Detection of virulent Rhodococcus equi in tracheal aspirate samples by polymerase chain reaction for rapid diagnosis of R. equi pneumonia in foals.

This research explored the use of polymerase chain reaction (PCR)-based tests to quickly and accurately detect a specific bacteria, Rhodococcus equi, causing pneumonia in young horses. The study found these tests were more time-efficient and specific than traditional culture methods.

Methodology and Purpose

• The study sought to explore a quicker and more specific method of detecting Rhodococcus equi, a harmful bacteria that can cause pneumonia in foals.
• An oligonucleotide primer pair from the sequence of the virulence-associated 15- to 17-kDa antigen gene, specifically associated with virulent R. equi, was used to amplify a 564 bp region through PCR.
• The PCR results were further confirmed using Southern blot hybridization, a method used to detect specific DNA sequence within a DNA sample.
• No positive reaction was found in DNA from 13 other microorganisms typically present in the foal’s respiratory tract, demonstrating the specificity of the PCR assay.

Study Findings

• The PCR assay, conducted on tracheal aspirates seeded with the bacteria, could detect between 10 to 100 bacteria per assay round, amounting to 1000 to 10,000 bacteria per milliliter of the aspirate.
• Meanwhile, a culture and colony blot analysis demonstrated the presence of virulent R. equi in 31 of 42 tracheal aspirates taken from the sick foals.
• Interestingly, of these 31 culture-positive samples, only 12 yielded a positive PCR result, indicating possible false-negatives.

Improving the Method

• To combat the issue of false negatives, researchers refined the methodology by introducing two new methods: nested PCR, which involves two rounds of PCR to increase the sensitivity, and a combination of PCR and enrichment cultures.
• These enrichment cultures allowed for an increase in the number of bacteria to 10,000/ml or more, enhancing the sensitivity of detection.
• These modifications ensured that all of the PCR-negative but culture-positive samples turned out positive, proving that PCR-based assays present a more specific and sensitive means of detecting virulent R. equi in foals’ tracheal aspirates.

Conclusion

• PCR-based assays represent a faster and more exact method for diagnosing R. equi pneumonia in foals compared to standard culture procedures, potentially promising quicker diagnoses and more effective treatments for infections caused by this bacteria in young horses.