Comparison of virulence plasmids among Clostridium perfringens type E isolates

Clostridium perftingens type E isolates produce iota-toxin, which is encoded by iap and ibp genes. Using Southern blot analyses, the current study identified iap/ibp plasmids of similar to 97 or similar to 135 kb among eight type E isolates. For most of these isolates, their iaplibp plasmid also encoded urease and lambda-toxin. However, the beta2-toxin gene, if present, was on a different plasmid from the iaplibp plasmid. For all isolates, the iaplibp plasmid carried a tcp locus, strongly suggesting that these plasmids are conjugative. Overlapping PCR analyses demonstrated some similarity between the iaplibp plasmids and enterotoxin-encoding plasmids of type A isolates. Additional PCR analyses demonstrated that the iaplibp locus is located near dcm sequences, an apparent plasmid hot spot for toxin gene insertion, and that two IS1151-related sequences are present in the iaplibp locus. To begin testing whether those IS1151-like sequences can mobilize iaplibp genes, a PCR assay was performed that amplifies a product only from circular DNA forms that could represent transposition intermediates. This PCR assay detected circular forms containing iaplibp genes and silent enterotoxin gene sequences, with or without an IS1151-like sequence. Collectively, these results suggest that a mobile genetic element carrying iaplibp has inserted onto a tcp-carrying enterotoxin plasmid in a type A isolate, creating a progenitor iaplibp plasmid. That plasmid then spread via conjugation to other isolates, converting them to type E. Further iaplibp plasmid diversity occurred when either the iaplibp genes later remobilized and inserted onto other conjugative plasmids or some iaplibp plasmids acquired additional DNA sequences.