Figure 1. Sample pulsed-field gel electrophoresis (PFGE) gel of representative Salmonella blockley isolates, indicating common and unique DNA fingerprints. Electrophoresis was through 1% agarose/0.5 x TBE, in a CHEF DRIII apparatus (BioRad Laboratories), at 14° C with a 120° switch angle and a run time of 20 hours, with a linear ramp of switching times from 5 to 32 seconds. Gels were stained with 0.5 mg/L ethidium bromide and documented under UV illumination by the EasyWin32 system (HeroLab, Germany). Images were assessed visually, and different PFGE subtypes (A1, A2, ...) were assigned to isolates with electrophoretic patterns differing by one to three DNA fragments (23). Gel images were also processed by the GelCompar software (Applied Maths, Kortrijk, Belgium), and on the basis of PFGE pattern similarities, a dendrogram was constructed by using the Dice coefficient and clustering by the unweighted pair group method, which uses arithmetic averages (UPGMA) with a 2% tolerance in band position difference.
Isol. no.: isolate number. PFGE: subtypes of PFGE type A. The sizes, in kb, of lambda phage DNA concatamers (New England Biolabs) are shown to the left of the gel. All lanes are from the same gel.