Classification of Bacteria

Bacteria are classified and identified to distinguish them from other organisms. The classification of bacteria serves a variety of different functions. Because of this variety, bacteria may be grouped using many different typing schemes.

The grounds for the classification commonly used may be:

1. Classification on the basis of Gram Stain and Bacterial Cell Wall
2. Classification of Bacteria on the Basis of Shape
3. Classification of Bacteria on the Basis of Mode of Nutrition
4. Classification of Bacteria on the Basis of Temperature Requirement
5. Classification of Bacteria on the Basis of Oxygen Requirement
6. Classification of Bacteria on the Basis of pH of Growth
7. Classification of Bacteria on the Basis of Osmotic Pressure Requirement
8. Classification of Bacteria on the Basis of Number of Flagella
9. Classification of Bacteria on the basis of Spore Formation

Classification on the basis of Gram Stain and Bacterial Cell Wall

Gram Staining is the common, important, and most used differential staining technique in microbiology, which was introduced by Danish Bacteriologist Hans Christian Gram in 1884. This test differentiates the bacteria into Gram-positive and Gram-negative Bacteria, which helps in the classification and differentiation of microorganisms.

Principle of Gram Staining

When the bacteria is stained with primary stain Crystal Violet and fixed by the mordant, some of the bacteria are able to retain the primary stain and some are decolorized by alcohol. The cell walls of gram positive bacteria have a thick layer of protein-sugar complexes called peptidoglycan and lipid content is low. Decolorizing the cell causes this thick cell wall to dehydrate and shrink, which closes the pores in the cell wall and prevents the stain from exiting the cell. So the ethanol cannot remove the Crystal Violet-Iodine complex that is bound to the thick layer of peptidoglycan of gram positive bacteria and appears blue or purple in colour.

In the case of gram negative bacteria, the cell wall also takes up the CV-Iodine complex but due to the thin layer of peptidoglycan and a thick outer layer which is formed of lipids, the CV-Iodine complex gets washed off. When they are exposed to alcohol, the decolorizer dissolves the lipids in the cell walls, which allows the crystal violet-iodine complex to leach out of the cells. Then when again stained with safranin, they take the stain and appears red in color.

Reagents Used in Gram Staining

• Crystal Violet, the primary stain

• Iodine, the mordant

• A decolorizer made of acetone and alcohol (95%)

• Safranin, the counterstain

The Procedure of Gram Staining

1. Take a clean, grease-free slide.

2. Prepare the smear of suspension on the clean slide with a loopful of the sample.

3. Air dry and heat fix

4. Crystal Violet was poured and kept for about 30 seconds to 1 minute and rinsed with water.

5. Flood the gram’s iodine for 1 minute and wash with water.

6. Then, wash with 95% alcohol or acetone for about 10-20 seconds and rinse with water.

7. Add safranin for about 1 minute and wash with water.

8. Air dry, Blot dry, and Observe under Microscope

Classification of Bacteria on the Basis of Shape

In the year 1872 scientist Cohn classified bacteria to 4 major types depending on their shapes are as follows:

A) Cocci: These types of bacteria are unicellular, spherical or elliptical shape. Either they may remain as a single cell or may aggregate together for various configurations. They are as follows:

Monococcus:– they are also called micrococcus and represented by single, discrete round Example: Micrococcus flavus.

Diplococcus:– the cell of the Diplococcus divides ones in a particular plane and after division, the cells remain attached to each other. Example: Diplococcus pneumonia.

Streptococcus: – here the cells divide repeatedly in one plane to form chain of cells. Example: – Streptococcus pyogenes.

Tetracoccus: – this consists of four round cells, which defied in two planes at a right angles to one another. Example: – Gaffkya tetragena. Staphylococcus: – here the cells divided into three planes forming a structured like bunches of grapes giving and irregular configuration. Example: – Staphylococcus aureus.

Sarcina: -in this case the cells divide in three planes but they form a cube like configuration consisting of eight or sixteen cells but they have a regular shape. Example: –Sarcina lutea.

B) Bacilli: – These are rod shaped or cylindrical bacteria which either remain singly or in pairs. Example: –Bacillus cereus.

C) Vibro: – The vibro are the curved, comma shaped bacteria and represented by a single genus. Example: – Vibro cholerae.

D) Spirilla: – These type of bacteria are spiral or spring like with multiple curvature and terminal flagella. Example: –Spirillum volutans.

Classification of Bacteria on the Basis of Mode of Nutrition

Nutritional Types of Bacteria

On the basis of energy source organisms are designated as:

Phototrophs:

• The organisms which can utilize light as an energy source are known as phototrophs. These bacteria gain energy from light.

Chemotrophs:

• These bacteria gain energy from chemical compounds. They cannot carry out photosynthesis.

