Assessment of Antimicrobial Susceptibility of Virulent Strains of Rhodococcus equi Isolated From Foals and Soil of Horse Breeding Farms With and Without Endemic Infections.

This research paper investigates the growing resistance of Rhodococcus equi, a bacterium causing serious pneumonia in foals, to certain antibiotics, and the effectiveness of 24 selected antibiotics against this bacterium. The researchers tested these antibiotics on samples collected from both deceased foals and breeding farms soil with and without endemic R. equi infections.

Research Objective and Method

• The goal of this research was to determine how the bacterium Rhodococcus equi reacts to 24 selected antibiotics. R. equi is known to cause pneumonia in foals and to live in the soil of horse breeding farms.
• The study focused on samples of the bacterium isolated from both deceased foals and environmental samples from horse breeding farms where R. equi was an endemic infection.
• The researchers used minimum inhibitory concentration (MIC) gradient strips to establish the smallest amount of each antibiotic that could effectively inhibit the growth of R. equi.

Results Analysis

• The results were analyzed by establishing an epidemiologic cutoff value (ECOFF) for each antibiotic based on normal MIC distribution. ECOFF is the boundary that distinguishes bacteria that are inherently susceptible from those that carry resistance mechanisms.
• The ECOFF results were compared to data generated for Staphylococcus aureus provided by the European Committee on Antimicrobial Susceptibility Testing.
• According to the findings, the highest level of bacterial inhibition was achieved with four antibiotics – clarithromycin, rifampicin, imipenem, and vancomycin.
• In relation to most of the antibiotics tested, the majority of the R. equi strains could be categorized as wild type, meaning they did not show any resistance to the antibiotics tested. However, an exception was found in the case of SXT (Trimethoprim/sulfamethoxazole), where 35% of the R. equi strains could be categorized as Non-Wild Type (NWT), indicating they are not as susceptible to the antibiotic.

Research Significance

• The lack of standard criteria for interpreting R. equi’s reactions to antibiotics is currently a significant challenge in both clinical and scientific assessment of antibiotic sensitivity. This study helps to address that challenge by establishing the effectiveness of a range of antibiotics against R. equi.
• However, the discovery of a Non-Wild Type response to SXT suggests that strains of R. equi may be developing a resistance to this antibiotic, which could have implications for future treatment methods.