Disc Diffusion Testing of Azithromycin Against Clinical Isolates of Typhoidal Salmonellae: A Diagnostic Conundrum.

Muhammad Shoaib,Luqman Satti,Ashfaq Hussain,Nazia Khursheed,Saba Sarwar,Abid H Shah

doi:10.7759/cureus.16777

Abstract

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.

Highlights

Typhoid remains a major healthcare problem in low and middle-income countries
Ten isolates had discordance between disc diffusion and minimum inhibitory concentrations (MICs) susceptibility results with major errors, i.e., false resistance by disc diffusion method compared to MIC value
Azithromycin resistance in isolates of S. typhi is increasing and it may present a threat to clinicians especially in typhoid endemic countries where XDR S. typhi is endemic

Summary

Introduction

Typhoid remains a major healthcare problem in low and middle-income countries. Azithromycin being the only oral option for XDR typhoid faces a threat of rapid resistance due to its overuse after the COVID-19 pandemic. Enteric fever is caused by typhoidal salmonellae, i.e., Salmonella typhi and Salmonella paratyphi A, B, and C [1]. Typhoid fever is a cause of significant morbidity and mortality in low and middle-income countries with poor quality of potable water and sanitation [2]. The disease is transmitted by contaminated food and water. It is a worldwide problem with endemic areas spread from South America, sub-Saharan Africa, the Middle East to South Asia and the Far East. The global incidence of the disease is estimated to be more than 11 million cases and 128,000 deaths annually [3]

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