DNA fingerprinting of plasmid-containing serotype A: 3,4 Pasteurella multocida isolated from cases of fowl cholera in chickens and turkeys

The live, attenuated vaccine strains of Pasteurella multocida have been hyp othesized to be responsible for homologous serotype outbreaks of fowl chole ra on farms that use the commercial vaccines. We have further hypothesized that the naturally occurring Clemson University (CU) vaccine strain may be transformed to virulence by the acquisition of plasmid DNA. To rest this hy pothesis, we obtained seven homologous serotype (A:3,4) P. multocida isolat es, all plasmid bearing, that were cultured from fowl cholera cases in vacc inated flocks and compared the isolates with the CU reference vaccine by mo lecular methods. Restriction fragment length polymorphisms (RFLPs) were det ected by DNA/DNA hybridization with labeled probes specific for the cya, ar oA, and rrn genes of P. multocida. The RFLPs obtained from BglII-digested g enomic DNA probed with cya demonstrated no differences among the isolates. Although three isolates probed with aroA showed a RFLP identical to the vac cine strain, five isolates were distinctly different. Isolates probed with rrn grouped into three different restriction patterns that were dissimilar from that of the vaccine strain. Therefore, we have shown that these fowl c holera isolates are different from the CU vaccine strain and that these out breaks were not vaccine related.