Incompatibility and surface exclusion properties of H1 and H2 plasmids.

Plasmids of the H incompatibility group showed two types of surface exclusion and incompatibility interactions. Strong incompatibility and surface exclusion were evident between plasmids within the same subgroup, and recombination frequently occurred between these plasmids after antibiotic selection for the presence of two plasmids in the same cell. Weaker interactions were seen between plasmids of the different subgroups, H1 and H2, and recombination was not detected. Incompatibility between H1 and H2 plasmids led preferentially to the loss of the H1 plasmid, irrespective of the order of entry of the plasmids. These data are consistent with the hypothesis that incompatibility is negatively controlled.