The emergence of cotrimoxazole and quinolone resistance in Shigella sonnei

Background: The emergence of cotrimoxazole resistance has been a dominant and consistent character in our isolates of Shigella sonnei. To study the behaviour of these emerging strains and characterise mechanisms of resistance to cotrimoxazole & ciprofloxacin the following study was performed. Methods & Materials: Isolates of Shigella sonnei confirmed by standard methods from 2012 to 2015 were subjected to antimicrobial susceptibility testing using the Kirby Bauer method as per Clinical laboratory standards institute and PCR for the detection of virulence genes. The degree of relatedness between the isolates was assessed by ERIC PCR followed by gel image analysis. Dendrogram was generated using Pyelph. PCR was carried out to determine the mechanisms of resistance to cotrimoxazole and ciprofloxacin. Results: Of 34 Shigella sonnei isolates, cotrimoxazole resistance was common (94.1%) followed by ciprofloxacin (47%). Majority carried the ipaH gene (97%) followed by ial (17.6%), sen (11.7%), set 1 & set2 (5.8%). No stx element was found. ERIC–PCR analysis of the isolates resulted in four major ERIC groups labelled Eric group I,II,III and IV. Type III was the dominant (44.1%) type. Majority harboured dhfr1 (94.1%), sul2 (85.2%) followed by sul3 (55.8%), sul1 (11.7%). Two isolates that were resistant to cotrimoxazole were negative for the sul genes but harboured the dhfr1 gene. All the phenotypically ciprofloxacin resistant isolates (47%) were positive for presence of gyr A,gyr B, parC and parE. Also, qnrB was the most prevalent PMQR gene (93.7%) while, qnrC was positive in 18.7% of isolates. None were positive for qnrA and qnrS. Two (0.1%) of the isolates were positive for aac(6’)-lb gene. The qepA gene regulating the efflux pump was negative in all the isolates studied. One isolate that was susceptible to all antibiotics tested negative for all the genes. Conclusion: The emergence of Shigella sonnei with a characteristic sulphonamide resistance needs to be addressed further in detail and the increasing trend of resistance to quinolones is a point of concern. This study also shows the emergence of a particular ERIC type in the background of this evolving resistance pattern.