Molecular biology of extended-spectrum beta-lactamase-producing Enterobacteriaceae responsible for digestive tract colonization.

Twenty-nine extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae strains (14 Klebsiella pneumoniae, 10 Escherichia coli and five Citrobacter diversus) isolated from April to July 1996 from faecal carriers in a surgical intensive care unit at the university hospital of Casablanca (Morocco) were studied. Plasmid content and DNA macrorestriction polymorphism determined by pulsed-field gel electrophoresis (PFGE) were used to compare the strains. Restriction profiles of total genomic DNAs cleaved by XbaI and compared by PFGE revealed nine, four and two clones in K. pneumoniae, E. coli and C. diversus, respectively. Plasmid profile analysis of ESBL-producing strains of K. pneumoniae showed that only seven of 14 isolates had a plasmid; four different plasmid profiles were observed. Three different plasmid profiles were observed in E. coli and two in C. diversus. Plasmids responsible for ESBL production could be transferred by conjugation to E. coli K(12) J53-2 from all E. coli isolates and from four of seven K. pneumoniae. No plasmid transfer could be obtained from C. diversus strains. Restriction enzyme digests of plasmids from transconjugants (four transconjugants of K. pneumoniae and five transconjugants of E. coli) showed different patterns. In the surgical intensive care unit where the survey was conducted, the dissemination of ESBLs was due to a mix of strain spread and strain diversity rather than to plasmid dissemination.