Five strains of enterobacteria (three of and two of ) were studied to assess and compare their phagocytic uptake and intracellular killing by mouse macrophages. Each strain was injected intraperitoneally into separate groups of mice and peritoneal exudate cells were harvested after 3 min for phagocytosis to occur . Acridine orange staining showed that there were approximately 10-fold fewer intracellular than . The average numbers of viable intracellular bacteria per leucocyte were 0.03 and 0.02 for strains M13 and H1, respectively, and 0.48, 0.45, and 0.28 for strains M14, A-D M5 and H40. Thus, both strains were ingested less readily than any of the three strains (p<0.01). The rates of in-vitro intracellular killing were similar for all five strains of bacteria. The intracellular killing constants (K) for the three mouse isolates were 0.017, 0.016 and 0.020 min for M14 and A-D M5, and M13, respectively; the Ks for the two human isolates were 0.026 and 0.029/min for H40 and H1, respectively. The Ks for all five strains were not significantly different. Assuming that the numbers of viable intracellular bacteria at the beginning of the assay represented 100% viability, 6-17% of the intracellular bacteria remained viable after 2 h, reflecting log 3.9-5.6 bacteria (6-8) x 10 peritoneal exudate cells. Intravenous injection of these five strains into separate groups of mice demonstrated that the strains were more virulent than the strains. Injection of each strain was associated with ruffled fur and death, whereas mice given any of the three strains remained visibly healthy and none died. Consistent with these observations, quantitation of viable bacteria in the liver and spleen showed that greater numbers of M13 than of M14 or A-D M5 persisted in these organs; similarly greater numbers of H1 than of H40 persisted in the liver and spleen. Because the rates of intracellular killing of these five strains were similar, the relative virulence of both strains of appeared to be associated with decreased phagocytic uptake rather than differences in intracellular survival.


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