Evaluation of phenotypic tests for the detection of metallo-beta-lactamase-producing Pseudomonas aeruginosa in a low prevalence country.
OBJECTIVE
To evaluate four phenotypic tests for the detection of metallo-beta-lactamase (MBL) production in Pseudomonas aeruginosa in a low MBL prevalence setting.


METHODS
Sixty clinical isolates of P. aeruginosa resistant to imipenem and/or meropenem and seven MBL-positive control strains were examined by: (i) MBL Etest; (ii) combined imipenem discs supplemented with EDTA (IPM-EDTA); (iii) beta-lactam discs on dipicolinic acid plates (DF-DIPI); and (iv) the Cica-beta test. Spectrophotometric analysis of crude cell extracts for imipenem hydrolysis along with consensus PCRs for bla(VIM) and bla(IMP) was used as reference methods.


RESULTS
Two clinical isolates (3%) were MBL-positive. The MBL Etest and IPM-EDTA test scored positive for all MBL-positive isolates, but showed specificities of 86% and 91%, and positive predictive values (PPVs) of only 20% and 29%, respectively. Adding resistance to ceftazidime (MIC >8 mg/L) as a criterion for MBL testing would reduce the number of isolates to be screened by 50% and increase the PPVs of the MBL Etest and IMP-EDTA test to 29% and 40%, respectively. The Cica-beta test correctly identified all MBL-negative isolates, but misidentified one MBL-positive clinical isolate as an extended-spectrum beta-lactamase (ESBL)-producer and one as inconclusive (producing multiple beta-lactamases). No reliable breakpoints could be defined for the DF-DIPI test due to overlapping inhibition zone diameters for MBL-positive and -negative isolates.


CONCLUSIONS
None of the phenotypic tests were optimal due to low sensitivity or specificity, resulting in low PPVs. Including ceftazidime resistance to the MBL-screening criteria would significantly improve the performance of the MBL Etest and IPM-EDTA disc test.
Samuelsen
Orjan
O
Reference Centre for Detection of Antimicrobial Resistance, Department of Microbiology and Infection Control, University Hospital of North Norway, Tromsø, Norway.
Buarø
Liselotte
L
Giske
Christian G
CG
Simonsen
Gunnar S
GS
Aasnaes
Bettina
B
Sundsfjord
Arnfinn
A
D001426:Bacterial Proteins / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D006801:Humans; D015378:Imipenem / Q000378:metabolism; D008826:Microbial Sensitivity Tests / Q000379:methods*; D016133:Polymerase Chain Reaction; D011237:Predictive Value of Tests; D011552:Pseudomonas Infections / Q000382:microbiology*; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000201:enzymology* / Q000302:isolation & purification; D012680:Sensitivity and Specificity; D013053:Spectrophotometry; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000235:genetics / Q000378:metabolism*
D001426:Bacterial Proteins; D004269:DNA, Bacterial; D015378:Imipenem; D001618:beta-Lactamases