Phenotypic and Molecular Characterization of Potentially New Shigella boydii Serotype in Bangladesh

Objective: Shigella is a major cause of bacillary dysentery among children under 5 years of age, especially in low- and middle-income countries. At Dhaka hospital of icddr,b, every year around 3% of Shigella isolates from patients with diarrhea are identified as Shigella-like organisms (SLOs). These isolates do not react with any of the existing and provisional Shigella antisera. The major aims of the present study were to characterize a unique cluster of SLOs using phenotypic and genotypic methods. Methods: A cluster of 6 SLOs was characterized using biochemical tests, serotyping, antimicrobial susceptibility, and virulence assay by using the Sereny test, PCR and compared their pulsed-field gel electrophoresis (PFGE) patterns with known serotypes of Shigella spp. Antisera against the isolate were raised in a rabbit model to determine cross-reactivity with known serotypes. Results: All six SLOs were mannitol fermenting, have invasive properties, positive for ipaH and Shigella enterotoxin 2 gene along with possession of virulence plasmid of ~ 140 MDa. All isolates were resistant to ampicillin, tetracycline, sulfamethoxazole-trimethoprim, but were sensitive to mecillinam, nalidixic acid, and ciprofloxacin. Plasmid and PFGE profiles showed that the genomic patterns of these isolates were identical but were different from all other S. boydii serotypes. Antiserum prepared against one of the isolates agglutinated with all isolates but did not cross-react with other Shigella and E. coli reference strains. Conclusion: This unique cluster of isolates might belong to a new provisional serovar of S. boydii and further analysis is required to incorporate it into the stereotypic scheme of Shigella.