Separation by CHEF gel electrophoresis of chromosomal restriction fragments and hybridization with the blaA-specific probe. (A) Chromosomal DNA digested with NotI was separated in 1% agarose gels run for 15 h at 6 V/cm and 14°C. The initial switch time was 10 s, and the final switch time was 50 s. Lane 1, bacteriophage lambda ladder concatemers; lanes 2, hyperresistant isolate Y100; lanes 3, strain Y56. (B) Chromosomal XbaI and SfiI fragments were separated in 1% agarose gels at 6 V/cm by using two consecutive sets of pulse conditions. In block 1, the initial switch time was 5 s and the final switch time was 10 s for 6 h. In block two, the initial switch time was 1 s and the final switch time was 6 s for 12 h. Lanes 1, unamplified strain Y56 digested with SfiI; lanes 2, strain Y56 digested with XbaI; lanes 3, hyperresistant isolate Y100 digested with SfiI; lanes 4; Y100 digested with XbaI; lanes 5 and 6, bacteriophage lambda ladder concatemers and pulse marker, respectively.