Enterotoxigenic (ETEC) strain P2200 of porcine origin possessed eight possibly plasmid-determined characters (K88 Raf HlyColSmTcSuSTa) and six plasmid DNA bands of 4·2–93 kb. Analysis of the spontaneous loss of characters and the results of matings with other strains revealed that the K88, Raf, Hly, Sm, Tc and Su characters could be transferred, and that the presence of the K88 and Raf characters was associated with an 83-kb plasmid. The presence and location of the STaI gene was investigated in several ETEC strains of bovine or porcine origin. Hybridisation with a non-radioactive polynucleotide probe associated the STaI gene with a plasmid in each strain; these plasmids were of 32–142 kb. In contrast, plasmids from a P2200 STa variant and plasmids from two STa variants of the bovine ETEC strain B41* (strain B41 obtained from a different source) did not hybridise with the probe. One of the B41*STa variants had lost the STa plasmid, whereas the second variant retained a plasmid of the same size which did not hybridise. In contrast, a third B41*STa variant retained a plasmid of the same size that still hybridised with the STaI probe. Plasmid DNA restriction fragment analysis, followed by hybridisation with the STaI probe, showed that the STaI gene was associated with 8·3-, 6·8- and 3·5-kb plasmid fragments in strain B41, and with 4·9-, 6·8- and 3·5-kb plasmid fragments in strain B41*, following digestion with RI, HI, or RI + HI, respectively. The STaI probe hybridised also with 12·2-kb RI and 4·6-kb HI fragments of the plasmid from the third B41*STa variant, that unexpectedly had given an initial positive hybridisation result. Different plasmid restriction fragment profiles were seen for strains B41 and B41*, the B41*STa variants and the transconjugant strains, thereby providing further evidence that molecular rearrangements of these plasmids can occur spontaneously.


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