Integrons And Antibiotic Resistance Of Shigella: A Correlativity Study

Shigella species are a common pathogen of diarrhoeal disease. With antimicroal drug widely used,bacterial drug resistance is very common and increasingly serious.More and more researches show that lateral gene transfer is the mainly approach to acquire exogenous DNA, which cause the bacterial resistant to drug and it was key to the evolution of bacteria.Integron,a genetic element of gene capture and expression in bacteria,expecially Gram-negative,has been showed to play an important role in acquired antibiotic resistance of bacteria. Integron consists of an integrase gene (intl), a integrase specific recombination site (attl),and gene cassetes with varied number.One gene cassete usually contains a open reading frame encoding antibiotic resistance and a specific recombination site called attC (or 59-base element). Gene cassete could be integrated into an integron with recombination between attl and attC sites by integrase (IntI),or excised from an integron and become a free circle molecule. Lately several investigations reported that class 1 and 2 integron exsists in Shigella strains and their association to antibiotic resistance.Objectives:To analysis the epidemic serotypes of shigella isolates and the change of their antibiotic resistance characteristics in Tianjin,China. To investigate carriage of class 1 and class 2 integrons of shigella isolates,their relationships to antibiotic resistance,and the gene cassettes they earried.Methods:A total of 162 clinical isolates of shigella identified in Tianjin during the years 1981-1983 and 2009-2010 were studied.Antibiotic susceptibility were detected by K-B disk diffusion method.Carriage of class 1 and class 2 integrons was investigated by polymerase chain reaction (PCR) with specific primers and confirmed by restriction endonuclease analysis of amplicons.Results:In the group duing the years 1981-1983, more than 72% of the Shigella isolates were resistant to Tetracycline,Streptomycin,Chloramphenicol,Multidrug resistance rate is 66.67%(22/33),all shigella strains were sensitive to third-generation cephlosporins and furaltadone; In the group duing the years 2009-2010, more than 77% of the Shigella isolates were resistant to Ampicillin,Piperacillin,Gentamycin,Trimethoprim-sulfamethoxazole,Tetracycline,Streptomycin,Multidrug resistance rate is 93.02%(120/129),the resistance rate to third-generation cephlosporins and fluoroquinolone is 3.88-12.40% and 1.55-2.33%.None shigella strains were sensitive to Imipenem.Class 1 integrons were found in 87.88%(29/33) of Shigella isolates in the group duing the years 1981-1983,27 of these integron 1-positive isolates contained gene cassette aadA which confer resistance to streptomycin.Only the Shigella sonnei isolates contained gene cassette dfrA17+aadA5 which confer resistance to trimethoprim and streptomycin.None integron 2-positive isolates were found;In the other group duing the years 2009-2010,Class 1 integrase were found in 79.17%(95/129) of Shigella isolates,however,variable region and 3'conserved segment of class 1 integron were negative in all isolates. Class 2 integrons were present in 62.79%(81/129) of the Shigella isolates. All of these integron 2-positive isolates contained constant gene cassette arrays of dfr Al+satl+aadAl which confer resistance to trimethoprim,streptothricin and streptomycin.Both class 1 and class 2 integrases were present in 62 shigella isolates. The resistance rates to streptomycin and chloramphenicol in the Class 1 integron positive group were significantly higher than that in negative group (P< 0.05); the resistance rates to ampicillin,piperacillin,streptomycin gentamycin and trimethoprim-sulfamethoxazole in the Class 2 integron positive group were significantly higher than that in negative group (P< 0.05).Conclusions:The main serotypes of Shigella spp. were S. sonnei and S. flexneri in Tianjin,China. Antibiotic resistance have been developed in the evolution of Shigella strains;Multidrug resistance rate of Shigella strains was 93.02%. Class 1 integron carriage was very common among the shigella isolates; Class 2 integrons was present only in the group duing the years 2009-2010.An atypical class 1 integron without 3'conserved segment was present in the group duing the years 2009-2010. The antibiotic resistance to Shigella is related to integrons. There is germination in the antibiotic resistance of Class 1 and 2 integron.