Increasing number of ESBL-producing Escherichia coli during the last three years in Stockholm – A challenge for the clinical laboratory

Abstract number: P615

Iversen A., Andersson M., Vondracek M., Giske C.G.

Objectives: This study aimed at describing the epidemiologic situation of ESBL-producing Escherichia coli (ESBL-EC) in the Stockholm area during 2005 to 2007. The PhenePlate System (PhP) was validated as a screening method used for epidemiological typing of all ESBL-EC isolates and genotypic characterisation of the phylogenetic subgroups of the CTX-M b-lactamase was performed in parallel.

Methods: Clinical isolates of ESBL-EC for the years 2005 (n = 217), 2006 (n = 315) and 2007 (until September, n = 290) were epidemiological typed with the PhP biochemical typing system for E. coli. The global congruence between PhP-typing and pulsed-field gel electrophoretic (PFGE) patterns on XbaI digested DNA was examined for a subset of the isolates (n = 56) by the Comparing Partitions computer software. Unique PhP and PFGE-types were defined as isolates with similarity indices above 0.96 and 0.85 (Dice coefficient), respectively. Index of diversity was determined by Simpson index, while the Wallace coefficient was used to evaluate global agreement between the two typing methods.

Results: A steady increase in the prevalence of ESBL-EC for the years 2005–2007 was observed, while the total number of susceptibility tested isolates between the years was comparable. A tendency towards increased multi-resistance was noted. Using the PhP System 28 major clusters (5 isolates of the same PhP-type) were found and two of these clusters have so far been confirmed with PFGE. In the subset of isolates subjected to parallel evaluation of PhP and PFGE greater discriminatory power was observed for PFGE, although not statistically significant. The Wallace coefficient showed that two strains belonging to the same cluster by PFGE had about 83% chance of having the same PhP-type, while conversely strains belonging to the same PhP-type had 74% chance of having the same PFGE-type.

Conclusion: Efficient epidemiological typing of ESBL isolates within a reasonable time-span is of great importance to clinical laboratories. For this purpose PhP, which is a less labour intensive method than PFGE was validated for epidemiological typing of ESBL-EC isolates. PhP-typing was found to be less discriminatory than PFGE, but can probably be used to exclude similarity by PFGE at the 85% cut-off level. Conversely, PhP-clusters should always be confirmed with PFGE. Genotyping revealed that CTX-M-15 and -14 are the most prevalent ESBL-variants in the Stockholm area.