The molecular sizes of Fime resistance plasmids from strains of Salmonella typhimurium, S. wien and S. typhi were within the range 87.9-102.6 x 106 daltons. DNA reassociation studies indicated that the plasmids from these hosts had at least 80% of their nucleotide sequences in common. A high proportion of Fame plasmids cannot mediate their own transfer.
The non-autotransferring property of such plasmids is the result of DNA deletion; a non-autotransferring Fame plasmid was about 10 x 10 6 daltons shorter than autotransferring representatives of the group, and its DNA showed 100% homology with them. Plasmids of the Fame group are incompatible with the F factor and with Fa R factors. Flme plasmids are incompatible with the fi+ MP10 plasmid of S. typhimurium, whereas F and Fa factors are compatible with MP10 (Anderson et al., 1977). There was no significant DNA homology between members of the Fime group and MP10, and these plasmids may share only a small region of DNA responsible for their incompatibility.
The Fame R factors examined had 29-37% DNA homology with the F factor, and 50-58% homology with the Fa resistance plasmid, R162. Molecular examination therefore supports the genetic differentiation of members of the Fame group from other F-like plasmids. Both types of investigation can thus be used in epidemiological studies of bacterial strains carrying resistance or other plasmids.