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Interactions of Escherichia coli and Proteus mirabilis with mouse mononuclear phagocytes
C L Wells1, R P Jechorek, R D Nelson
1Department of Laboratory Medicine, University of Minnesota, Minneapolis 55455.
Abstract:
Five strains of enterobacteria (three of Escherichia coli and two of Proteus mirabilis) 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 in vivo. Acridine orange staining showed that there were approximately 10-fold fewer intracellular P. mirabilis than E. coli cells. The average numbers of viable intracellular bacteria per leucocyte were 0.03 and 0.02 for P. mirabilis strains M13 and H1, respectively, and 0.48, 0.45, and 0.28 for E. coli strains M14, A-D M5 and H40. Thus, both P. mirabilis strains were ingested less readily than any of the three E. coli strains (p less than 0.01). The rates of in-vitro intracellular killing were similar for all five strains of bacteria. The intracellular killing constants (Kk) for the three mouse isolates were 0.017, 0.016 and 0.020 min for E. coli M14 and A-D M5, and P. mirabilis M13, respectively; the Kks for the two human isolates were 0.026 and 0.029/min for E. coli H40 and P. mirabilis H1, respectively. The Kks 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 log10 3.9-5.6 bacteria (6-8) x 10(6) peritoneal exudate cells. Intravenous injection of these five strains into separate groups of mice demonstrated that the P. mirabilis strains were more virulent than the E. coli strains. Injection of each P. mirabilis strain was associated with ruffled fur and death, whereas mice given any of the three E. coli 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 P. mirabilis M13 than of E. coli M14 or A-D M5 persisted in these organs; similarly greater numbers of P. mirabilis H1 than of E. coli 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 P. mirabilis appeared to be associated with decreased phagocytic uptake rather than differences in intracellular survival.
Insights
Proteus mirabilis bacteria are ingested less by mouse macrophages than Escherichia coli. Despite similar intracellular killing rates, P. mirabilis showed higher virulence, suggesting reduced phagocytic uptake contributes to its pathogenicity.
Area of Science:
- Immunology
- Microbiology
- Bacterial Pathogenesis
Background:
- Macrophages are key immune cells responsible for phagocytosing and eliminating bacterial pathogens.
- Enterobacteria, including Escherichia coli and Proteus mirabilis, are common causes of infection.
- Understanding bacterial interactions with macrophages is crucial for elucidating mechanisms of virulence and host defense.
Purpose of the Study:
- To compare the phagocytic uptake and intracellular killing of mouse macrophage by different strains of Escherichia coli and Proteus mirabilis.
- To assess the in vivo virulence of these enterobacterial strains in a mouse model.
- To determine the relationship between phagocytic efficiency, intracellular survival, and overall bacterial virulence.
Main Methods:
- Five enterobacterial strains (3 E. coli, 2 P. mirabilis) were studied.
- In vivo phagocytosis was assessed by intraperitoneal injection into mice and subsequent peritoneal exudate cell harvesting.
- Intracellular killing rates were determined in vitro, and bacterial virulence was evaluated by intravenous injection and monitoring survival and organ colonization.
Main Results:
- Proteus mirabilis strains exhibited significantly lower phagocytic uptake by mouse macrophages compared to all tested Escherichia coli strains (p < 0.01).
- In vitro intracellular killing rates were similar across all five bacterial strains.
- In vivo, P. mirabilis strains were more virulent, causing death and higher organ colonization, while E. coli strains did not cause mortality or significant organ burden.
Conclusions:
- The reduced phagocytic uptake of Proteus mirabilis by macrophages, rather than differences in intracellular killing, appears to be a key factor contributing to its higher virulence.
- These findings highlight the importance of bacterial entry into host cells as a critical determinant of pathogenicity.
- The study provides insights into differential host-pathogen interactions relevant to enterobacterial infections.