Molecular Characterization of Antimicrobial Resistance in Clinical Shigella Isolates During 2014 and 2015: Trends in South India

Background: Shigella species are an important cause of acute diarrheal disease worldwide. This study describes the prevalence of Shigella spp. serotypes and their resistance profile in Vellore, South India from 2014 to 2015. Methods: From 2014 to 2015, 338 Shigella strains were isolated from stool samples at Christian Medical College, Vellore, India. Identification and serotyping was carried out using standard protocols. Antimicrobial susceptibility testing was done against commonly used antibiotics. Multidrug resistance was detected in 157 isolates. A subset of 73 isolates was randomly characterized further for acquired antimicrobial resistance genes in this study. Results: The resistance profile of the study isolates varied by species and year. S. sonnei isolates were 100% resistant to all tested antibiotics in 2014, whereas in 2015, resistance was found for AMP-NAL-TAX-SXT-FIX. The resistance phenotypes among S. flexneri isolates for the year 2014 and 2015 were AMP-SXT-NAL-NOR-FIX-TAX and AMP-NAL-SXT-TAX-NOR-FIX respectively. Screening for antimicrobial resistance genes in S. flexneri found dhfr1A, sulII, bla_OXA, bla_TEM, bla_CTX-M-1, qnrB, qnrS and AmpC genes while S. sonnei were found to have only dhfr1A, sulII, bla_CTX-M-1 and qnrS genes respectively. Antimicrobial resistance genes were predominantly seen in AMP-SXT-NAL and AMP-SXT-NAL-NOR resistance phenotypes. Conclusion: Shigella prevalence of 4.8% to 4.6% was documented between the years 2014 to 2015 in this study. We show evidence that resistance to commonly used antibiotics continues to increase among Shigella spp. in South India. The presence of qnrS and bla_CTX-M-15 in the study isolates further indicates the threat of spreading resistance to quinolones and third-generation cephalosporins.