. DNA-DNA hybridisation was used to examine 89 isolates of serogroup O138 for toxin gene content and location. Sixty-six isolates encoded toxin genes; 29 were STpa STpb VT2, 24 were VT2, four were STpa STpb, three were STpb VT2, three were STpb VT2 LT and one was STpa. None were K88-, K99- or F41-positive. The VT toxin gene was chromosomally located in all VT isolates tested whereas STpa, STpb and LT were plasmid borne. In co-transfer experiments with pRP4 as the mobilising plasmid, VT2 genes were not transferred to recipient K12 whereas STpa, STpb and LT genes were. Two VT2-positive isolates inhibited pRP4 transfer to K12 by a factor of 10 and rare transconjugants harboured a cointegrate plasmid comprising pRP4 (54kb) and a cryptic host plasmid (145 kb). In a K12 background, the co-integrate had transfer and incompatibility properties of pRP4 whereas in both progenitor O138 backgrounds the co-integrate was transfer inhibited and in one case resolved to give original plasmids.


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