Reporting of Azithromycin Activity against Clinical Isolates of Extensively Drug-Resistant Salmonella enterica Serovar Typhi

This study aimed to evaluate the minimum inhibitory concentration (MIC) of azithromycin (AZM) in clinical isolates of extensively drug-resistant (XDR) Salmonella Typhi (i.e., resistant to chloramphenicol, ampicillin, trimethoprim-sulfamethoxazole, fluoroquinolones, and third-generation cephalosporin) using the E-test versus the broth microdilution method (BMD). From January to June 2021, a retrospective cross-sectional study was carried out in Lahore, Pakistan. Antimicrobial susceptibility was performed initially by the Kirby-Bauer disk diffusion method for 150 XDR Salmonella enterica serovar Typhi isolates, and MICs of all the recommended antibiotics were determined by the VITEK 2 (BioM?erieux) fully automated system using Clinical Laboratory Standard Institute (CLSI) 2021 guidelines. The E-test method was used to determine AZM MICs. These MICs were compared with the BMD, which is the method recom-mended by the CLSI but not adopted in routine laboratory reporting. Of 150 isolates, 10 (6.6%) were resistant by disk diffusion. Eight (5.3%) of these had high MICs against AZM by the E-test. Only three isolates (2%) were resis-tant by E-test, having an MIC of 32 mg/mL. All eight isolates had a high MIC by BMD with different MIC distributions, but only one was resistant, having an MIC of 32 mg/mL by BMD. The sensitivity, specificity, negative predictive value, positive predictive value, and diagnostic accuracy of the E-test method versus BMD were 98.65%,100%, 99.3%, 33.3%, and 98.6%, respectively. Similarly, the concordance rate was 98.6%, negative percent agreement was 100%, and positive percent agreement was 33%. The BMD is the most reliable approach for reporting AZM sensitivity in XDR S. Typhi compared with the E-test and disk diffusion methods. Potentially, AZM resistance in XDR S. Typhi is around the corner. Sensitivity patterns should be reported with MIC values, and if possible, higher values should be screened for the presence of any potential resistance genes. Antibiotic stewardship should be strictly implemented.

Automated summary

This study aimed to evaluate the minimum inhibitory concentration (MIC) of azithromycin (AZM) in clinical isolates of extensively drug-resistant (XDR) Salmonella Typhi using the E-test versus the broth microdilution method. The study was conducted in Lahore, Pakistan and included 150 XDR Salmonella Typhi isolates. The results showed a high concordance between the E-test and broth microdilution method for determining the MIC of azithromycin.