Clonal origin of aminoglycoside-resistant Citrobacter freundii isolates in a Danish county

*Department of Clinical Microbiology, Aalborg Hospital, DK-9000 Aalborg, Department of Clinical Microbiology, Aarhus University Hospital, DK-8000 Aarhus C, Danish Veterinary Laboratory, DK-1345 Kbh V, Department of Molecular and Structural Biology, Aarhus University, DK-8000 Aarhus C, ||Department of Gastrointestinal Infections and #Clinical Microbiology, Statens Serum Institut, DK-2300 Copenhagen S, Denmark Corresponding author: Dr N. Nørskov-Lauritsen (e-mail: prof1{at}aas.nja.dk ). Received 14 Aug. 2000; revised version accepted 8 Dec. 2000. Abstract During 1997, attention was drawn to an increased frequency of aminoglycoside-resistant Citrobacter freundii in a Danish county, when a total of 24 resistant C. freundii isolates was detected. In this study, 15 such isolates were typed by pulsed-field gel electrophoresis, riboprinting and partial sequencing of the gene encoding translation initiation factor 2. Fourteen of the 15 isolates were identical, as evaluated by their antibiograms and by all these typing methods. This epidemic strain harboured the aminoglycoside resistance genes aac(3)-II and ant(3'')-I , with the latter located in tandem with a dihydrofolate reductase gene in a class I integron. The source of the strain remains unresolved. Representative isolates were obtained from various specimens from hospitals and general practice throughout the county, with no evidence of patient-to-patient transmission.