Molecular detection of azithromycin resistance mechanisms in typhoidal salmonellae.

Background: Antibiotic resistance in enteric fever continues to pose major therapeutic challenge with increase in ciprofloxacin resistance. Azithromycin has been used to treat enteric fever as an alternate treatment option without any guidelines for laboratory interpretation. In 2015 issue CLSI has added breakpoints for testing azithromycin susceptibility in typhoidal Salmonellae. We aimed to assess the prevalence of resistance to azithromycin in S. Typhi and S. Paratyphi A in a collection of strains to characterize the mechanisms underlying resistance. Methods & Materials: A total number of 224 S. Typhi and S. Paratyphi A isolates were available in the cryopreservated stock which were isolated from the patients presented with enteric fever during 1990 to 2015. Antimicrobial susceptibility testing was done by disk diffusion method as per CLSI 2015 and MICs were determined by E test method as per manufacturer's guidelines (AB Biodisk, Sweden). For molecular chaeacterization of azithromycin resistance, PCR detection was done to screen for the presence of genes responsible for azithromycin resistance i.e, ereA, ereB, ermB, mefA, mphA, mphB and mphD from plasmid and genomic DNA. Sequence analysis was done to detect mutations in acrR, rlpD and rlpV from genomic DNA. Results: It was observed that 96.11% of the isolates were susceptible to azithromycin using disk diffusion method. There was a linear trend observed with time in azithromycin susceptibility (X² = 5.240, P value <0.02). The MIC 50 and MIC 90 values were 6 and 12μg/ml respectively while the resistance breakpoint is ≥16μg/ml. There was no acquired gene for macrolide resistance in plasmid or genomic DNA of any isolate and DNA sequences of acrR, rlpD and rlpV genes did not show any mutations. Total use of azithromycin in pediatric population was only 13,125 mg given to 16 patients in 2014-15. Conclusion: Azithromycin is a good promising agent against typhoid fever on the basis of MIC distribution in India at present. More studies are required to study the use of azithromycin in complicated infections as at present it is being used only for uncomplicated cases. [ABSTRACT FROM AUTHOR]