[Biology] Bacteria

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INTRODUCTION

Bacteria are unicellular, microscopic prokaryotes that are placed in the kingdom Monera

CLASSIFICATION OF BACTERIA

Bacteria is divided into two sub-categories:

  • Archaebacteria
  • Eubacteria

ARCHAEBACTERIA

  • 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:

  1. Methanogens
  2. Halophiles
  3. Thermo-acidophiles

METHANOGENS

  • 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

  • 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

THERMO-ACIDOPHILES

  • 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.

EUBACTERIA

  • 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;
    1. On the basis of nutrition
    2. On the basis of structure
    3. 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

  • 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

  • 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;

  1. Cocci (rounded)
  2. Bacilli (capsule)
  3. Spirillum (spiral shaped)
  4. Vibrio (coma shaped)

Cocci

  • 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

Example

  • Staphylococcus aureus 
  • Streptococcus pyogenes

Bacillus

  • 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.

Example

  • Bacillus anthracis
  • Escherichia coli

SPIRILLUM

  • 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.

FOR EXAMPLE

Spirillum minus, which causes rat-bite fever.

VIBRIO

  • 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.

FOR EXAMPLE

Vibrio cholera which is responsible for cholera.

Learn Bacteria introduction and shape of bacteria in 3 minutes.

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)
    • Water
  • 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.

Gram Staining | BioNinja

REPRODUCTION IN BACTERIA

  • Bacteria undergoes only asexual reproduction.
  • Bacteria reproduce asexually by;
    • Binary fission
    • Budding
    • Conidia formation

BINARY FISSION

  • 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

BUDDING

  • 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

  • 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

AUTOTROPHIC 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
    • Chemoautotrophs

PHOTOAUTOTROPHS

  • 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

EXAMPLE

  1. Purple Sulphur bacteria
  2. Green Sulphur bacteria

CHEMOAUTOTROPHS

  • 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.

EXAMPLE

  1. Nitrosamines
  2. Methanogens

HETEROTROPHIC BACTERIA

  • 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
  • Chemo-heterotroph
  • Organotrophs
  • Lithotrophs

PHOTO-HETEROTROPHS

  • 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-HETEROTROPHS

  • 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

  • Organotrophs are bacteria that obtain their energy from an organic source
  • Bacteria involved in composting and decomposition process are mainly Organotrophs

LITHOTROPHS

  • 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

REFERENCES

  • Arihant’s handbook of biology. Biological classification. Page no: 7-23.
  • NCERT biology; textbook for class 11. Biological classification. Page no: 16-28.
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