Genetic relatedness in the family Enterobacteriaceae.

Five criteria of genetic relatedness are considered. The first, transfer of plasmids between groups, is frequently not a good criterion, because transfer is possible between all genera of the Enterobacteriaceae and also to genera in other families. Though transfer to closely related groups is most frequent, host restriction and the properties of the plasmid may influence the transfer frequency as much as the relatedness of the donor and recipient. The second criterion is interspecies recombination (integration) of chromosomal genes transferred by Hfr strains. Crosses between closely related genera (E. coli and Shigella) gave high frequency of stable hybrids, but crosses between less related genera (E. coli and Salmonella) result in lower recombination, with the donor genes frequently integrated in nonallelic positions on the chromosome, or remaining as autonomous CCC-DNA. In crosses between distantly related genera such as E. coli and Proteus, all the donor DNA remained as CCC, with no detectable integration into the chromosome. Third, the linkage maps of different strains of a species such as E. coli or of closely related species are very similar. The linkage maps of E. coli and S. typhimurium are also similar, with one gene rearrangement, an inversion, distinguishing them. There are some indications of differences in gene order between E. coli and Yersinia and between E. coli and S. marcescens and considerable evidence for rearrangements in gene order between E. coli and P. mirabilis. No similarity between the linkage maps of E. coli and of nonenteric bacteria such as Pseudomonas was observed. Thus within the enteric bacteria there is striking similarity in order of genes between closely related genera, but major changes when less related genera, such as E. coli and P. mirabilis, are observed. are observed...