Discrepancy in the metallo-beta-lactamase phenotypic tests results associated with diversity in the promoter region of class 1 integrons

Abstract number: p933

Castanheira  M., Picao  R.C., Mendes  R.E., Pignatari  A.C.C., Sader   H.S., Gales  A.C.


The expression of metallo-beta-lactamase (MBL) genes was studied in MBL-producing Enterobacter cloacae and Klebsiella pneumoniae isolates due to discordant phenotypic test results for detection of MBL production.


Four E. cloacae and two K. pneumoniae isolates that possess blaVIM and blaIMP (see table below) isolated between 2001 and 2005 were evaluated. All isolates were submitted to MBL phenotypic detection: disk approximation method with a beta-lactam substrate (ceftazidime or imipenem) associated with a MBL inhibitor (EDTA and mercaptopropionic acid) and hydrolysis assay against imipenem. Search for the MBL genes was carried out by PCR followed by DNA sequencing. Class 1 integron promoter regions were also sequenced.


The results are summarized in the table shown below. The disk approximation test detected all isolates as MBL producers. In contrast, only 3 of 6 isolates demonstrated positive hydrolysis results against imipenem. The isolates showing imipenem hydrolysis also had a significant inhibition of imipenem activity when previously treated with 25 mM EDTA. All MBL genes were located in the first position of class 1 integrons and just one promoter, Pant, was present in the MBL carrying integrons. However, the DNA sequence of the promoter region was different among the MBL carrying integrons.


Hydrolysis assays against imipenem and meropenem have been considered an important tool for MBL detection; however, in the current study the disk approximation method demonstrated better performance than the imipenem hydrolysis assay for detection of MBL production. Our findings suggest that differences in the promoter DNA sequence directly influenced the expression of MBL genes, and consequently the proper phenotypic method to detect them. This is the first report to correlate the diversity of the promoter strength with MBL expression in Enterobacteriaceae isolates.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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