Dr Kardos is also at the Department of Medical Microbiology, University of Debrecen, H-4032 Debrecen, Nagyerdei krt, 98, Hungary to 20 ng/μl; 5 μl of this DNA solution was used as a template in the PCRs. ERIC-PCRs were carried out according to Amonsin and others (1997, 2002) in 50 μl volumes. For the BOX-PCRs, the protocol described by van Belkum and Hermans (2003), using the BOX A1R primer, was followed without modification. SERE-PCR was performed according to the method of Rajashekara and others (1998). The reproducibility of the ERIC-PCR was tested in repeated experiments. For PFGE, 4 x 108 bacterial cells were mixed with 2 per cent low-melting-point agarose (BioRad) containing 1 per cent sodium-lauryl-sarcosyl sulphate and 1 mg/ml proteinase K (Sigma). Plugs were incubated overnight in 100 mM EDTA containing 0·2 per cent sodium deoxycholate, 1 per cent sodium-lauryl-sarcosyl sulphate and 1 mg/ml proteinase K. The next day the plugs were washed for 15 minutes with distilled water, then for one hour with Tris-EDTA buffer (100 mM Tris, 250 mM EDTA) containing 0·35 mg/ml phenylmethylsulphonyl fluoride (PMSF, Sigma), and finally three times for one hour with Tris-EDTA buffer. Plugs were stored in Tris-EDTA at 4°C. Before digestion with restriction enzyme, plugs were washed for one hour in the buffer appropriate for the restriction enzyme used. Digestion of the DNA was performed with ApaI and SmaI restriction enzymes using the buffer and conditions recommended by the manufacturer (Promega). Separation of fragments was performed in Dice (Opt 1·00%) (Tol 2·0%-3·0%) (H>0·0% S>0·0%) (0·0%-100·0%)