Characterization of defective beta-lactamase genes in Yersinia enterocolitica.

OBJECTIVES

To study at the molecular level the heterogeneity of expression of the two chromosomal beta-lactamases, BlaA and BlaB, in Yersinia enterocolitica strains isolated from clinical samples.

METHODS

MIC determination by the agar dilution method and beta-lactamase assays was performed to determine the resistance level conferred by these enzymes. DNA cloning, PCR and direct sequencing were used to detect the presence of mutations.

RESULTS

The blaA allele from strain IP97 (blaA97) was found to carry a deletion of 51 bp which entirely abolished its beta-lactamase activity. Both the ampR gene and the promoter region of strain Y56 were shown to be functional by a gene swapping experiment. The blaB allele from strain Y56 was found to carry two point mutations, only one of them resulting in a change in the amino acid sequence of the protein. This single amino acid change created a practically inactive BlaB or AmpC cephalosporinase in Y. enterocolitica Y56.

CONCLUSIONS

The lack of activity observed in the beta-lactamases of some Y. enterocolitica isolates was due to the presence of point mutations or small deletions in the corresponding genes.