Bacteria are unicellular, microscopic prokaryotes that are placed in the kingdom Monera
CLASSIFICATION OF BACTERIA
Bacteria is divided into two sub-categories:
- Archaebacteria are the most ancient living organisms on Earth.
- They are microscopic organisms with diameters ranging from 0.5–1.0 micrometer.
- They lack peptidoglycan in their cell walls.
- Their cell wall is composed of glycoproteins and polysaccharides.
- They consist of a cell membrane inner to the cell wall, which is made of ether-linked phospholipids
TYPES OF ARCHEABACTERIA
There are three basic types of Archeabacteria:
- Methanogens usually exist in marshy areas
- They convert formic acid and carbon dioxide into methane by the aid of hydrogen.
- They are exploited commercially in the production of methane and fuel gas e.g., Methanobacterium, Methcinococcus.
- Some of the methanogen Archeabacteria occur as symbionts (e.g., Methanobacterium)
- These Archeabacteria are helpful to the ruminants in fermentation of cellulose.
- Halophiles are named so because they usually occur in salt rich habitats like salt pans, salt beds and salt marshes e.g., Halo bacterium.
- They are aerobic chemo-heterotrophs.
- Their cell membranes have red carotenoid pigment for protection against harmful solar radiations.
- Under anaerobic conditions, they use ATP synthesized by membrane pigment system from solar radiations.
- Halophiles can survive under high salt conditions
- A Thermoacidophiles is an extremophilic bacteria that is both thermophilic and acidophilic
- It can survive under conditions of high temperature and low ph.
- Thermoacidophiles occur in hot springs or in environments of geothermal activity.
- They are unicellular, prokaryotic microscopic organisms.
- The cell wall of Eubacteria is made up of Peptidoglycan (Murien)
- Their cell membrane contains glycol-lipids.
- Chromosome in eubacteria is circular and nucleosomes maybe present
- Eubacteria can be classified in three ways such as;
- On the basis of nutrition
- On the basis of structure
- On the basis of staining behavior
ON BASIS OF MODE OF NUTRITION
On the basis of nutrition, eubacteria are classified into two groups
- Autotrophic bacteria
- Heterotrophic bacteria
- Autotrophic bacteria are capable of synthesizing their own food from simple inorganic nutrients like H2S, CO2 etc.
- They are essential to all life because they are the primary producers at the base of all food chains.
- Heterotrophic bacteria cannot synthesize their own food
- They depend on autotrophs for nutrition
ON THE BASIS OF SHAPE
On the basis of shape, bacteria are divided into four groups;
- Cocci (rounded)
- Bacilli (capsule)
- Spirillum (spiral shaped)
- Vibrio (coma shaped)
- The cocci are bacteria that are spherical, oval or round in shape.
- Cocci may remain attached after cell division and can grow in pairs, chains, or clusters, depending on their orientation.
- Most cocci bacteria do not possess flagella and are non-motile.
- Cocci bacteria can be pathogenic, symbiotic, or commensals.
- These types of bacteria can exist in several different arrangements that include:
- Diplococci are pairs of cocci
- Streptococci are chains of cocci.
- Tetrad is a cluster of four cocci arranged in the same plane
- Sarcina cuboidal arrangements of eight cocci.
- Staphylococci are irregular grape like clusters of cocci
- Staphylococcus aureus
- Streptococcus pyogenes
- Bacillus (bacilli plural) are rod-shaped bacteria.
- They use flagella for surface motility
- These bacteria occur as facultative anaerobes.
- They also form endospores
- Bacillus frequently occur in chains.
- Bacillus bacteria can exist in several different arrangements that include:
- Mono-bacillus it is a single bacillus bacterium
- Diplo-bacilli are pairs of bacilli bacteria
- Strepto-bacilli are chains of bacilli
- Cocco-bacilli have a shape in between cocci and bacilli. They are short rods and can be mistaken for cocci.
- Bacillus anthracis
- Escherichia coli
- Spirillum (plural spirila) is an elongated, spiral-shaped bacterium
- These bacteria are rigid and capable of movement.
- They lack outer sheath and endo-flagella, but have a typical bacterial flagellum.
- Spirochete is a special group of spirila. They are long, slender, and flexible.
Spirillum minus, which causes rat-bite fever.
- Vibrio bacteria resemble the shape of a comma.
- Vibrio bacteria are Gram-negative
- These are facultative anaerobes and can survive without oxygen.
- They also comprise of a flagellum, which is used for movement.
Vibrio cholera which is responsible for cholera.
ON THE BASIS OF STAINING BEHAVIOR
- Gram staining is a technique used to differentiate two large groups of bacteria on the basis of their different cell wall constituents.
- The Gram stain technique was devised by Hans Christian Gram in 1884.
- This technique distinguishes different bacterial cells by coloring them pink or violet.
- Following chemicals are used in gram staining procedure for differentiating bacterial cells:
- Crystal violet (primary stain)
- Iodine solution/Gram’s Iodine (mordant that fixes crystal violet to cell wall)
- Decolorizer (e.g. ethanol)
- Safranin (secondary stain)
- The color acquired by bacterial cells during gram staining depends on their cell wall structure and composition.
