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Effect of type 1 piliation on in vitro killing of Escherichia coli by mouse peritoneal macrophages
B R Keith1, S L Harris, P W Russell
1Department of Microbiology, Pathology and Parasitology, School of Veterinary Medicine, North Carolina State University, Raleigh 27606.
Abstract:
Escherichia coli K-12 mutants possessing defined lesions affecting type 1 pilus production, receptor binding, or length were examined for their ability to resist killing by mouse peritoneal macrophages in vitro. Mutants were mixed pairwise at known ratios in wells containing macrophages, and after incubation, the ratio of the survivors was assayed. The difference in phagocytic killing between type 1 piliated cells and isogenic nonpiliated cells was significant, the piliated cells being approximately threefold more resistant. Pilus length had little effect upon survival, as the long-piliated mutants were no more resistant to killing than the normal-length parents. Interestingly, the receptor-binding function of type 1 pili was most important in effecting resistance, as mutants lacking the ability to bind receptor were killed as effectively as nonpiliated mutants. These data are consistent with the notion that pili actually impede killing by macrophages rather than serve as passive physical barriers to uptake.
Insights
Type 1 pili on Escherichia coli enhance resistance to macrophage killing, with receptor binding being crucial. Pili appear to impede, not block, macrophage engulfment.
Area of Science:
- Microbiology
- Immunology
- Bacterial Pathogenesis
Background:
- Type 1 pili are surface appendages of Escherichia coli.
- These pili play roles in bacterial adhesion and host-pathogen interactions.
- The role of type 1 pili in evading host immune defenses, specifically macrophage phagocytosis, requires further elucidation.
Purpose of the Study:
- To investigate the influence of type 1 pilus production, receptor binding, and length on the resistance of Escherichia coli K-12 to phagocytic killing by mouse peritoneal macrophages.
- To determine whether type 1 pili act as physical barriers or actively impede macrophage-mediated killing.
Main Methods:
- Generation of Escherichia coli K-12 mutants with defined defects in type 1 pilus production, receptor binding, or length.
- In vitro co-incubation assays of pairwise mixed bacterial mutants with mouse peritoneal macrophages.
- Quantification of surviving bacterial populations to determine relative resistance to phagocytosis.
Main Results:
- Type 1 piliated Escherichia coli exhibited approximately threefold greater resistance to macrophage killing compared to isogenic nonpiliated cells.
- Bacterial mutants lacking type 1 pilus receptor-binding function were killed as effectively as nonpiliated mutants.
- Pilus length did not significantly impact bacterial survival rates.
Conclusions:
- The presence of type 1 pili significantly enhances Escherichia coli resistance to macrophage-mediated killing.
- The receptor-binding capability of type 1 pili is critical for conferring this resistance.
- These findings suggest that type 1 pili actively impede macrophage killing mechanisms rather than acting solely as passive barriers to uptake.