Evaluation of direct Etest for antimicrobial susceptibility testing of bacteria isolated from synovial fluid of horses using enrichment bottles.

The paper describes a research study investigating the effectiveness of the Etest for determining the resistance of bacteria, taken from horse synovial fluid, to certain antimicrobials. The Etest was found to be a fairly reliable and quicker method, although less accurate than the standard method for determining minimum inhibitory concentrations (MICs) of those antimicrobials.

Study Background and Goals

• This study aimed to evaluate the efficacy of the Etest, a method used in direct antimicrobial susceptibility testing (AST) to predict the susceptibility or resistance of bacterial isolates from horse synovial fluid.
• The Etest was evaluated for its capacity to determine minimum inhibitory concentrations (MICs) for a range of 13 antimicrobials, in comparison with several other established testing methods including the standard Etest, the agar dilution reference method, and the disk diffusion method.

Methodology

• The researchers used culture-positive broths sampled from horse synovial fluids and incubated in BACTEC enrichment bottles.
• The broths were then inoculated onto agar to determine the MICs of the 13 chosen antimicrobials through the Etest.
• In order to compare the results, the researchers translated MICs into susceptibility categories using clinical breakpoints, which offer a standardized way of determining the resistance or susceptibility of bacteria to different drugs.
• The Etest’s reliability was compared against the other testing methods, based on its predictive accuracy and the consistency of the results obtained.

Results and Conclusion

• The results showed that the Etest prediction had an overall accuracy of 91%, making it more reliable than the disk diffusion method.
• However, it was found to be less accurate than the standard Etest for producing results within 1 log dilution of the reference method.
• For two of the tested antimicrobial agents specifically, trimethoprim and sulfadiazine, the Etest was notably inaccurate.
• The direct Etest method facilitated quicker results in comparison to the standard Etest as it did not require subculturing and preparation of a standardized inoculum. This allowed for faster diagnosis of infections.
• While providing quicker results, for situations where accuracy is prioritized, the standard Etest was recommended over the direct Etest.