Bacteria
Bacteria are the sole members of the Kingdom Monera. They are the most abundant microorganisms. Bacteria occur almost everywhere. Hundreds of bacteria are present in a handful of soil. They also live in extreme habitats such as hot springs, deserts, snow-covered regions, and deep oceans, where very few other life forms can survive. Many of them live in or on other organisms as parasites.
1.0Main characteristics of Bacteria
Shape of Bacteria
● Bacteria are grouped under four categories based on their shape:
● The spherical Coccus (pl.: cocci), e.g. Streptococcus
● The rod-shaped Bacillus (pl.: bacilli), e.g. Escherichia coli
● The comma-shaped Vibrio (pl.: vibrio),e.g. Vibrio cholerae
● The spiral Spirillum (pl.: spirilla), e.g. Spirillum
Motility in Bacteria
● Bacteria are motile and non-motile. Bacterial movement occurs via flagella.
● The number and arrangement of flagella is called flagellation.
● Peritrichous – When flagella are found on the whole body of a bacterium, e.g. E.coli, Salmonella typhi
Pili (Sing. Pilus)
● The bacterial cell wall is covered by numerous hair-like structures called pili.
● Pili are smaller than the flagella.
● They are of two types: (A) Longer pili, (B) Shorter pili.
● Longer pili are also known as 'F' pili or 'sex' pili. Longer pili are found only on the donor (F+ or male) bacteria and facilitate conjugation.
● These are absent in the recipient bacteria or the female.
● The shorter pili take part in attachment to rocks in streams and to the host tissue.
● These are also known as 'infective' pili or Fimbriae. These are found only in pathogenic bacteria.
2.0Internal Structure of Bacteria
● Though the bacterial structure is very simple, they are very complex in behaviour
● Bacterial cell has a chemically complex cell envelope. The cell envelope consists of a tightly bound three layered structure.
● (a) Glycocalyx (b) Cell wall (c) Cell membrane
● Although each layer of the envelope performs distinct function they act together as a single protective unit.
1 . Glycocalyx
● Capsule - Thick, nonsticky, regular and smooth layer.
● Slime layer - a thin, sticky, irregular, and rough layer.
● When a capsule, called a capsule bacteria surrounds bacteria.
● The cell membrane performs formation of a capsule.
● Capsulated bacteria are mostly pathogenic.
● A capsule is made up of polysaccharides and polypeptides.
● When the molecules of polysaccharides are very loosely arranged, they form a thin, sticky layer known as the slime layer.
● The glycocalyx protects bacteria from W.B.C. and also facilitates colony formation.
2. Cell Wall
● The bacterial cell wall is rigid and composed mainly of peptidoglycan, or murein, a type of mucopeptide.
● Gram (+) and Gram (-) bacteria:
● In Gram (+) bacteria, the cell wall is single-layered and thick. It is made up of peptidoglycan. Lipids are also present but in smaller quantities.
● While in Gram (-) bacteria cell wall is double-layered.
● The inner layer is thin and composed of peptidoglycan, while the outer layer is thick and made up of lipopolysaccharide.
● Some lipoproteins and phospholipids are also present in the outer layer.
3. Cell membrane or Plasmalemma or Plasma membrane
● This is the innermost layer.
● The bacterial cell membrane is composed of lipoproteins, as is the eukaryotic membrane.
4. Cytoplasm
● In bacterial cytoplasm, membrane-bound cell organelles, viz. Mitochondria, Chloroplasts, E.R., Lysosomes, Golgi body, Microbodies are absent.
● The bacterial cytoplasm does not exhibit streaming or cyclosis.
5. Cytoplasmic Structures
Mesosomes
● F. James discovered the mesosome. A mesosome is a special membranous structure formed by the extension or infolding, or invaginations of the plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae.
● These are functionally mitochondria-like structures. Oxidative enzymes are found in the mesosome.
Storage granules/Inclusion bodies
● Glycogen granules –They store carbohydrate
● Volutin granules – also known as metachromatic granules or phosphate granules.
Chromatin material (Nucleoid) –
● The nucleus of a bacterial cell is called the Nucleoid, or Genophore or Incipient nucleus or fibrilar nucleus. The nuclear membrane and nucleolus are absent.
● True chromosomes are also absent in bacterial cells. Instead of a true chromosome, they contain a single false chromosome or prochromosome. It consists of a double-stranded circular naked DNA (without histone) + Non-histone proteins. Non-histone proteins are polyamines. The nucleoid is connected to the mesosome.
● Besides this DNA, another small, circular DNA is present in bacterial cells, called a Plasmid. It is also known as extrachromosomal, extranuclear, or extragenomic genetic material. The term 'plasmid' was coined by Lederberg.
