Griffith has carried out his experiments on mice and guinea pigs that
have been infected with pneumococci (Streptococcus pneumoniae). One of
the strains was strongly virulent causing serious disease of the respiratory
organs, while the other one was avirulent and thus harmless. The virulent
strain cultivated on an agar nutrient medium has been forming typical
brilliant colonies with a smooth surface. The avirulent strain under the same
conditions has been forming turbid colonies with a roughly surface. Except
for this, the two strains were differing in yet another genetically determined
feature — the virulent one had a polysaccharide capsule, while the
avirulent did not form such a capsule.
When the experimental animals were injected with the avirulent strain
they have all remained alive, while when the pneumococci introduced in
them have belonged to the virulent strain the animals were all dead. The
variation in which mixed injections were given containing the native
avirulent strain and killed by heating pneumococci of the virulent one
proved to be more intriguing from a biological point of view. Most
unexpectedly all animals died. The pneumococci isolated from them have
been forming colonies typical of the virulent strain and displaying
polysaccharide capsules. Griffith has suggested that this transforming
factor could have been the polysaccharide itself.
The reason for this phenomenon was later elucidated by the American
microbiologist O. T. Avery and co-workers (1944). It has been established
that DNA extracted from capsulated smooth colonies (S-type) has
transformed the uncapsulated bacterial cells of the R-type (rough) into
capsulated ones with well-expressed virulent capacities. This phenomenon
was given the name of bacterial transformation. A great number of other
researchers have shown that it is characteristic of lots of bacteria.
After two years J. Lederberg and E. Tatum (1946 a, b) discovered a
sexual process in bacterial cells leading to genetic recombinations. This
process was called conjugation. It was studied in detail on Escherishia
coli but was confirmed also on other bacterial strains, which is a proof for
its wide spreading in prokaryotic cells. It is quite possible for this
phenomenon to be a major biological tool for exchange of genetic
information in the independently existing unicellular organisms.
As a process conjugation has become the target of many studies
(Nelson, 1951; Hayes 1952—57; Clowes, Rowley, 1954; Wollman, Jacob,
1955—58 a, b; Jacob, Wollman, 1956, 1958, etc.). The monographs of E.
Wollman and F. Jacob (1959, 1961) are remarkable in that respect. By the
help of electron microscopy the existence of cytoplasmic bridges in
bacterial cells and the exchange of genetic information between them has
been convincingly proved (Fig. 2–12 A, B).
The studies in that direction has disclosed the existence of “sexual”
differences in bacterial cells called F-factors. The cells which donate
chromosomal markers are designated F+
and the cells receiving them — F⁻