Antimicrobial activity of tulathromycin and 14 other antimicrobials against virulent Rhodococcus equi in vitro.

The research discusses the performance of the antibiotic tulathromycin, among others, against the bacterium Rhodococcus equi. The antibiotic was found to perform poorly in laboratory conditions, regardless of whether the bacterial strains were resistant or susceptible to a group of antibiotics known as macrolides.

Study Overview

The study focused on demonstrating the ability of tulathromycin to combat Rhodococcus equi under laboratory conditions. The experiment used:

• Ninety-eight virulent isolates of the bacterium R. equi, which is often responsible for severe respiratory infections in horses. Twenty of these were already identified as resistant to macrolides, a group of antibiotics including tulathromycin.
• A custom 96-well antimicrobial susceptibility testing plate was used, with each section receiving different concentrations of antimicrobials.

Methodology and Findings

The researchers cultured the bacterial isolates amidst different concentrations of the antimicrobials. They then used the resulting data to establish the minimum inhibitory concentration (MIC) for each antimicrobial – essentially, the minimum quantity at which the growth of the bacterium was significantly inhibited.

• The results highlighted that tulathromycin was not very effective against R. equi, with the required concentration (MIC) being more than 64 µg/mL for both macrolide-resistant and susceptible strains. This result refers to the MIC50 and MIC90 values, which are common indicators of the amount of the drug needed to inhibit 50% and 90% of the bacterium growth, respectively.
• The study also found that the MIC values for other macrolides in the test were consistent with past published data, meaning their results were not outlying or unexpected.

Implications of Results

This study’s findings suggest that tulathromycin may not be as effective as hoped in dealing with R. equi infections, especially considering the high concentration required for it to inhibit bacterial growth. These results could be crucial to the treatment of equine clinical cases caused by R. equi, allowing for the adjustment of strategies including the selection of a more effective antibiotic. The consistency of the results with past data also offers reassurance about the reliability of their findings and methodology.