Phagocytosis and intracellular killing of the contagious equine metritis organism by equine neutrophils in serum.

The research article investigates the ability of horse neutrophils to consume and kill Haemophilus equigenitalis and E.coli bacteria in serums with varying antibody levels. The findings suggest that E.coli is more quickly consumed than H. equigenitalis, but the latter is more rapidly killed once inside the neutrophils.

Research Methods and Findings

• The study began with the combination of equine neutrophils (white blood cells) with Haemophilus equigenitalis (the bacterium that causes contagious equine metritis) or Escherichia coli in serum environments with low and high concentrations of antibodies.
• The researchers’ attention was on the ability of the neutrophils to phagocytose (ingest) and kill these bacteria. They found that more E. coli were consumed than H. equigenitalis during each time period examined.
• In a low-antibody-concentration serum incubation for 120 minutes, 56.3% of E. coli and 34.3% of contagious equine metritis organisms (CEMO) were consumed. In contrast, 74.9% of E. coli and 45% of CEMO were consumed by neutrophils in a high-antibody-titer serum within 120 minutes, suggesting that antibody concentration plays a role in the phagocytosis process.
• Subsequent examination revealed that more than 75% of eaten E. coli and 90% of ingested CEMO were killed within 210 minutes of incubation. Even more curious, fewer E. coli were killed compared to CEMO despite being more readily consumed.

Pathogenesis and Intracellular Survival

• Upon further examination of the neutrophils at the ultrastructural level, the researchers found that CEMO was degraded, particularly in neutrophils in high-antibody-titer serum. Neutrophils are thus capable of destroying H. equigenitalis once inside.
• From the findings, the article suggests that CEMO is an extracellular pathogenic bacterium incapable of prolonged intracellular survival. This is likely due to the neutrophil’s ability to quickly degrade the CEMO upon ingesting it.
• Moreover, it is also suggested that CEMO is slower to be ingested than a nonpathogenic E.coli, which could be due to structural differences between the bacterium or a particular interaction with the equine neutrophils.