ABSTRACT
Background: Shigella is one of the common genera of pathogens responsible for bacterial diarrhoea in humans. According to World Health Organisation (WHO), 800,000–1,700,000 patients in China were infected with Shigella spp. in 2000, and Shigella flexneri is the most common serotype (86%). Objectives: We investigated the transfer patterns of integron‑associated and antibiotic resistance genes in S. flexneri during different time intervals in the city of Tianjin in the People’s Republic of China. Materials and Methods: The integrase‑encoding and variable regions of the integrons of the bacterial strains were amplified by polymerase chain reaction (PCR), followed by gene sequencing. Fifty‑six S. flexneri strains, 32 of which were stored in our laboratory and the other 24 were isolated from tertiary hospitals in Tianjin during different time intervals, were tested for their sensitivity to 12 antibiotics by using the Kirby–Bauer antibiotic testing method (K‑B method). Results and Conclusion: Of the 32 strains of S. flexneri isolated from 1981 to 1983 and stored in our laboratory, class 1 integron was detected in 28 strains (87.50%), while 27 strains (84.37%) harboured an aminoglycoside resistance gene, aadA, in the variable region of their integrons. Class 1 integron was identified in 22 (91.67%) of the 24 S. flexneri strains isolated from 2009 to 2010, whereas the variable region and 3′‑end amplification were not present in any of the strains. Class 2 integron was not found in the 1981–1983 group (group A) of strains; although 19 (79.17%) of the 24 strains in the 2009–2010 group (group B) possessed class 2 integron, and the variable region of the integron harboured dfrA1 + sat1 + aadA1 genes, which, respectively, mediate antibiotic resistance to trimethoprim, streptothricin and streptomycin. Seventeen strains of the total 56 possessed both class 1 and 2 integrons. Strains belonging to group A were highly resistant to tetracycline, chloramphenicol and a combination of trimethoprim‑sulfamethoxazole; 65.63% of the strains were multi‑resistant to three or more antibiotics. In group B, the strains showed high resistance to ampicillin, trimethoprim‑sulfamethoxazole, piperacillin and tetracycline; 83.33% of the strains were multi‑resistant to three or more antibiotics. Class 1 and 2 integrons exist extensively in S. flexneri, and the 3′‑conserved segments of class 1 integron may have deletion or other types of mutations. Comparing the antibiotic and multi‑drug resistance of group A with that of group B, it is apparent that the antibiotic resistance and the incidence of genes that confer multi‑drug resistance have increased over the years in S. flexneri.
ABSTRACT
Background & objectives: The four species of the genus Shigella, namely, S. dysenteriae, S. flexneri, S. boydii and S. sonnei cause a wide spectrum of illness from watery diarrhoea to severe dysentery. Genomes of these four species show great diversity. In this study, NotI, XbaI or I-CeuI restriction enzyme digested genomes of two Shigella dysenteriae isolates belonging to the serotypes 2 and 7 were extensively analyzed to find their relatedness, if any, with the whole genome sequenced strains of S. dysenteriae type 1 and S. flexneri type 2a. Methods: Pulsed-field gel electrophoresis (PFGE) technique was used to determine the diversity of Shigella genomes by rapid construction of physical maps. DNA end labelling, Southern hybridization and PCR techniques were also applied for mapping purposes. Results: The intron-coded enzyme I-CeuI cuts the bacterial genome specifically at its rrn operon. PFGE of I-CeuI digested S. dysenteriae genomes were found to carry seven rrn operons. However, I-CeuI profiles showed distinct restriction fragment polymorphism (RFLP) between the isolates as well as with the whole genome sequenced isolates. Further studies revealed that the genome sizes and I-CeuI linkage maps of the S. dysenteriae type 7 and type 2 isolates were similar to that of S. dysenteriae type 1 and S. flexneri type 2a genomes, respectively. Interpretation & conclusions: Our findings indicate that the type 7 and type 1 isolates of S. dysenteriae were probably evolved from a same precursor, while the type 2 and S. flexneri type 2a were probably evolved and diversified from a common progenitor.