Description:
<p>My data and other reports indicate that R factors are widespread in clinically significant gram-negative bacteria and are now widespread all over the world. The biological properties of R factor pose important biological as well as medical problems. R factors are transferable by conjugation, they render a wide range of gram-negative bacteria multiple-resistant, and they replicate autonomously and more rapidly than host chromosomes. The R factors are also a useful tool to study genetic and biological properties of episomes, because the factors can be easily traced by their resistance markers. Although much has been learned concerning the genetic properties of R factor and the kinetics of R transfer, many problems still remain to be solved, especially the origin of R factor. R factor consists of 2 species of DNA, the base composition of which is 58% and 52% G+C, respectively [61]; the 58% component of the R factor comprises about 80% of the episomic DNA, the other 20% being 52% in G+C content. These data are in agreement with the thought herein presented that R factor consists of 2 components: the r determinant and T factor. The R factor does not appear to be integrated into the chromosome. However, the r determinants such as tet, chl, and (str, sul, chl ) may be integrated into the chromosome under appropriate conditions of Escherichia, Shigella, and Salmonella species, and it is possible that this part of the R factor (perhaps the 52% G+C component) may have evolved in this grouping of bacteria. On the other hand, the major portion of the R factor, the 58% G+C component, may be the T factor itself, which is different from the majority of F factor, which is 50% G+C. The 58% GEC component of R factor also suggests that the origin of this component should be ascribed to species which share this DNA base composition. R factor has increasingly acquired resistance to such drugs as AB-PC and KM. This fact suggests that the origin of R factor should be ascribed to the recombination of 2 components: the r determinant and the episomic element. Several types of penicillinase in substrate profiles and different types of KM inactivation, that is, acetylation and phosphorylation, indicate that rapid increases in R-factor resistance and rapid changes in resistance determinants of R factor may tae places as a consequence of gene evolution.</p>