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Disc Diffusion Testing of Azithromycin Against Clinical Isolates of Typhoidal Salmonellae: A Diagnostic Conundrum
Muhammad Shoaib1, Luqman Satti1, Ashfaq Hussain1
1Medical Microbiology, Pakistan Navy Ship (PNS) SHIFA Hospital, Bahria University Medical and Dental College, Karachi, PAK.
Introduction:
Typhoid remains a major healthcare problem in low and middle-income countries. The emergence of extremely drug-resistant (XDR) typhoid strains from the Indian subcontinent has led to very limited therapeutic options. Azithromycin being the only oral option for XDR typhoid faces a threat of rapid resistance due to its overuse after the COVID-19 pandemic.
Objective:
To evaluate the reliability of azithromycin disc diffusion testing against clinical isolates of typhoidal salmonellae in comparison with E-test minimum inhibitory concentrations (MICs).
Study Design:
This is a cross-sectional validation study. Place and duration of the study: The Department of Microbiology, Pakistan Navy Ship Shifa hospital, Karachi from June 1 to December 31, 2020.
Methodology:
Antimicrobial susceptibility was performed by Kirby Bauer disc diffusion method for 60 isolates including Salmonella enterica ser. Typhi and Paratyphi A using Clinical Laboratory Standard Institute (CLSI) guidelines. MICs by the E-test method were determined for Azithromycin only.
Results:
A significant proportion of the isolates (55%) had high azithromycin MIC in the wild-type distribution range (8-16 µg/ml). Ten (16.6%) isolates showed false resistance, i.e., zone diameter <13 mm by disc diffusion method when compared to E-test MIC results. Isolates with MICs close to breakpoint, i.e., 16 µg/ml were more likely to show discordant results. The sensitivity, specificity, negative predictive value, positive predictive value, and diagnostic accuracy of the disc diffusion method versus E-test were 100%, 83%, 100%, 9%, and 83%, respectively.
Conclusions:
Disc diffusion method as recommended by CLSI is not reliable for azithromycin susceptibility testing particularly for isolates with high MICs in the susceptible range. The E-test method may be a better alternative to disc diffusion provided appropriate training is done prior to its application.
Insights
The disc diffusion method is unreliable for testing azithromycin resistance in typhoid-causing bacteria, especially with high minimum inhibitory concentrations (MICs). The E-test offers a more accurate alternative for susceptibility testing of these critical pathogens.
Area of Science:
- Medical Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- Typhoid fever poses a significant global health challenge, particularly in low and middle-income countries.
- The rise of extremely drug-resistant (XDR) typhoid strains necessitates reliable susceptibility testing for effective treatment.
- Azithromycin, a key oral antibiotic for XDR typhoid, faces increasing resistance threats due to widespread use.
Purpose of the Study:
- To assess the accuracy of azithromycin disc diffusion testing against clinical isolates of typhoidal Salmonella.
- To compare disc diffusion results with E-test minimum inhibitory concentrations (MICs) for azithromycin susceptibility.
Main Methods:
- A cross-sectional validation study was conducted involving 60 isolates of Salmonella Typhi and Paratyphi A.
- Antimicrobial susceptibility was determined using the Kirby Bauer disc diffusion method according to Clinical Laboratory Standard Institute (CLSI) guidelines.
- E-test was employed to determine the MICs for azithromycin.
Main Results:
- 55% of isolates exhibited high azithromycin MICs within the wild-type distribution range (8-16 µg/ml).
- The disc diffusion method showed a 16.6% false resistance rate compared to E-test MICs.
- Discordant results were more frequent for isolates with MICs near the breakpoint (16 µg/ml), with diagnostic accuracy at 83%.
Conclusions:
- The CLSI-recommended disc diffusion method is unreliable for azithromycin susceptibility testing, particularly for isolates with high MICs.
- The E-test method is a more dependable alternative for azithromycin susceptibility testing, provided adequate training.
- Accurate susceptibility testing is crucial for managing typhoid fever and combating antimicrobial resistance.
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