A pseudo-outbreak of a Citrobacter with an extended-spectrum b-lactamase in a haemato-oncology ward

Abstract number: 1733_60

Jansen L., Bakkers J., Wulf M.

Objective: To investigate a sudden increase of Citrobacter species, which were reported to have an extended spectrum b-lactamase (ESBL) on a haemato-oncology ward.

Design: A case-series of 16 patients colonised with a Citrobacter ESBL between January and December 2005 were investigated. Strains were typed by pulsed field gel electrophoresis. On the 22 strains identified by the automated Phoenix system as a possible ESBL, a disk diffusion test with cefpodoxim, ceftazidim, cefotaxim combined with clavulanic acid was performed and ESBL Etest strips were used to confirm the ESBL phenotype. In addition molecular detection of TEM, SHV, GES and CTX-M was done.

Results: Of two 22 strains the Phoenix System indicated the resistance pattern as `possible ESBL', 9 strains had a phenotype on disk diffusion and E-tests consistent with this result. Of these 9 strains 4 (44%) could be confirmed by PCR as ESBL positive: 2 strains with SHV, 2 with TEM-26. 3 strains with PCR positive did had a negative disk-diffusion test: 2 strains from one patient carried a GES, one a CTX-M.

Conclusions: The increase of reported Citrobacter ESBL was partly due to a change in laboratory practice where all strains indicated by the Phoenix to have a resistance pattern indicative of ESBL were reported as such. The `outbreak' was not due to a single strain and different genes coding for ESBL were found. 4 of 9 (44%) strains with a susceptibility pattern indicative of ESBL could be confirmed by molecular techniques, whereas 3 strains were found to contain an ESBL that did not have a typical resistance pattern. Detection of ESBL in Gram-negative bacteria other than E. coli and Klebsiella species remains difficult and results should be interpreted with caution.