SUMMARY: Plasmids which determine I pili were subdivided by their compatibility properties. The original examples, R64, R144 and Col Ib-P9, together with others more recently isolated, constituted one group, termed Iα. Another group, Iβ, was compatible with Iα plasmids. The 1β group was represented by some newly observed trimethoprim-resistance factors. A group incompatible with both of these (possibly the ancestral form) was designated Iω. Yet another group, designated Iγ was compatible with all the above plasmids, and with members of all other known compatibility groups. Its exemplar repressed piliation by some, but not all, mutant Iα plasmids derepressed for pilus synthesis. Exclusion was not necessarily correlated with incompatibility.


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