The following points highlight the three main processes involved in the genetic recombination of bacteria. The processes are: 1. Conjugation 2. Transformation 3. Transduction.
Process # 1. Conjugation:
Lederberg and Tatum (1946) discovered conjugation in E. coli and its detailed studies were made by Woolman and Jacob (1956). Conjugation, is a process by which genetic material is transferred from one bacterial cell (“donor cell” or “male cell”) to another (“recipient cell” or “female cell”) through a specialized intercellular connection called sex-pilus or conjugation tube.
The maleness and femaleness of bacterial cells are determined by the presence or absence of F-plasmid (also called F-factor or sex factor). F- plasmid, an extra chromosomal genetic material, is always present in the cytoplasm of donor or male cells, and the latter develop specialized cell surface appendages called F-pili or sex-pili under the control of F-plasmid. Recipient or female cells always lack F-pIasmids and, therefore, F-pili are not present on their surface.
F-plasmid or F-factor can exist in two different states:
(i) The autonomous state in which it lies free in the cytoplasm and replicate independent of the bacterial chromosome (DNA); a donor or male cell containing F- factor in autonomous state is called F+ cell, and
(ii) The integrated state in which it is integrated (inserted) into the bacterial chromosome (DNA) and replicate along with it; a donor or male cell containing F-factor in integrated state is called Hfr cell (for high frequency recombination) or high frequency male cell. However, the recipient or female cell lacks F-factor and this is called F– cell.
1. Conjugation between a F+ (donor) cell and a F– (recipient) cell:
In conjugation between a F+ (donor) cell and a F– (recipient) cell, it is the autonomous F-factor (F-plasmid) which is transferred, never the bacterial DNA (Fig. 29.2). When the two cells (F+ and F–) come close to each other, the F-pilus of the F+ (donor) cell attaches with the F– (recipient) cell and acts as a conjugation tube.
Simultaneously, the double-stranded circular F-factor DNA is nicked at a specific point, and begins to replicate producing a single-stranded copy of the F- factor DNA, which migrates through the tube into the cytoplasm of the F– (recipient) cell.
It becomes double- stranded, and circulars and lies free in the cytoplasm thus rendering the recipient cell to become F+ donor cell. In this way, mixing a population of F+ (donor) cells with a population of F+ (recipient) cells results in the conversion of virtually all the cells in the population becoming F+ (donor) cells.
2. Conjugation between Hfr Donor Cells and Recipient (F–) Cell:
The Hfr donor cells are considered to be fertile because, unlike F+ (donor) cells, their chromosomal segments are transferred from donor to recipient cells and the F-factor remains in situ.
When the two cells (Hfr and F–) come in contact, a conjugation tube develops between them. The circular DNA of Hfr donor cell is nicked and replication is initiated. The integrate F-factor always lies at the rear end of the DNA molecule. The replication of DNA starts towards the end near the conjugation tube and the newly synthesized single strand starts migrating through the tube into the recipient (F–) cell.
In nature, the mating of two cells exists for a short period and gets interrupted resulting in the migration of only a portion of the donor DNA into the recipient cell. Since the F-factor lies at the rear end of the molecule, it is rarely transferred to the recipient cell.
The genes of the newly entered DNA fragment may replace the homolgous genes of the DNA of the recipient cell, resulting in a recombinant genetic material. The newly formed recombinant genetic material now possesses those male characters that have been transferred through recombination with the migrated DNA fragment (Fig. 29.3).
3. Conjugation between F’ (F-prime) Male and F– (Recipient) Cell (Sex-Duction):
Existence of Hfr donor cells is not absolute. The F-factor integrated into the bacterial DNA of Hfr donor cells may dissociate and become free in the cytoplasm (Fig. 29.4).
The dissociation may be occasionally anomalous during which the dissociated F-factor may bring with it some genes of the bacterial chromosome. Adelberg and Burns (1958) first identified such a modified F-factor and called it F’ (“F-prime”) factor; the donor cell possessing this factor is called F’ (F-prime) male.
