Transfer patterns of integron-associated and antibiotic resistance genes in S. flexneri during different time...

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%).

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.

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).

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.