Molecular characteristics of class 1 and class 2 integrons and their relationships to antibiotic resistance in clinical isolates of Shigella sonnei and Shigella flexneri.
OBJECTIVE
To analyse the gene cassettes and determine the roles of class 1 and class 2 integrons in antibiotic-resistant strains of Shigella sonnei (n=31) and Shigella flexneri (n=33).


METHODS
Various molecular techniques, including PCR and Southern-blotting analysis, were used to analyse various markers of class 1 and class 2 integrons in these 64 S. sonnei and S. flexneri isolates collected in Hangzhou, China. The gene cassette arrays in integrons were identified by DNA sequencing and/or restriction fragment length polymorphism. Two genomic DNA fragments, one containing intI1 from a S. flexneri isolate that contains intI1 but lacks 3'-conserved region and another containing intI2 from a S. sonnei isolate, were cloned into pUC19 vectors and sequenced. The links between integron gene cassette arrays and antibiotic resistance were analysed.


RESULTS
Class 2 integrons were present in 80.6% (25/31) of the S. sonnei isolates and 87.9% (29/33) of the S. flexneri isolates. All of these integron 2-positive isolates contained constant gene cassette arrays of dfrA1+sat1+aadA1 which confer resistance to trimethoprim and streptomycin. It was demonstrated that the class 2 integron was located in the Tn7 region inside the attTn7 locus downstream of glmS in Shigella. Class 1 integrons were found in 9.4% (6/64) of Shigella spp. isolates. An atypical class 1 integron without a 3'-conserved segment on the Shigella chromosome, termed Shigella atypical class 1 integron (SAI), was present in 84.9% (28/33) of S. flexneri isolates. The SAI contained two gene cassettes, bla(OXA30) and aadA1; however, the SAI conferred resistance to ampicillin, but not to streptomycin, in Escherichia coli host. The bla(OXA30) and aadA1 cassettes of the SAI seemed to be always coordinately excised or integrated.


CONCLUSIONS
Multiple and complex mechanisms involving mobile genetic elements in class 1 and class 2 integrons and antibiotic resistance have been developed in the evolution of Shigella strains.
Pan
Jing-Cao
JC
Microbiology Laboratory, Hangzhou Center for Disease Control and Prevention Hangzhou 310006, People's Republic of China. jingcaopan@sina.com
Ye
Rong
R
Meng
Dong-Mei
DM
Zhang
Wei
W
Wang
Hao-Qiu
HQ
Liu
Ke-Zhou
KZ
D000668:Ampicillin Resistance; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001483:Base Sequence; D015139:Blotting, Southern; D002681:China; D003001:Cloning, Molecular; D004251:DNA Transposable Elements / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D004405:Dysentery, Bacillary / Q000382:microbiology*; D005822:Genetic Vectors; D006801:Humans; D032023:Integrons / Q000235:genetics*; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D012150:Polymorphism, Restriction Fragment Length; D017422:Sequence Analysis, DNA; D012763:Shigella flexneri / Q000187:drug effects / Q000235:genetics* / Q000302:isolation & purification; D012764:Shigella sonnei / Q000187:drug effects / Q000235:genetics* / Q000302:isolation & purification; D013307:Streptomycin / Q000494:pharmacology; D014296:Trimethoprim Resistance
D000900:Anti-Bacterial Agents; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D013307:Streptomycin