REPRODUCTION IN BACTERIA

The Bacteria reproduce asexually by fission, endospores, cysts,

fragmentation, sporangiospores and conidia. Sexual reproduction is
represented by transformation, conjugation, transduction and
lysogenic conversions.
ASEXUAL REPRODUCTION
a. Binary fission: Fission involves formation of a septum and
chromosome division (replication of genome). Both the events
occur simultaneously.

b. Endospore: It is an extremely resistant asexual spore (e.g.

Species of Bacillus and Clostridium).

Only one spore is formed in a cell. A septum is laid down around

the concentrated genome. Additional walls are laid down around
the septum. This new cell is formed within the parent cell. The
endospore may be central, lateral or terminal in position.
c. Cyst: These are less resistant than endospores. The whole cell
is transformed into a cyst. Cysts are formed in Azatobacter.

d. Fragmentation, Sporangiospores and Conidia: These

methods are common in Actinomycetes.
SEXUAL REPRODUCTION
 a. Transformation: In this method, DNA is absorbed by the
bacterium from external medium to bring about recombination.

b. Conjugation: Here DNA is transferred from one bacterium to

another by direct contact through conjugation tube. The
recipient female cell (F-) receives the DNA from a donor
bacterium Hfr (High fertility rate male) through cellular contact.

c. Transduction: The DNA is acquired through a virus. Thus, it is

a phage mediated genetic transfer.

TRANSFORMATION
In 1928, Fred Griffith, an English bacteriologist made an important
observation. he was working with the bacterium Pneumococcus
pneumoniae the causal organism of pneumonia. He worked with
both virulent and avirulent strains which differed in morphology and
colony characters. The virulent strain was capsulated and formed a
smooth colony on growth medium. The avirulent strain was non-
capsulated and formed a rough colony.

The virulent strain caused death of mice on injection while the

avirulent strain was harmless. The heat killed virulent strain failed to
kill the mice. However, when the living avirulent strain and the heat
killed virulent strain was mixed and injected, the mice died. Griffith
could isolate virulent bacteria from the dead mice. The avirulent
strain was transformed into virulent strain in association with dead
virulent bacteria, Griffith called the phenomenon,
“TRANSFORMATION”. The heat killed bacteria provided the
transforming principle. He thought (erroneously) that the factor was
the polysaccharide of the capsule. The nucleic acids as bearers of
heredity were not known then.
MECHANISM OF TRANSFORMATION
A fragment of DNA present in the medium gets absorbed on the
recipient cells and then enters the bacterial cell. Inside the cell, it
replaces the homologous part of the bacterial genome. This results in
the development of new characters which pass on to the progeny as
stable heritable characters.
Avirulent  Mice (Mice alive)
Virulent  Mice (Mice Dead)
Heat killed virulent  Mice (Mice alive)
Mixture of Avirulent & Heat killed virulent  Mice (Mice dead - isolation
of virulent strain from body of dead mice)

Avery, McLeod & McCarty identified the transforming principle as

the DNA. This was a big achievement which proved for the first time
that DNA was the genetic material. For pinpointing the transforming
principle, they extracted the DNA, polysaccharides and proteins from
the killed bacterial cells in pure form and added each separately to the
avirulent cultures. But only DNA brought about transformation.

CONJUGATION
Mutation is a sudden heritable change of the genetic material.
Edward Tatum, by inducing mutation with the help of ultraviolet and
X-Ray irradiations, developed strains of E-coli which showed
deficiency for some growth factors. One parental strain (A- B- C+ D+)
requires factors A and B for growth. The other parent (A+ B+ C- D-)
requires factors C and D. Individually, both the strains were unable to
grow on minimal media (has only basic requirements).
But when the two strains were mixed and then spread on the same
minimal medium, colonies appeared and grew in the absence of any
growth factors A, B, C and D. The growth of the colonies suggests
the development of recombinant bacteria which must be A+ B+ C+ D+
genetically.

Note:
A medium which supplies all the minimal nutritional requirements is
called a minimal medium. The organisms that grow on minimal
medium are called prototrophs. A biochemical mutant of prototroph
which fails to grow on the minimal medium is called an auxotroph.
The medium that supplies the deficient growth substance is called an
enriched medium or a complete medium.
MECHANISM OF CONJUGATION
The exact mechanism of the genetic exchange has been worked out.
It is demonstrated that physical contact is established between the
cells and the DNA from one passed into the other cell through a
conjugation tube.
Strains of E-coli showed sexual differences – one acting as donor of
genes (male) and the other as recipient of genes (female). The
maleness is bestowed by a fertility factor called F factor. The fertility
factor is autonomous and lies free in the cytoplasm. It is a circular
DNA molecule.
The F factor is an infective element, now called as episome which can
live with the bacterial chromosome. When inserted in the
chromosome, the F+male becomes Hfr male (high fertility rate). It is
a type of male which shows enormous increase in its frequency of
recombinations. When such a Hfr male conjugates with a female F-
the genetic material is transferred. The female is not converted into a
male as the F factor is not passed on. The genetic material replaces a
portion of the female genome. This brings about genetic
recombination.
The bacterial chromosome breaks at the site of attachment of the F
factor and becomes linear DNA molecule having F factor always at
the rear end. Chromosomal replication starts at the end which is
directed towards the conjugation tube. One of the daughter
chromosomes enters the female cell. The F factor being always at the
rear end never goes to the recipient cell.
The F factor because of its episomal nature can spontaneously
dissociate and go back to the cytoplasm again. The Hfr males are
then reverted to F+males.
 TRANSDUCTION
Lederberg & Zinder in 1951, obtained nutritionally deficient mutants
of Salmonella typhimurium (auxotroph) which failed to grow on the
minimal medium. When a mixture of two mutants were placed
together, recombinants appeared in a few cases. When they analysed
the cause, they discovered a new type of gene exchange which
involved the mediation of bacteriophages and this method is
transduction.
In this experiment, conjugation was ruled out by the following
experiment. A U-tube was taken which had a sintered glass filter
between two arms, through which bacteria could not pass. The two
auxotrophs (nutritionally deficient mutants) were grown in the two
arms (Auxotroph A in one arm and Auxotroph B in the other).
Auxotroph A is a lysogenic strain and Auxotroph B is a lytic strain.
The medium could flow freely from one arm to another.
Recombinants appeared in the bacterial population of one of the arms
only.
THE MECHANISM OF TRANSDUCTION
It was found that the Auxotroph A was a lysogenic strain harbouring a
temperate phage. A small amount of the phage was always present in
the medium as some cells lysed and released phage particles. The
phage particles are small enough to pass through the filter and attack
the Auxotroph B strain in the other arm which are sensitive to the
phage. During the lytic cycle of the Strain B, the viruses accidentally
contained a piece of bacterial DNA. These phages called transducing
phages, when attacked the Strain A behaved as temperate phages
(lysogenic). The DNA of Strain B brought by the virus integrates
with the DNA of Strain A. This integration results in recombination
between the recipient and acquired DNA. The genome of the
recipient is modified.