect and influence the pH changes occurring within PMN phagosomes and, thereby, modulate the efficiency of intracellular destruction of S. aureus.