The contact between the F⁺ and F⁻ cells is accomplished by F-pilli. If the
F-factor is integrated in the bacterial chromosome (it can also be found in a
separate structure as is the case with the plasmids) and has a high
frequency of recombination, then the strains are designated with the
symbols Hfr (high frequency of recombination).
image a
image b
Figure 2–12. Electron micrographs of conjugation in cells of Escherichia coli (After Anderson et al., 1957; Wollman and Jacob, 1959).
(A) Conjugation between K-12 Hfr H anf GF⁻. The K-12 Hfr bacterium is
elongated and in the process of division, while the G F⁻ bacterium is
thick and rounded. The two bacteria are connected by a cytoplasmic
bridge. In the contact area the cell walls seem obliterated. The K-12 Hfr
bacterium is flagellated and is therefore mobile, while the G F⁻
bacterium has no flagella and is not mobile; (B) Conjugation between
bacteria Hfr F⁻λʳ and F⁻λˢ. The two bacteria are close to each other. In
the contact area the walls seem obliterated. The F⁻ bacterium is in a
process of division and is covered by a number of bacteriophages λ,
which are fixed by the help of their tails.
The transfer of genetic factors (markers) takes place under certain
conditions and for a definite period of time. The method of starting and