Minimum inhibitory concentration (MIC) of Ceftriaxone and Azithromycin for blood culture isolates of Salmonella enterica spp.

INTRODUCTION

Enteric fever caused by Salmonella enterica continues to be a major public health problem worldwide. In the last decade, ceftriaxone and azithromycin have become the drugs of choice for treating enteric fever caused by Nalidixic acid resistant Salmonella (NARS) enterica. This has led to reports of drug resistance to both drugs. Since enteric fever is endemic in India, accurate drug susceptibility surveillance is crucial to ensure empiric management of enteric fever is appropriate. The aim of this study is to evaluate the minimum inhibitory concentration (MIC) of ceftriaxone and azithromycin for blood culture isolates of NARS isolated at our centre.

METHODOLOGY: This is a retrospective study conducted in a tertiary care center in Mumbai for blood culture isolates of NARS from 2016 to 2018. Isolates were tested for antimicrobial susceptibility testing (AST) against ceftriaxone and azithromycin using a manual broth microdilution method (BMD).

RESULTS

Of 155 blood culture isolates of NARS: S. Typhi (n = 112) and S. Paratyphi A (n = 43) were included in the study. 81.9% (127 / 155) isolates were susceptible, 6.4% (10 / 155) isolates were intermediate while 11.6% (18 / 155) isolates were resistant to ceftriaxone. 100% susceptibility of NARS was observed to azithromycin.

CONCLUSIONS

This study documents an alarming increase in resistance to ceftriaxone among NARS in Mumbai while azithromycin continues to be susceptible in vitro. It is essential to know MICs to understand epidemiological trends and choose appropriate treatment regimens for treating enteric fever.