When a F’ male conjugates with F– (recipient) cell, the F’-factor is transferred from donor to the recipient cell, and such a recipient bacterial cell becomes heterozygous (merozygous) for that part of the bacterial chromosome, which the F’- factor had obtained during its anomalous dissociation.
Transfer of F’-factor to recipient cell apparently occurs by the same mechanism as F-factor, transfers during in F+ and F– mating and chromosome transfer in Hfr and F– cell mating. Genetic recombination of this type, mediated by F’-factor, is called sex-duction or F-duction.
Process # 2. Transformation:
This process of genetic recombination was first studied by Griffith (1928), an English bacteriologist. He took two strains of the bacterium Streptococcus pneumoniae (= Pneumococcus pneumoniae), then called Diplococcus pneumoniae. One of the two strains was virulent or pathogenic and capsulated normal; it formed smooth colonies. The other strain was non-pathogenic or avirulent and non-capsulated; it formed rough colonies on the culture medium.
He experimented on mice as summarised below:
It is obvious from Griffith’s experiment that the avirulent or non-pathogenic strain becomes virulent or pathogenic when mixed with heat-killed (dead) virulent strain thus causing the death of mice. Griffith named this change of avirulent into virulent strain as ‘transformation’.
Griffith reasoned that there was a transfer of some factor from heat-killed (dead) virulent strain to the avirulent strain and called it “transforming principle”. Further, he said that the transforming principle was the polysaccharide of capsule of heat-killed virulent strains.
The idea of polysaccharide as transforming principle came to an end in 1944 when Avery, MacLeod and McCarty showed that it is the DNA which works as transforming principle not the polysaccharide of the capsule. They proved for the first time that DNA is the genetic material in organisms.
In transformation (Fig. 29.5) a free (naked) DNA molecule is transferred from a donor to a recipient bacterial cell. The donor bacterium undergoes lysis to free the DNA molecule and the recipient bacterium must be competent to receive it.
This competence of bacterial cell is not a permanent feature; it has been demonstrated in relatively few bacterial genera and depends upon the growth phase of bacteria and the environmental conditions.
When donor DNA comes into contact with the competent bacterial cell, it first binds on the cell surface and then is taken up inside the cell. In some of the cases, it is observed that the double- strand (ds) DNA enters inside the bacterial cell as such and its one strand is degraded by endonuclease enzyme therein leaving single-strand (ss) DNA whereas in others such as some species of Bacillus and Streptococcus it appears that only single-strand (ss) DNA enters the recipient bacterial cell.
An endonuclease enzyme now degrades one of the strands of dsDNA of recipient bacterial chromosome in corresponding region and this gap is filled by the donor ssDNA with the help of ligase enzyme which joins it with the DNA of the recipient bacterial chromosome. If the allelic forms of the donor and recipient genes are not identical, the donor DNA forms a heteroduplex with the recipient bacterial DNA.
When the bacterial cell containing ‘heteroduplex’ undergoes binary fission the heteroduplex replicates forming two ‘homoduplexes’. One of these is a normal duplex which is all recipient in origin and the daughter cell containing it is like the recipient bacterial cell.
The other homoduplex is a transformed duplex (hybrid genome) different from that of either the donor or the recipient bacterial genome. The daughter cell containing transformed duplex is a ‘transformed cell’ and contains some of the characteristics of the donor bacterial cell which are inherited progeny to progeny. 
Process # 3. Transduction:
This process of genetic recombination was discovered by Zinder and Lederberg (1952) in Salmonella typhimurium during their experiments with the objective of discovering whether E. coli type of genetic exchange also existed in S. typhimurium.
In contrast to transformation, wherein free (naked) DNA is transferred, fragments of DNA are transferred from one bacterial cell to the other with the help of a viral carrier (bacteriophage) during transduction i.e., the transduction is a phage-mediated process of genetic material transfer in bacteria.
