FIG. 1.

PFGE resolution of ApaI (A) and AscI (B) restriction fragments of genomic DNA from 12 L. monocytogenes strains (lanes 2 to 7 and 9 to 14). Lanes 1, 8, and 15 of each gel contained XbaI restriction fragments of genomic DNA from an E. coli reference strain (G5244) as molecular size markers. PFGE was performed at 200 V with pulse times ramped from 4 to 40 s over 22 h. The dendrograms showing the relationships between ApaI (C) and AscI (D) profiles for the 12 strains were created using software supplied by Nonlinear Dynamics, Newcastle upon Tyne, United Kingdom. The software uses the neighbor-joining method of Saitou and Nei (17) to compare profiles on the same gel. The numbers on the branches of the dendrograms denote the lengths of the branches. The shorter the distance, the more similar the lanes.