Southern blot analysis was performed to verify a specific single integration into the genome. Genomic DNA was extracted using a Phenol-chloroform extraction [42] . Four mg of genomic DNA was digested with EcoRV enzyme and separated on a 0.8% agarose/0.5xTris-acetate-EDTA gel by electrophoresis. DNA was transferred onto a Hybond N+ filter (Amersham) in 10x saline sodium citrate (SSC) solution. The filter was washed in 2X SSC and baked at 80uC for 2 hours. Prehybridization of the filter was done for 2 hrs at 60uC in 5x SCC, 5x Denhart and 100 mg/ml of yeast tRNA. Probes specific for the group II intron (EBS2-tcdC623as/Sal-R1), ermB gene (oWKS1131/oWKS1132) and tcdC gene (tcdC5-tcdC6) were generated. Primers are listed in Table 2 . The generated probes (100 ng) were radiolabeled ( 32 P dATP) using Klenow enzyme (Roche) and overnight hybridized in 10 ml fresh pre-hybridization buffer at 60uC. The filter was washed for 30 min in 2x SCC, 0.5% SDS, 30 min in 1X SSC, 0.5% SDS and 30 min in 0.5X SSC, 0.5% SDS and analyzed using phosphorimage screen and a Typhoon 9410 scanner (GE healthcare).