PFGE profiles of E. coli clinical isolates hyperproducing chromosomal β-lactamase. Lanes 1, 2, 7, and 8: markers. Lanes 3, 4, 5, 6, 9, 10, 11, and 12: isolates HUS4/94, HUS7/94, HUS23/94, HUS31/94, HUS34/94, HUS36/94, HUS42/94, and HUS47/94, respectively.