Strains of Salmonella typhimurium of predominantly Middle Eastern origin, but distributed from England to India, were found to carry at least three types of resistance plasmid. The most important was initially identified as an FI plasmid by compatibility tests, but differs from the F factor on the one hand and the FI factors R162 and CoIV on the other. The three groups of FI plasmids can be distinguished by their compatibility reactions with the MP10 plasmid of S. typhimurium (Smith, Humphreys, Grindley, Grindley & Anderson, 1973) and group H1 factors: the F factor is unilaterally incompatible with group H1 (Smith, Grindley, Humphreys & Anderson, 1973; Anderson, 1975b); the FI factors are compatible with MP10 and group H1 and FIme factors are incompatible with MP10 but compatible with H1. The majority of S. typhimurium cultures belonged to phage type 208; most of those that did not, belonged to types related to 208. Only a minority of their FIme plasmids were autotransferring. The remainder were mobilizable by F-like plasmids, and by group H1 and H2 factors, but not by the fi–I1 facter δ, or by plasmids of the I2, B, P, W, N and com 7 groups. The compatibility reactions of the autotransferring F1me plasmids were identical with those of the non-transferring members of the group, and both were large, single-copy plasmids.
The S. typhimurium strains of this series carried A or AK, and SSu resistance determinants: small, probably multicopy, non-transferring plasmids similar to those originally described in phage type 29 of S. typhimurium (Anderson & Lewis, 1965b).
These S. typhimurium cultures probably represent a clone of wide geographical distribution. The accurate epidemiological study of such clonal outbreaks requires, in addition to phage typing, precise identification of the plasmids harboured by the epidemic strains, and may have to be carried to the molecular level.
FIme plasmids were identified in other drug-resistant salmonellas, notably in a strain of S. wien which caused large outbreaks of mainly paediatric infection in Algeria, and also spread to Britain. An FIme plasmid was found in S. typhi phage type 44 from Algeria, in which the phage-restricting properties of the plasmid are responsible for the specificity of the type.