If you ask someone which creature is most important on the earth, we may get different answers, man, plants other animals and so on. One cannot suddenly think of those minute microscopic creatures that are invisible to the naked eye. A wide range of those microscopic organisms usually referred as micro organisms include bacteria, fungi, viruses, algae and protozoans. Though some of them are harmful many of them are useful and very important for survival of life on this planet. Bacteria are a group of microscopic organisms which are present everywhere omnipresent and omnipotent and are considered to be the most abundant microorganisms on earth. Different types of bacteria are included under Archaebacteria, Eubacteria and Cyanobacteria under kingdom Monera of Whittaker’s Five Kingdom classification.
Important persons in microbiology and bacteriology:
* Leewen hoek, Louis Pasteur and Robert Koch – foundation for microbiology.
* Louis Pasteur – developed technique of pasteurization and preparation of vaccine.
* Anton Van Leewenhoek – discovered bacteria and called them animalcules.
* Anton Van Leewenhoek – Father of Microbiology.
* Louis Pasteur – Father of Bacteriology.
* Ehrenberg – coined the term Bacteria.
* Louis Pasteur – showed that bacteria are chemical factories that bring changes in nature.
* Koch – Germ theory of disease.
Bacteria - Key concepts:
* Escherichia coli bacteria are present in human intestine.
* Rhizobium is present in the nodules of roots of leguminous plants.
Morphology of Bacteria:
* The size of bacteria ranges from 2.0 to 5.0µm in length and 0.5 to 1.0 µm in breadth.
* Shapes of bacteria may be
Cocci – spherical
Bacilli – Rod shaped
Spirillum – helical rods (more than one complete twist)
Vibrios – comma shaped (less than one complete twist)
Pleomorphic – bacteria change their shape depending on the type of environment or available nutrients.
Spirochaets – flexible spiral bacteria (slender long an cork-screw shaped)
Filamentous – long thread like or filament like chains
* Bacteria cells may be single or in groups like
Monococcus – single spherical cell
Diplococcus – a pair of spherical cells
Tetra coccus – a group of four spherical cells
Streptococcus – linear chain of spherical cells
Staphylococcus – bunches of spherical cells
Sarcinae – eight spherical cells arranged as a cube
Mono bacillus – single rod shaped cell
Diplo bacillus – a pair of rod shaped cells
Strepto bacillus – chains of bacilli appearing like straws
Bacteria – Cell structure:
* A bacterium shows prokaryotic cell structure. (studied in first year)
* Some bacteria contain flagella which help in movement towards favourable environment and against adverse conditions.
* 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.
* The main genetic material or the bacteria chromosome is called genophore.
* Small circular, double stranded DNA molecules in the bacterial cell are called plasmids.
* Plasmids
- contain few genes
- confer resistance to drugs
- produce toxins and enzymes
- used as vectors in genetic engineering
Nutrition in bacteria:
According to the source of carbon and energy four nutritional groups of bacteria are recognized:
* Saprophytes are free living and feed on organic debris and dead organisms.
e.g.: Bacillus
* Parasites derive nutrients from the living cells or tissues of the host.
e.g.: Xanthomonas, Salmonella (Bio-medically important)
* Microbial purity of Ganges waters is due to Bdellovibrio bacteriovorus which is a parasite on other harmful bacteria.
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.
