Bacteria – Cell structure:
1) A bacterium shows prokaryotic cell structure. (studied in first year)
2) Some bacteria contain flagella which help in movement towards favourable environment and against adverse conditions.
3) Depending on the number and arrangement of flagella bacteria may be:
Monotrichous – single polar flagellum
Amphitrichous – single flagellum at each end
Lophotrichous – two or more flagella at one pole
Peritrichous – flagella distributed all over the cell.
4) The main genetic material or the bacteria chromosome is called genophore.
5) Small circular, double stranded DNA molecules in the bacterial cell are
called plasmids.
6) Plasmids
– contain few genes
– confer resistance to drugs
– produce toxins and enzymes
– used as vectors in genetic engineering
Reproduction in Bacteria:
Bacteria reproduce frequently by binary fission resulting in two genetically similar daughter cells. The rate of multiplication is very high in bacteria as binary fission is said to occur once in every 20 minutes.
Sexual reproduction:
As there is no true sexual reproduction bacteria reproduce by exchange of genetic material also called genetic recombination in the following three ways:
1. Conjugation
2. Transformation
3. Transduction
1. Conjugation:
It is the direct transfer of DNA between the living donor and the
living recipient cells.
It was first observed by Lederberg and Tatum in Escherichia coli.
Two bacterial cells designated as F+ and F- come into contact and a
conjugation tube or sex pili connect the two cells.
The F+ cell contains F plasmid and F- cell lacks F plasmid.
The pilus or conjugation tube shortens and the F+ cell replicates its F plasmid DNA.
The copy of replicated DNA passes through the bridge formed by the
pilus into the recipient cell or F-cell and the conjugation is said to have completed.
This is a conservative process where the donor cell retains the
complete copy of the DNA transferred.
2. Transformation:
It is the uptake of naked DNA fragments from the surrounding
environment and expression of that genetic information in the
recipient cell.
It was discovered by Frederick Griffith in Streptococcus pneumoniae.
3. Transduction:
It is the transfer of genetic material from one bacterium to another
through a bacteriophage virus.
It was discovered by Lederberg and Zinder in Salmonella
typhimurium.
Importance of Bacteria to Humans:
Bacteria are called “friends and foes of man” as they are both beneficial and sometimes causing harm to human beings.
Human diseases caused by bacteria:
Tetanus – Clostridium tetani
Cholera – Vibrio cholera
Typhoid – Salmonella typhi
Diphtheria – Corynebacterium diphtheria
Tuberculosis – Mycobacterium tuberculosis
Pneumonia – Diplococcus pneumoniae
Leprosy – Mycobacterium leprae
Gonorrhoea – Neisseria gonorrhoea
Syphilis – Treponema pallidum
Clostridium botulinum caused spoilage of food called Botulism.
Plant diseases caused by bacteria:
Blight of rice – Xanthomonas oryzae
Citrus canker – Xanthomonas axonopodis pv. citri.
Crown gall of apple and pear – Agrobacterium tumefaciens
Importance of microbes in modern science:
Microbes are presently used in bio-mining for extraction of metals like uranium.
Bacterial DNA components are used as Biosensors to detect toxic pollutants. They are also used in medical diagnostics, food and fermentation operations.
Bacteria are the real tools in genetic engineering.