Serological classification of conjugative plasmids by indirect haemagglutination

The indirect haemagglutination reaction was evaluated in the classification of conjugative plasmids. A simple and sensitive method was worked out using pili and in some cases whole bacteria as antigens. Antibodies were prepared against pili coded for by plasmids from incompatibility groups IncFI, IncFII, IncI alpha and IncN. The antisera were tested against pili from 35 strains harbouring plasmids. The test differentiated clearly between plasmids from unrelated incompatibility groups, whereas cross-reaction occurred with closely related groups such as FI and FII. Minor antigenic variation could be seen within the IncFII group.