Investigation of falsely reported resistance of Streptococcus equi subsp. zooepidemicus isolates from horses to trimethoprim-sulfamethoxazole.

This research investigates the accuracy of reported resistance in a specific bacterium found in horses to a common antimicrobial drug, finding that flaws in laboratory procedure led to significantly skewed results. The study highlights the importance of rigorous quality control in lab testing and thorough understanding of different test results for accurate interpretation.

Study Objective

• The study aimed to investigate a supposed increase in resistance of a bacteria called Streptococcus equi subsp. zooepidemicus (S. zooepidemicus), taken from the lower respiratory tract of horses, to a common antimicrobial drug called trimethoprim-sulfamethoxazole (SXT).

Methodology

• The researchers did this by comparing recorded SXT susceptibility results of 50 S. zooepidemicus isolates from two distinct time periods (1987-1990 and 1997-2001).
• These bacteria were obtained from the tracheal fluids of horses examined at the Colorado State University Veterinary Teaching Hospital.
• A statistical analysis was then conducted to investigate any significant differences in their resistance to the drug.

Findings

• There was a significant statistical difference between the bacteria’s resistance in the two time periods, with the 1997-2001 samples showing much higher resistance (42%) compared to the 1987-1990 samples (8%).
• However, upon closer inspection of the methodology, the researchers found flaws in the laboratory’s quality control in the second time period. This was leading to falsely high resistance results.

Implications and Conclusions

• This research shows the major impact that even minor deviations from standard lab procedures can have on test results – dramatically skewing our understanding of bacterial resistance to drugs.
• By extension, it emphasises the importance of regular surveillance and monitoring of trends in antimicrobial susceptibility to detect and correct such issues.
• Furthermore, it underscores the need for epidemiologists and data collectors to interact closely with laboratories to ensure accurate analysis and conclusions are drawn from various different types of testing methods.