Journal of Laboratory Physicians
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ORIGINAL ARTICLE
Year : 2019  |  Volume : 11  |  Issue : 1  |  Page : 39-44

Reconsidering azithromycin disc diffusion interpretive criteria for Salmonellae in view of azithromycin MIC creep among typhoidal and nontyphoidal salmonella


1 Department of Microbiology, Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, Kochi, Kerala, India
2 Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, Kochi, Kerala, India
3 Department of General Medicine, Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, Kochi, Kerala, India

Correspondence Address:
Dr. Sadia Khan
Department of Microbiology, Amrita School of Medicine, Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, Kochi - 682 041, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JLP.JLP_99_18

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PURPOSE: Enteric fever continues to be an important public health challenge for the developing world. With the emergence of fluoroquinolone resistance in Salmonellae spp. azithromycin is increasingly being used for oral treatment of enteric fever. We investigated the antibiotic susceptibility pattern of azithromycin in Salmonellae spp. isolates from a tertiary care hospital to detect emerging resistance. METHODS: The study assessed the reliability of disc diffusion as a screening test to detect azithromycin resistance by comparing it with the minimum inhibitory concentrations (MICs) of the drug in 100 Salmonellae spp. strains. The strains of Salmonellae spp. showing resistance to azithromycin were further investigated for resistance markers – mphA, mphB, and mef B genes. RESULTS: This study was conducted on 100 Salmonella enterica strains recovered from blood culture samples between 2013 and 2017. Among these isolates, 18 showed resistance to azithromycin by disc diffusion methodology with zones of inhibition <13 mm. MIC of 6 of these isolates were ≥32 mg/L. The mean MIC of azithromycin increased from 5 mg/L in 2013 to 24 mg/L in 2017. Azithromycin consumption as defined daily doses per 1000 patient days also showed an increase over the past 4 years. CONCLUSION: Azithromycin disc diffusion diameter interpretations as recommended by Clinical and Laboratory Standards Institute can mislabel a few sensitive strains as resistant. Azithromycin resistance is emerging in typhoidal and nontyphoidal Salmonella. MphA gene is associated with high MICs in nontyphoidal Salmonella spp.


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