Tests of phagocytosis and killing by polymorphonuclear neutrophil leucocytes (PMNL) are usually done with pre-opsonised organisms. Phagocytosis of 11 strains of , pre-opsonised, and in the stationary phase, resulted in the killing of only one strain although all the organisms were phagocytosed. However, when the same strains were added unopsonised to a PMNL-serum mixture, eight were killed after phagocytosis. With two of these strains, the amount of killing was inversely proportional to the time of preopsonisation. incubated for 30 min in dilute peptone water in Hanks's Balanced Salts Solution before phagocytosis also became resistant to killing; bacterial division did not occur during this period. Experiments with bacteria in urine confirmed these findings and showed that exposed to serum or urine before phagocytosis became resistant to killing by PMNL. rapidly changes its sensitivity to phagolysosome killing during transition from stationary to lag phase in a nutrient medium. This resistance is retained through the exponential phase but is lost during the stationary phase. The killing of , , and by PMNL was unaffected by varying the method of opsonisation or the phase of growth. If this phenomenon occurs it may affect the outcome of infections caused by strains of that survive killing by PMNL.


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