Comparison of two selective media for the recovery, isolation, enumeration and differentiation of Rhodococcus equi.

This study explores two selective media methods, NANAT and modified CAZ-NB, for isolating and enumerating the bacteria Rhodococcus equi from soil samples in horse farms. The findings indicate that while both methods had similar recoverability rates, the modified CAZ-NB method resulted in a higher number of virulent R. equi colonies, suggesting it might be more suitable in studies requiring optimal recovery of the bacteria.

Objective of the Research

• The purpose of this research was to compare the effectiveness of two selective media—NANAT and modified CAZ-NB—for isolating and counting the bacteria R. equi from environmental and clinical samples. This bacteria affects horses and is of concern to farmers and horse caregivers. The team performed a side-by-side comparison of the two methods using detailed protocols and examined their performance under similar conditions.

Methodology and Findings

• The researchers used 60 paired soil samples from horse farms for the study. They employed methods like colony blotting and DNA hybridisation for bacterial identification.
• The findings showed that there was no apparent difference between the two media in recovering defined strains of R. equi. Similarly, the proportion of soil cultures positive for R. equi was roughly the same for the two methods.
• The NANAT medium was less inhibitory of bacterial growth from soil culture compared with mCAZ-NB (P = 0.001). Yet, this didn’t result in a significant difference in the number of R. equi colonies recovered.
• Soil cultured on mCAZ-NB medium produced a larger number of virulent R. equi colonies than NANAT (P = 0.03).

Conclusion and Recommendations

• The study concluded that, in terms of the number of virulent R. equi colonies, the mCAZ-NB method was more promising than the NANAT method. In mCAZ-NB, the proportion of R. equi that were virulent (32%) was over three times that observed in NANAT cultures (9%).
• The researchers suggest that modified CAZ-NB appears to be a better selective media for research where high recovery of virulent R. equi is necessary. This includes studies of gastrointestinal carriage of virulent R. equi and of subclinically infected foals.