- On this basis, bacteria are divided into two major groups:
- Gram positive bacteria
- Gram negative bacteria
GRAM NEGATIVE BACTERIA
- Gram negative bacteria have a comparatively thin peptidoglycan layer in their cell wall
- When such bacteria are stained with primary stain, they do not retain the violet color
- They are thus stained pink with the secondary stain, safranin
- Thus, such bacteria appear to be pink when viewed under microscope
- Gram-negative bacteria more resistant to antibiotics than Gram-positive ones
GRAM POSITIVE BACTERIA
- Gram positive bacteria have thick layer of peptidoglycan in their cell walls
- When these cells are stained with primary stain, they retain the primary stain.
- Thus they appear violet when viewed under microscope.
REPRODUCTION IN BACTERIA
- Bacteria undergoes only asexual reproduction.
- Bacteria reproduce asexually by;
- Binary fission
- Conidia formation
- In binary fission, single parent cell divides equally into two daughter cells.
- Binary fission is a rapid process and cell undergoes division after every interval of 20-30 minutes
- In such reproduction, the bacterial cell develops a small swelling at one side which gradually increases in size
- As the nucleus undergoes division, one cell remains with the parent and other one goes to the swollen outgrowth.
- This outgrowth is in the form of bud, which gets separated from the parent cell by the growth of cell wall and therefore give rise to a new bacterial cell
- Conidia formation occurs in filamentous bacteria like Streptomyces etc.
- This is due to the formation of a transverse septum at the apex of the filament
- After detachment from the parent, conidium finds a suitable substratum where it germinates and gives rise to new mycelium.
NUTRITION IN BACTERIA
- Like all other living organisms, Bacteria also require energy and nutrients to drive various biochemical processes.
- They require sources of carbon, nitrogen, phosphorous, iron and a large number of other molecules.
- Considering the mechanisms of nutrition, bacteria are further classified into two forms:
- Autotrophic bacteria
- Heterotrophic bacteria
- These bacteria are capable of synthesizing their own food from inorganic substances like H2O, C02, H2S salts.
- They are further categorized into two groups:
- Photoautotrophs capture the energy of sunlight and convert it into the chemical energy.
- During this process the CO2 is reduced to carbohydrates.
- Free oxygen is also produced along with carbohydrates.
- Sunlight is captured by photoautotrophs by means of chlorophyll pigment present in them
- Purple Sulphur bacteria
- Green Sulphur bacteria
- Chemoautotrophs do not require sunlight and pigmentation for their nutrition.
- They oxidize certain inorganic substances with the help of atmospheric oxygen.
- This is an exothermic process and the energy produced is used to drive the synthetic processes of the cell.
- Heterotrophic bacteria are unable to produce their own food and rely on external sources for nutrition.
- These bacteria are found widely in soil, water, foods and the bed soil of water.
- They take an active part in the natural recycling of materials.
TYPES OF HETEROTROPHIC BACTERIA
They can be classified into various types:
- Photo-heterotrophs are heterotrophic organisms that utilize light energy as their energy source.
- They cannot acquire carbon dioxide as their sole carbon source.
- They obtain energy from sunlight but acquire organic compounds like sugars from the environment to survive.
- Examples of photoheterotrophic bacteria include
- Green non-sulfur bacteria
- Purple non-sulfur bacteria
- Chemo-heterotroph are the bacteria that acquire energy from chemical reactions.
- Like all heterotrophs, they require organic compounds from the environment in order to survive and cannot manufacture their own.
- Chemo-heterotrophs are often found at thermal vents in the deep ocean.
- Organotrophs are bacteria that obtain their energy from an organic source
- Bacteria involved in composting and decomposition process are mainly Organotrophs
- Lithotrophs are the bacteria that require an inorganic source for obtaining energy
- These type of heterotrophic bacteria occur rarely in environment.
RESPIRATION IN BACTERIA
- Like all other living things, respiration also takes place in bacteria.
- They oxidize food materials present in the cytoplasm to acquire energy.
- Most bacteria utilize free oxygen of the atmosphere or oxygen dissolved in the liquid environment.
- On the basis of respiration there are four different types of bacteria
- Aerobic bacteria utilize oxygen for respiration e.g.: Pseudomonas
- Anaerobic bacteria respire in the absence of oxygen e.g.: spirochete
- Facultative bacteria can respire in both absence and presence of oxygen e.g.: E. coli
- Microaerophilic bacteria require very low concentration of oxygen to grow e.g.: Campylobacter
ECONOMIC IMPORTANCE OF BACTERIA
- Soil bacteria play a vital role in bringing about decomposition of organic matter. They play role in maintaining and increasing soil fertility
- Nitrogen fixing bacteria can fix the free nitrogen of the atmosphere into absorbable form of nitrates
- Some bacteria act as bio-remediators as they remove pollutants from contaminated water, soil and subsurface materials.
- Bacteria are important in manufacturing multiple antibiotics such as Streptomycin
- Arihant’s handbook of biology. Biological classification. Page no: 7-23.
- NCERT biology; textbook for class 11. Biological classification. Page no: 16-28.