Bacteria reproduce mainly by asexual method and also show sexual recombination( True sexual reproduction is absent).
1. Asexual Reproduction : Bacteria produce several types of asexual spores like , sporangiospores, oidia, conidia and endospores. However, the most common mode of asexual reproduction is binary fission.
Under favourable conditions of nutrient availability , moisture and temperature, daughter cells may repeat binary fission many times and may forms a large population. Fortunately, such a rapid rate is seldom achieved. The process gradually slows down and ultimately stops because of :
i) Shortage of space.
ii) Lack of nutrient availability.
iii) Accumulation of waste products (making environmental conditions unfavourable for growth).
iv) Development of bacteriophages, destroying bacteria.
a) Binary Fission : it is the most common method under favourable conditions of temperature. Moisture and availability of nutrients. Mature bacteria cell divides into two daughter cells. In this process the cell division is amitotic type i.e., not involving the spindle formation.
The binary division of a bacterial cell involves mainly 3 steps:
i) Replication of DNA. The bacterial chromosome divides (replicates) resulting in the formation of two ‘circular’ chromosomes. Since at one stage. The replicating chromosome appears like the Greek letter , this mode of replication is called theta model. This mechanism of replication was suggested by Cairn and is also known as Cairn’s
ii) Mesosome division and membrane formation : The parent chromosome is attached to the mesosome. The mesosome begins to divide because of the synthesis of membrane between the DNA-mesosome attachment sites. As a result of the synthesis of cytoplasmic membrane between the mesosomes, each mesosome is pushed to the middle of a daughter cell. Because one chromosome is attached to each mesosome, the two daughter chromosomes get properly partitioned into the daughter cells.
iii) Cross- wall formation : A peripheral ring of plasma membrane invaginates and continues to grow until the two cells are separated. The cell wall materials are also deposited between the membranes completing the division of the cell. The two events, septum and cross wall synthesis, occur simultaneously.
b) Endospores : Cells of certain bacteria, e.g., Bacillus, Clostridium etc. form thick-walled, highly resistant bodies within the cell, called endospores. One bacterial cell normally produces only a single endospore. The endospores may be spherical or oval in shape and are terminal or central in position. Anticoagulant nature of endospore is due to the presence of Ca-dipicolinic acid in cortex layer of wall.
1. Sexual Recombination (Genetic Recombination) : The bacteria exhibit a primitive from of sexual reproduction which differs from eukarotic sexual reproduction because there is no gamete formation and fusion. However, the essential feature of sexual reproduction, i.e., exchange of genetic material does take place and is called genetic recombination.
Three methods are known by which genetic recombination is achieved by bacteria. In the order of their discovery, these are transformation, conjugation and transduction.
a) Transformation : Griffith (1928) worked on the effect of Diplococcus or streptococcus pneumonia bacteria on mice and discovered the process of transformation.
In transformation , the donor and recipient do not come in contact. The donor cell releases a piece of DNA which is actively taken up by the recipient cell from the solution. This ability to pick up DNA from the solution is called competence.
Two strains of D. pneumonia are :
Capsulated or S-III (Virulent strain ) and Non- capsulated or R- II (non – virulent strain).
Four steps were performed in experiment:
- i) S-III bacteria – injected into – Healthy mice – Mice died.
- ii) R-II bacteria – injected into – Healthy mice – Mice survived.
iii) S-III bacteria – injected into – Healthy mice – Mice survived (Heat killed)
- iv) R-II (living) + S- II (heat killed) bacteria – injected into – Healthy mice – Mice died.
(But R-II is not virulent and S-III lost its virulent capacity upon heating).
Griffith concluded that something passed from heat killed S-III to R-II bacteria, so that non virulent strain changed or transformed into virulent bacterial strain.
Avery, MacLeod and McCarty (1948) repeated this experiment using various enzymes and proved that the transformation principle is DNA of heat killed S-III strain. They proved that DNA is a genetic material.
b) Conjugation : Lederberg and Tatum (1946) demonstrated in E. coli that during conjugation, one cell containing F– plasmid acts as donor (F+ or male ) cell and the other lacking F- plasmid as recipient (F– or female) cell. The plasmid contains fertility factor or F gene which produces protrusions termed sex pill. These help the donor F + cell in attaching to the recipient cell. The plasmid replicates and a replica is transferred to recipient cell, changing it into F+. often the plasmid integrates with bacterial chromosome, converting it into Hfr (High frequency of recombination) cell or super male and a part or whole of bacterial chromosome is transferred to recipient cell through conjugation tube. Such association of episome with the endogenote increases the efficiency of genetic transfer. The number of genes transferred depends upon the time for which the two cells remain joined together.
When F– conjugates with super male, the frequency of recombination increases by 1000 times, that is why it is called as Hfr (super male).
c) Transduction :During transduction, a small double stranded piece of DNA is transferred from donor to recipient by a bacteriophage. This mode of genetic recombination in bacteria was first demonstrated by Zinder and Lederberg (1952) while working with Salmonella typhimurium.
Some viruses have the ability to integrate their DNA with bacterial DNA, which is replicated at the same time as the host DNA and is passed from one bacterial generation to the next. Such bacteria carrying phage (viral) DNA with their won DNA are called lysogenic bacteria. Occasionally, the phage DNA becomes active and codes for the production of new virus particles. A number of phage particles are synthesized followed by the destruction of the host cell and release of phage particles. Upon release, the phage particles attack sensitive bacterial cells, multiply and release more phage particles. However, sometimes faulty detachment of phage DNA from bacterial DNA results in the incorporation of a small amount of bacterial DNA into the phage DNA. Subsequent infection of another bacterium with this aberrant phage called transducing phage, introduces the piece of foreign bacterial DNA into the recipient’s chromosomes, producing a genetic change.
Types of transduction : the ability of the bacteriophage to carry the genetic material from any region of bacterial DNA is called generalized transduction, e.g., T4-phage. On the other hand, there are bacteriophages such as lambda phage () of E. coli which can carry only a specific region of the bacterial DNA to a recipient. This is called specialized transduction (or restricted transduction). Sometimes , the DNA brought by the phage does not integrate with the genome of the recipient bacterium and is lost after one or two generations. Such a transduction is called abortive transduction.