Comparison of Etest and combined disk for detection of metallo-b-lactamase-producing isolates

Abstract number: P864

Campana E., Picão R., Nicoletti A., Girardello R., Andrade S., Gales A.C.

Objective: The aim of this study was to comparatively evaluate the ability of E-test MBL strip (AB BIODISK, Solna, Sweden) and combined disk (CD) assay for detection of MBL-producing Pseudomonas aeruginosa (PSA), Acinetobacter spp. (ACB) and enterobacterial clinical isolates.

Methods: A total of 46 genetically unrelated clinical isolates were tested, including 28 P. aeruginosa (10 SPM-1, 14 IMP-type, 3 VIM-type and 1 GIM-1), 1 P. putida (IMP-1), 10 Acinetobacter spp. (9 IMP-1, 1 SIM-1), and 7 enterobacterial strains [Enterobacter cloacae (1 IMP-1 and 3 VIM-1), Klebsiella pneumoniae (2 IMP-1) and Serratia marcescens (1 IMP-1)]. Nineteen imipenem (IMI)-resistant isolates but non-MBL producers were included as negative controls. The isolates were classified as MBL producers by E-test according to the manufacturer's instructions. The CD assay was performed adding 2, 4, 6, 8 and 10 ul of EDTA (100 mM) to IMI disks. The isolates were categorised as MBL producers considering an increase of geqslant R: gt-or-equal, slanted5 mm in inhibition zone of IMI/EDTA disks, compared to the IMI disk alone.

Results: The sensitivity (S) and specificity (E) of E-test and CD assay are shown in the table below. Results of S and E (100% and 33.3%, respectively) for E-test were the same regardless of the MBL type produced. The best E results obtained by Etest and CD were observed for the Enterobacteriaceae group (80% and 100%, respectively). Two positive controls (Acinetobacter spp.) and four negative controls (2 P. aeruginosa and 2 enterobacterial strains) were categorised as non determinable (ND) by E-test. The best S and E results for CD assay were achieved after dropping 6 ul of the EDTA.

Conclusions: Our results indicated that, although the E-test was able to correctly identify almost all MBL-producing isolates, false positive MBL results may occur when using this methology. On the other hand, false negative results may arise when CD test is used to screen for MBL-producing isolates. In summary, these two methods are suitable for preliminary MBL screening for some selected species, but should not be used as the unique indicator of such resistance determinants production.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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