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False resistance of salmonellae to trimethoprim-sulphamethoxazole in disc diffusion method
M V Jesudason1, R Pandian, U Mukundan
1Department of Microbiology, Christian Medical College Hospital, Vellore.
Abstract:
Apparent resistance to trimethoprim-sulphamethoxazole (TMP-SMZ) in disc diffusion method was found in 14 strains of Salmonella typhi and 2 strains of S. paratyphi A grown on Mueller Hinton agar (MHA) of one manufacturer. On MHA from another manufacturer and on diagnostic sensitivity test agar (DSTA), these 16 strains were sensitive to TMP-SMZ. The minimum inhibitory concentration of trimethoprim (TMP) assayed by agar dilution method, on DSTA medium for these 16 strains was in the range of 0.01 to 0.04 micrograms/ml. Thus, the apparent resistance was a spurious result due to the unsatisfactory quality of the batch of MHA of the first manufacturer. Presumably the medium contained thymine or its derivatives which act as antagonists to TMP and sulphonamide drugs.
Insights
False trimethoprim-sulphamethoxazole resistance in Salmonella strains was due to poor quality Mueller Hinton agar. The agar likely contained thymine, interfering with antibiotic testing results.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Resistance
Background:
- Accurate antimicrobial susceptibility testing is crucial for guiding treatment decisions.
- Mueller Hinton agar (MHA) is a standard medium for susceptibility testing.
- Variations in MHA quality can potentially impact test results.
Purpose of the Study:
- To investigate the cause of apparent trimethoprim-sulphamethoxazole (TMP-SMZ) resistance in Salmonella typhi and Salmonella paratyphi A.
- To determine if the observed resistance was a true phenomenon or a result of testing conditions.
Main Methods:
- Disc diffusion method using MHA from two manufacturers and diagnostic sensitivity test agar (DSTA).
- Agar dilution method to determine minimum inhibitory concentrations (MICs) of trimethoprim (TMP).
Main Results:
- 16 Salmonella strains showed apparent TMP-SMZ resistance on MHA from one manufacturer.
- The same 16 strains tested sensitive to TMP-SMZ on MHA from another manufacturer and on DSTA.
- MICs of TMP for these strains were low (0.01-0.04 µg/ml) when tested on DSTA.
Conclusions:
- The observed TMP-SMZ resistance was a spurious result attributed to the poor quality of a specific batch of MHA.
- The MHA batch likely contained thymine or related compounds, acting as antagonists to TMP-SMZ.
- Ensuring the quality and consistency of diagnostic media is essential for reliable antimicrobial susceptibility testing.