The bacteriophage acquires a portion of the bacterial DNA of the host cell in which it reproduces and then transfers this acquired DNA to another bacterial cell to which it infects. Such bacteriophage is called ‘transducingphage’. Transduction is of the following two types: generalized (non- specialised) and specialized (restricted).
1. Generalized Transduction:
Transduction, which results in transfer of any bacterial gene from one bacterial cell to the other is referred to as generalized or non-specialized transduction. It is mediated by some virulent phages and certain temperate phages; E. coli phage P1, Salmonella phage P22, and Bacillus subtilis phages PBS1 and SP10 are such phages.
In generalized transduction (Fig. 29.6), some of the developing progeny phages, during their normal lytic- cycle may accidentally acquire pieces of bacterial DNA. Such phages, after the lysis of the host bacterial cell and their release, attach to and inject their DNA into a new recipient cell but fail to re-establish lytic-cycle therein.
Once inside the recipient bacterial cell, the injected DNA may be degraded by nucleases, in which case genetic exchange does not occur. The injected DNA, however, may undergo integration resulting in homologous recombination, as a result, the transduced cell may possess new combination of genes. The transduced bacterial cell now undergoes usual binary fission and produces progeny cells containing new combination of genes.
2. Specialized Transduction:
In contrast to generalized (non-restricted) transduction, which results in transfer of any gene from donor to recipient bacterial cell, specialized (restricted) transduction is that which leads to the transfer of only specific (restricted) genes from donor to recipient cell.
Specialized transduction (Fig.29.8) is mediated by those temperate bacteriophages (e.g., lambda (λ) phage, mu (μ) phage and ɸ80 phage) that usually incorporate (integrate) their DNA into the bacterial chromosome.
The phage-DNA is called ‘prophage’ in its integrated state with the bacterial chromosome; the bacterium having a prophage is said to be lysogenic, and this phage-host-relationship is called lysogeny.
Lysogenic temperate phages spontaneously switch over from lysogenic to lytic state at a low rate (about one in 195 cell divisions) in nature, or they may be induced to do so by irradiation with ultraviolet light. During this transition, the prophage is usually excised precisely from the specific site of integration in its exactly original form.
But occasionally, it may excise imprecisely so that it takes with it that specific portion of bacterial chromosome which lies close to the site of prophage insertion and leaves a portion of its own DNA remaining integrated within the bacterial chromosome.
Such prophage is called ‘specialized transducing principle’ and is packaged into a developing phage particle inside the host bacterial cell. Phage particle so developed is called ‘specialized transducing phage’ and is released after the host bacterial cell undergoes lysis.
Only those specialized transducing phages are viable that contain an amount of greater than 73% and less than 110% of the phage-DNA. When a viable specialized-transducing-phage infects a new bacterial cell, its specialized-transducing principle that already contains specific portion of bacterial chromosome inserts into the recipient bacterial chromosome thus making the latter diploid for that specific bacterial gene (partial diploid or heterogenote or merogenote).
Since the specialized transducing phage is ‘defective’ phage as it has lost some genes during the excision, it functions in recipient bacterial cell only when the latter is already infected by another phage (termed as helper phage) that contains the missing genes and thereby complements the lost phage-functions of the specialized transducing-phage.
The partial diploid contains two copies of the concerned genes, one from donor bacterium and other from recipient bacterium, and is unstable. As a result, bacterial cells containing gene of donor bacterium and those containing gene of recipient bacterium segregate at a frequency of about one in 1,000 cell divisions. 
For example, lambda (λ) phage integrates between the gal genes (required for the utilization for galactose as an energy source) and the bio genes (essential for the synthesis of biotin amino acid) in the E. coli chromosome.
It transduces, therefore, only gal or bio genes thus making the recipient bacterial chromosome diploid for either gal or bio genes. Similarly, phage ɸ80 integrates near the trp genes (required for the synthesis of tryptophan amino acid) and transduces them.