Detection of extended spectrum beta-lactamases in the routine clinical microbiology laboratory.
OBJECTIVE
To compare three methods of confirming the presence of an extended spectrum beta-lactamase (ESBL) enzyme with the initial detection (i.e., screening) by the Vitek AMS.


METHODS
Gram-negative bacteria which flagged as ESBL-positive in the Vitek GNS card, or were suspected of harbouring an enzyme, were further tested by each of the following methods: (a) combination disc test using cefpodoxime, ceftazidime and cefotaxime with and without clavulanate; (b) cefotaxime ESBL Etest; and (c) Jarlier keyhole method with cefpodoxime (10 microg), cefotaxime (5 microg) and aztreonam (30 microg) placed 15mm away from an augmentin (30 microg) disc.


RESULTS
A total of 52 isolates were investigated, representing an 18-month time period. Fifty of these were positive by Vitek. Twenty-eight (56%) were confirmed by other methods (true positives). Of the 44% Vitek-positive/confirmatory test-negative (false positives), eight were Escherichia coli which was 53% of all E. coli tested. The majority of other false-positive isolates were Klebsiella oxytoca (24% overall) which were all Vitek- and Etest-positive but negative by the combination disc test.


CONCLUSIONS
All ESBL-positive strains by Vitek should be confirmed by the combination disc test using all three antibiotics. This will enable differentiation of 'true' ESBLs from false-positive organisms, including K1 hyperbetalactamase-producing Klebsiella oxytoca and AmpC-producing organisms. The cefpodoxime combination discs gave the best differentiation in this study with only one ESBL organism being missed.
Microbiology Unit, Southern Cross Pathology, Monash Medical Centre, Clayton, Victoria, Australia. p.midolo@southernhealth.org.au
P D
PD
Midolo
D
D
Matthews
C D
CD
Fernandez
T G
TG
Kerr
D000328:Adult; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001398:Aztreonam / Q000494:pharmacology; D002511:Cephalosporins / Q000494:pharmacology; D002648:Child; D024881:Drug Resistance, Bacterial / Q000502:physiology*; D005189:False Positive Reactions; D006090:Gram-Negative Bacteria / Q000201:enzymology / Q000502:physiology*; D006801:Humans; D008828:Microbiological Techniques; D001618:beta-Lactamases / Q000032:analysis* / Q000502:physiology*
D000900:Anti-Bacterial Agents; D002511:Cephalosporins; D001618:beta-Lactamases; D001398:Aztreonam