Evaluation of clonal relatedness of extended-spectrum β-lactamase-producing Proteus mirabilis isolates by quantitative antibiogram and RAPD typing

Objective: To delineate, using two different typing systems, the clonal relatedness of 40 isolates of extended-spectrum β-lactamase (ESβla)-producing Proteus mirabilis obtained over a period of 7 years in six hospitals in the Paris area and two in Pas-de-Calais.Methods: Random amplified polymorphic DNA (RAPD) polymerase chain reaction typing was applied by using three random primers on the ESβla-producing P. mirabilis isolates and on isogenic Escherichia coli strains with or without plasmids encoding the representative resistance pattern transferred from P. mirabilis. Quantitative antibiogram typing, which was also applied to the P. mirabilis isolates, was used to define the euclidean distance between these strains.Results: After having demonstrated that P. mirabilis plasmids did not influence chromosomal DNA amplification, we could classify the ESβla-producing P. mirabilis isolates into 12 groups based on RAPD fingerprints. The same isolates were classified into 19 groups by quantitative antibiogram typing. Despite this difference in group numbers, general concordance between the typing systems was observed. This allowed us to show that the greater number of isolates in some hospitals belonged to a single strain and that single isolates obtained in different hospitals generally represented unique strains.Conclusions: A small number of ESβla-producing P. mirabilis strains was isolated during 7 years in the eight medical centers studied, and the number of different strains identified suggested that inter-hospital transfer had not occurred