The interaction of Rhodococcus equi and foal neutrophils in vitro.

The research paper investigates the interaction of Rhodococcus equi, a bacterium, with foal neutrophils, part of the immune system, both in healthy and pneumonia-affected foals. The study concludes that the foal neutrophils are effective in killing both the R. equi and Staphylococcus aureus bacteria, indicating that a defect in the neutrophils’ ability to kill bacteria is not likely a contributing factor in foals developing pneumonia.

Methodology and Procedure

• The researchers took polymorphonuclear neutrophil leukocytes (PMNL), a type of white blood cell in the immune system, from eight healthy foals aged between two and fourteen weeks and two foals affected by bacterial pneumonia.
• The PMNL were then isolated from the collected whole blood through a two-step Percoll gradient process, which is a method used to separate different cell types.
• These purified PMNL were then tested for their bactericidal function, or ability to kill bacteria, against two types of bacteria, Rhodococcus equi and Staphylococcus aureus, in the presence of normal horse serum, acting as an external environment.
• The researchers also calculated the percentage uptake, essentially how many of the bacteria the PMNL would take in and attempt to kill, after a 15-minute pre-incubation of the PMNL with the bacteria.

Results

• The study found that the PMNL from the foals could effectively phagocytose (consume) and destroy both R. equi and S. aureus in the presence of normal horse serum.
• On average, 99.3% of R. equi and 99.9% of S. aureus were consumed by the PMNL. Moreover, the majority of these consumed bacteria, 97.8% of R. equi and 98.4% of S. aureus, were destroyed within the 15 minutes of incubation.
• After a longer incubation period of 3 hours, 91.9% of the remaining R. equi were killed, but only 49.2% of the remaining S. aureus, however, the final total bactericidal effect of the foal PMNL was 99.3% against R. equi and 99.9% against S. aureus.
• Neutrophils from the pneumonia-affected foals also showed a greater than 95% uptake and total bactericidal efficacy, which means they took in and killed over 95% of the bacteria.

Conclusion

• An ultrastructural examination was performed on the PMNL after their interaction with the R. equi after the 15-minute incubation. The examination showed that the bacteria were consumed by the neutrophils and that there were morphological changes consistent with neutrophil degranulation, which is the release of antimicrobial molecules to fight the bacteria.
• Based on the evidence gathered and analysed, the researchers concluded that a potential defect in PMNL’s ability to kill bacteria is not likely a contributing factor to the development of R. equi pneumonia in foals. This finding dispels a potential theory about the cause of the pneumonia in foals.