A novel class 1 integron containing an aadA aminoglycoside resistance gene cassette resulting from in vivo homologous recombination

Abstract number: 1134_03_38

Cano M.E., Benito I., García-Lobo J.M., Agüero J.

Objective:  

To describe a new variant of the aadA gene encoding resistance to aminoglycosides, as the result of a sequence reassortment between the aadA2 and aadA1a gene cassettes.

Methods:  

A total of 78 clinical isolates of Yersinia enterocolitica collected from different patients in our area between 1994 and 2003, were analysed for the presence of class 1 integrons. Identification and susceptibility testing were performed with the Microscan Walkaway System (Dade Behring). Susceptibility to chloramphenicol, nalidixic acid, tetracycline, kanamycin, streptomycin and spectinomycin was determined by disk-diffusion. Strains were screened by PCR using primers specific for IntI1 gene, and gene cassettes were identified by sequencing the PCR products of the integron variable regions obtained with primers 5’CS and 3’CS.

Results:  

Sixty out of the 78 Y. enterocolitica isolates (77%) showed the presence of integrons, and they could all be grouped into two clusters of 1000 bp (45 isolates) and 1900 bp (15 isolates) according to the two patterns obtained from the amplification of the variable region. The sequencing of the 1000 bp products revealed the presence of a new aadA gene cassette the sequence of which was a mosaic built up from pieces from both aadA2 and aadA1a. All the isolates were resistant to streptomycin, spectinomycin and chloramphenicol, and susceptible to gentamicin, tobramycin, amikacin and kanamycin.

Conclusions:  

The new aadA gene described here enlarges the reported examples of integron containing gene cassettes. Furthermore, nucleotide sequence analysis revealed that new gene variants can be produced in nature by exchange mechanisms probably mediated by homologous recombination.