Effect of pH and osmolality on in vitro phagocytosis and killing by neutrophils in urine

Abstract

Phagocytosis and intracellular killing of two strains of Escherichia coli and a Staphylococcus saprophyticus by polymorphonuclear neutrophils (PMN) in pooled sterile urine at three osmolalities (800, 485, and 200 mosM/kg of H2O) between pHs 5 and 8 was investigated. Urine at 800 mosM virtually abolished phagocytosis of both E. coli strains, regardless of pH, and reduced the phagocytosis of S. saprophyticus to 30%; no killing of any organisms took place at this osmolality. On the other hand, phagocytosis was a good in urine as in Hanks balanced salt solution at both 485 and 200 mosM between pHs 6 and 8. Phagocytosis of all three strains was virtually abolished at pH 5. Killing of the strains by PMN was optimal between pHs 6.5 and 7.5 in urine at 485 mosM (being at least 90% of the control values in Hanks balanced salt solution), whereas at 200 mosM killing was reduced to 50 to 70% of these values. Reduced killing of all three strains occurred at pH 8, whereas at pH 6 only S. saprophyticus was killed. Thus, the bactericidal activity of PMN in urine was more sensitive than phagocytic function to alterations in pH. The dominant modulating factor affecting PMN function in urine of 500 mosM or less was pH, but osmolality had a greater influence at 800 mosM. Thus, raising the pH of urine and reducing the osmolality may increase the ability of natural defense mechanisms to eliminate infecting organisms.