Bacteria (singular bacterium) are unicellular microorganism which are of microscopic size and cannot be seen with unaided eyes.
Constitute large domain-prokaryotes.
They are among the first life form evolve on the earth and present in most of the habitat.
Bacteria inhabit normal to the extreme habitat like air, water, soil, radioactive waste, hot springs, deep seas, even in human gut, etc.
Also live in symbiotic and parasitic relationship with plants and animals. Example rhizobium associate with leguminous plants.
Length of bacteria ranges in few micrometres. E.coli (1.0-2.0 micrometre long and 0.5 micrometre in radius), Mycobacterium tuberculosis (2-4 micrometre long and 0.2-0.5 micrometre width), V cholerae (1-3 micrometre long and 0.5-0.8 micrometre radius).
The study of this discipline of microbiology is called bacteriology.
Bacteria are beneficial for human and other animals in a way that they produce various kind of vitamins, enzyme and food products. Example- vitamin B12, lactic acid, alcohol.
They are key components of our biosphere playing important role in biogeochemical cycles, removal of toxic substance and decomposition of waste materials.
Involve in nitrogen fixation hence improve soil fertility.
With beneficial characteristics several bacteria are pathogenic and cause various kind of disease in human, plants and animals. Example- cholera, tuberculosis, syphilis, anthrax, and more.
In industries bacteria are useful in waste water treatment and industrial fermentation for cheese and yogurt production.
Structure
Bacteria is a prokaryotic organism their body lacks nucleus and cellular components.
Bacteria are covered by a membrane called cell wall chiefly made up peptidoglycan (murein layer).
Peptidoglycan layer mainly constitute of polysaccharide which are cross linked by peptide bonds.
Peptidoglycan layer is made up of two glucose derivative N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) chain. The chain is linked with tertapeptide bonds
Four protein in tetrapeptide bond are L-alanine, D-alanine, L-lysine or meso-diaminopimelic acid (DPA) and D-glutamine.
Cell wall contributes to the survival of the bacteria, protection from harsh environment and antibiotics.
Peptidoglycan layer in gram positive bacteria is 20-80 nm thick in contrast gran negative bacteria contain 2-7 nm thick layer.
Gram negative bacteria contain acidic substance known as teichoic acids which provide rigidity to the cell wall.
Outer Membrane
Over the cell wall gram negative bacterium contain an external layer called outer membrane it contain lipopolysaccharide, phospholipids, lipoprotein and proteins.
Glycocalyx
It’s the carbohydrate enriched layer that covers the outside of the bacteria.
It provides protection against host
This glycocalyx layer associate with the pathogenic property to the bacteria.
Glycoclayx in a tightly packed form is called as capsule, in contrast in loose packing it is called as slime layer.
Surface Auxiliary
FLAGELLA
FLAGELLA
Hairlike structure, attach on the surface of the cell, main function is to provide mobility to the bacteria
Both gram positive and gram negative bacteria contain flagella. Consist of three parts filament, hook and basal body.
PILLI
PILLI
Thin hairlike structure on the surface of gram negative bacteria
Play an Important role in conjugation process
Fimbrae
Present on both kind of cells gram negative an gran positive
Helps in attachment to the surface
Classification
Shape
Type
Example
Bacillus (Rod-Shaped)
Escherichia coli (E. coli)
Coccus (Sphere)
Streptococcus pneumoniae
Vibrio (Comma Shaped)
Vibrio cholerae
Spirilla or spirochete (Spiral)
Spirillum volutans
Mode of Nutrition
Type
Example
Autotrophic Bacteria
Purple bacteria
Heterotrophic Bacteria
All disease-causing bacteria
Cell Wall
Type
Example
Gram positive
Staphylococcus aureus
Gram negative
Enterobacteriaceae
Mode of Respiration
Types
Example
Aerobic
Psuedomonas aeruginosa
Anaerobic
Clostridium
Reproduction
The mode of replication in bacterium is binary fission.
In this process the parent bacterial cell divided into two identical daughter cells.
Replication of DNA starts in the parent cell and each copy is transfer into the daughter cell.
Rate of reproduction is depend on the conditions like temperature, nutrient availability, moisture this is called favourable condition. E.coli generation rate is 2 million bacteria in 7 hrs.
In some rare cases they undergo sexual reproduction by conjugation, transduction and transformation. Helps in genetic modification in bacteria which results in the antibiotic resistant property.
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