After electrophoresis, the gel was stained with ethidium bromide 1 lg mL )1 and washed for 10 min. Subsequently the gel image was acquired with a Gel Doc 2000 (Biorad). The amplified products (with a size of approximately 336 bp) were cloned, sequenced, and sequence comparison was performed. Hence, a reference ladder was made by adding an equal amount (about 200 ng of PCR fragments) of the rpoB amplicons and used in the DGGE analysis.