A method of plasmid classification by integrative incompatibility

A method of plasmid classification by integrative incompatibility has been developed. The characteristics of this system are as follows: (i) The conventional plasmids usually used as standards for incompatibility grouping were integrated into the host chromosome to increase stability and to minimize recombination with the superinfecting plasmid. Strains were constructed by integrative suppression which was in some cases facilitated by the introduction of Tn5 into the plasmid. (ii) The resulting Hfr strains were made deficient in the rec A function to eliminate homologous recombination between the resident and the superinfecting plasmids. A test plasmid is introduced into these rec A Hfr test strains in the stationary phase of growth. In an incompatible cross, the number of transconjugant colonies was usually less than 10^?2 of that in a compatible cross. Occasionally, an inhibitory mechanism, other than incompatibility was coded by the resident plasmid [e.g., restriction in R124 (inc FIV)]. This complicated the interpretation, but did not invalidate the experiment. The colonies arising in incompatible crosses were shown to carry drug resistance determinants coded by both the resident and superinfecting plasmids. These were presumably the result of rec-independent integration of all or part of the superinfecting plasmid into the host chromosome. Thus the reduced frequency of superinfectant formation in an incompatible cross is usually the consequence of incompatibility between the resident and the superinfecting plasmids. This integrative incompatibility system should be useful for epidemiological studies of R plasmids.