3.0Bacterial Nutrition
Bacteria, as a group, exhibit the greatest metabolic diversity. Most of the bacteria are heterotrophic, but some are autotrophic. Based on nutrition, bacteria are classified into the following three categories.
Autotrophs
● These bacteria use light or chemical energy to synthesise their own food.
● Based on their energy source, autotrophs are of the following two types.
1.Photosynthetic autotrophs = Photoautotrophs = Phototrophs
● These bacteria use light energy for food synthesis. In bacterial photosynthesis, the H-donor is not water, so they do not release oxygen; hence, this is called Non-oxygenic photosynthesis, e.g., Rhodospirillum.
2. Chemosynthetic autotrophs = Chemoautotrophs = Chemotrophs
● These are non-photosynthetic autotrophs i.e., photosynthetic pigments are absent. They use chemical energy instead of light energy for food synthesis. These bacteria oxidise some chemical compounds and release energy, which is used for food synthesis. e.g. Nitrifying bacteria, Nitrite bacteria, Nitrate bacteria.
Heterotrophs
● Most bacteria are heterotrophic, i.e., they cannot manufacture their own food. They receive their own food from dead organic matter or living organisms. These are of the following types.
1. Saprotrophic bacteria
● These bacteria obtain food from dead and decaying organic matter. e.g. Bacillus vulgaris, Clostridium botulinum, Pseudomonas
2. Parasitic bacteria
● They obtain their food from living organisms. e.g. Mycobacterium leprae, Mycobacterium tuberculosis.
3. Symbiotic bacteria
● They form symbiotic relationships with other organisms. These bacteria convert atmospheric nitrogen into ammonia and then into nitrogenous compounds such as amino acids, NO3, or salts of ammonia. e.g. Rhizobium
Respiration
● Based on respiration, bacteria are of two types
(1) Aerobic bacteria: They use oxygen in respiration. e.g. Azotobacter
(2) Anaerobic bacteria: They do not use oxygen in respiration. e.g. Clostridium
Reproduction
● Reproduction leads to increase in number of individuals.
● Bacteria reproduce only by asexual reproduction but they also perform a primitive type of gene transfer from one bacterial cell to the other which is called genetic recombination.
4.0Asexual Reproduction
The following methods of asexual reproduction are found in bacteria.
1. Binary fission
This is the most common method of bacterial reproduction. First, DNA replication occurs in a bacterial cell. Under favourable conditions bacterial cell divides into two cells due to the formation of a septum (partition) in the centre of the cell. Each daughter cell grows into a new bacterium. Thus, the bacterial cell divides by amitosis, which is a faster process than mitosis or meiosis.
2. By Endospore
Endospore formation occurs under unfavourable conditions. Only one endospore is formed in a bacterial cell. It is a highly resistant structure. It is resistant to high temperature, radiation, antibiotics and chemicals. Endospores have the highest resistance due to the presence of Ca-dipicolinate in the wall.
Genetic Recombination
Conjugation - Gene transfer by physical contact between two bacterial cells.
Conjugation between F+ and F–
● First, the donor cell attaches to the recipient cell via sex pili. Sex pili help in the formation of the conjugation tube.
● The 'F' factor now replicates, and the replica moves to F– through the conjugation tube.
● Both cells are then separated. Due to the transfer of the 'F' factor F-cell now becomes an F+ cell.
Transformation
● The transfer of genetic material from one bacterium to another through the surrounding medium is called transformation.
Transduction
● When the gene transfer process between two different bacteria is carried out by a virus (bacteriophage) called transduction.
5.0ECONOMIC IMPORTANCE OF BACTERIA
Harmful Activities
- Diseases in Human beings
- Disease in Animals
● Anthrax – Bacillus anthracis
- Disease in plants
● Citrus canker – Xanthomonas citri
● Crown gall in many plants – Agrobacterium tumefaciens
- Food poisoning
●Botulism – Clostridium botulinum – It is the most lethal type of food poisoning. These bacteria survive in the absence of O2. These bacteria multiply in canned food. Their toxins damage the parasympathetic nervous system. It leads to paralysis of both smooth and striped muscles, resulting in immediate death.
Useful Activities
(i) Dairy products –
Dairy products are formed with the help of bacterial fermentation.
(ii) Antibiotics –
● Many antibiotics are derived from bacteria.
● For example, streptomycin is obtained from Streptomyces griseus (It is an actinomycete)
(iii) Pollution indicating bacteria: -
● Water containing E. coli is considered polluted. Water quality depends on the number of E. coli. If E. coli is present in the water it will be highly contaminated. So, E. coli is known as a pollution-indicating bacterium.
Frequently Asked Questions (FAQs)
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