Molecular serogrouping of serologically atypical shigella isolates from South India

Abstract

Background: Shigellae are the causative agents of bacillary dysentery, represents a significant public health problem worldwide especially in developing countries. New serotypes or subserotypes in Shigella are not uncommon and are reported from different parts of the world. Notably, atypical Shigella exhibit greater antibiotic resistance than typical Shigella serotypes and also found to harbour integrons which has the ability to acquire resistance genes. The purpose of this work was to amplify rfb regions (contains the genes coding for the enzymes responsible for O-antigen synthesis) of non-agglutinating Shigella isolates to characterize these by endonuclease restriction and to study the presence of antimicrobial resistance genes (AMR). Methods & Materials: A total of 3647 faeces specimens were processed between January to December 2014, among these, 27.5% (n = 176) were identified as Shigella spp. Eight non-agglutinable Shigella isolates obtained were included in the study for molecular characterization. These strains were tested for their antimicrobial susceptibility against six antibiotics by Kirby Bauer disc diffusion method. All the isolates were screened for the AMR genes (dhfr1A, sulII, bla-OXA, bla-TEM, bla-CTX-M, AmpC and qnrA,B,S) by PCR. The isolates were also amplified for their O-antigen gene cluster using Expand long template PCR system and the amplicons were further restricted using MboII enzyme. Results: The prevalence of shigellosis during the study period was 4.8%. The antimicrobial resistance for the atypical Shigella were 50% for ampicillin, co-trimoxazole (87%), nalidixic acid (37%), cefixime (12.5%) and all were susceptible to norfloxacin and cefotaxime. The AMR gene PCR results showed 25% of dhfr1A (n = 2), 62.5% sulII (n = 5), 50% bla-TEM (n = 4), 50% qnrS (n = 4) and all the isolates were negative for bla-OXA, bla-CTX-M and AmpC genes. rfb-RFLP results showed clearly identifiable and reproducible pattern, six different patterns were obtained. Conclusion: This study revealed the description of O-specific patterns allowing serotype identification without the use of antisera. Although the number of atypical Shigella strains in this study was only eight, thorough and strict monitoring of isolation of such atypical strains would help to understand the actual disease burden caused by the new Shigella serovars and consequently to study the epidemiology of shigellosis.