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Nalidixic acid susceptibility test to screen ciprofloxacin resistance in Salmonella typhi
Arti Kapil1, Renuka, Bimal Das
1Department of Microbiology, All India Institute of Medical Sciences, New Delhi, India.
Background & Objectives:
Clinical non response to ciprofloxacin therapy in enteric fever is increasingly being encountered in endemic areas possibly due to the increase in the levels of resistance to ciprofloxacin in Salmonella typhi. The antimicrobial susceptibility tests for S. typhi performed by the disc diffusion method using NCCLS breakpoints fail to detect the increasing MIC of ciprofloxacin, leading to the inappropriate treatment of enteric fever with ciprofloxacin. We explored the possibility of testing S. typhi strains for their susceptibility to nalidixic acid by disc diffusion method as a marker for high MIC to ciprofloxacin.
Methods:
Isolates (94) of S. typhi were tested for in vitro susceptibility to nalidixic acid (30 micrograms) and ciprofloxacin (5 micrograms) by disc diffusion method using NCCLS guidelines. The MIC of these strains to ciprofloxacin was also determined by E-test.
Results:
Of the 94 strains tested, 56 were NARST (nalidixic acid resistant S. typhi) and 34 were NASST (nalidixic acid sensitive S. typhi). MIC of ciprofloxacin in the NASST strains varied from 0.002-0.125 microgram/ml while that for NARST strains varied from 0.023-0.38 microgram/ml, which is about 10-folds higher than that of NASST strains.
Interpretation & Conclusion:
Our study shows that resistance to nalidixic acid is associated with a high MIC to ciprofloxacin in S. typhi. These strains would have been interpreted as ciprofloxacin sensitive by routine antimicrobial susceptibility testing by disc diffusion method. Hence screening of S. typhi isolates by the nalidixic acid susceptibility test may be incorporated in a clinical bacteriology laboratory to alert the treating physicians of the possibility of the failure to ciprofloxacin therapy in patients with enteric fever.
Insights
Nalidixic acid resistance in Salmonella typhi is a marker for high-level ciprofloxacin resistance. This finding helps predict treatment failure in enteric fever, improving patient outcomes.
Area of Science:
- Clinical microbiology
- Infectious diseases
- Antimicrobial resistance
Background:
- Ciprofloxacin treatment failure in enteric fever is rising due to Salmonella typhi resistance.
- Standard disc diffusion tests may not detect elevated ciprofloxacin Minimum Inhibitory Concentrations (MIC).
Purpose of the Study:
- To evaluate nalidixic acid disc diffusion as a marker for high ciprofloxacin MIC in Salmonella typhi.
Main Methods:
- Tested 94 Salmonella typhi isolates for susceptibility to nalidixic acid and ciprofloxacin using disc diffusion.
- Determined ciprofloxacin MIC using E-test.
Main Results:
- 56 isolates were nalidixic acid-resistant (NARST) and 34 were nalidixic acid-sensitive (NASST).
- NARST strains showed approximately 10-fold higher ciprofloxacin MICs (0.023-0.38 µg/ml) compared to NASST strains (0.002-0.125 µg/ml).
Conclusions:
- Nalidixic acid resistance strongly correlates with high ciprofloxacin MIC in Salmonella typhi.
- Nalidixic acid susceptibility testing can identify strains likely to be resistant to ciprofloxacin, aiding clinical decisions.
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