On the basis of electron source organisms are designated as:

Lithotrophs:

• Some organisms can use reduced organic compounds as electron donors and are termed as Lithotrophs.

• They can be Chemolithotrophs and Photolithotrophs

Organotrophs:

• Some organisms can use organic compounds as electron donors and are termed as organotrophs.

• Some can be Chemoorganotrophs and Photoorganotrophs.

Thus, bacteria may be either:

• Photo-lithotrops: These bacteria gain energy from light and use reduced inorganic compounds such as H₂S as a source of electrons. eg: Chromatium okeinii.

• Photo-organotrophs: These bacteria gain energy from light an d use organic compounds such as Succinate as a source of electrons.eg; Rhodospirillum.

• Chemo-lithotrophs: These bacteria gain energy from reduced inorganic compounds such as NH3 as a source of electron eg; Nitrosomonas.

• Chemo-organotrophs: These bacteria gain energy from organic compounds such as glucose and ammino acids as a source of electrons.eg; Pseudomonas pseudoflora.

• Some bacteria can live ether chemo-lithotrophs or chemo-organotrophs like Pseudomonas pseudoflora as they can use either glucose or H2S as electron source.

On the basis of carbon source bacteria may be:

• All organisms require carbon in some form for use in synthesizing cell components.

• All organisms require at least a small amount of CO2.

• However, some can use CO2 as their major or even sole source of carbon; such organisms are termed as Autotrophs (Autotrophic bacteria).

• Others require organic compounds as their carbon source and are known as Heterotrophs (Heterotrophic bacteria).

Autotrophic Bacteria

These bacteria synthesize all their food from inorganic substances (H2O, C02, H2S salts).

The autotrophic bacteria are of two types:

(i) Photoautotrophs

• These bacteria capture the energy of sunlight and transform it into the chemical energy.

• In this process, CO2 is reduced to carbohydrates.

• The hydrogen donor is water and the process produce free oxygen.

• Photoautotroph has Chlorophyll pigment in the cell and its main function is to capture sunlight e.g., Cyanobacteria.

(ii) Chemoautotrophs

• These bacteria do not require light (lack the light phase but have the dark phase of photosynthesis) and pigment for their nutrition.

• These bacteria oxidize certain inorganic substances with the help of atmospheric oxygen.

• This reaction releases the energy (exothermic) which is used to drive the synthetic processes of the cell.

Heterotrophic Bacteria

• The heterotrophic bacteria obtain their-ready made food from organic substances, living or dead.

• Most of pathogenic bacteria of human beings, other plants and animals are heterotrophs.

• Some heterotrops have simple nutritional requirement while some of them require large amount of vitamin and other growth promoting substance. Such organisms are called fastidious heterotrophs.

• Heterotrophic bacteria are of three types:

a. Photoheterotrophs

• These bacteria can utilize light energy but cannot use CO2 as their sole source of carbon.

• They obtain energy from organic compounds to satisfy their carbon and electron requirements. Bacteriochlorophyll pigment is found in these bacteria.

• e.g., Purple non-sulphur bacteria (Rhodospirillum, Rhodomicrobium, Rhodopseudomonas palustris).

b. Chemoheterotrophs

• Chemoheterotrophs obtain both carbon and energy from organic compounds such as carbohydrates, lipids and proteins.

Glucose or Monosaccharide [(CH2O)n] + O2 → CO2 + H2O + Energy

There are three main categories that differ in how chemohetrotrophs obtain their organic nutrients:

(i) Saprophytic bacteria.

(ii) Parasitic bacteria.

(iii) Symbiotic bacteria.

i) Saprophytic bacteria

• Saprophytic bacteria obtain their food from the dead and organic decaying matter such as leaves, fruits, vegetables, meat, animal feces, leather, humus etc.

• These bacteria secrete enzymes to digest the food and absorb it.

• The enzymes secreted to break down the complex compounds such as carbohydrate and protein, into simpler soluble compounds, which are easily absorbed.

• Examples are Bacillus mycoides, B. ramosus, Acetobacter etc.

ii) Parasitic bacteria

• These bacteria obtain their nutrition from the tissues of the hosts on which they grow.

• They may be harmless or may cause serious diseases.

• Parasitic bacteria which cause various diseases in plants and animals are known as pathogens, e.g., Bacillus typhosus, B. anthracis, B.tetani. B.diplheriae, B.tuberculosis, B. pneumoniae, Vibrio cholerae, Pseudomonas citri etc.

iii) Symbiotic bacteria

• Symbiotic bacteria live in close association with other organisms as symbionts.

• They are beneficial to the organisms.

• The common examples are the nitrogen-fixing bacteria, e.g., Bacillus radicicola, B. azotobacter, Rhizobium, Clostridium, Rhizobium spp., B. radicicolaand B. azotobacter.

• These bacteria live inside the roots of leguminous plants.

• These bacteria fix free atmospheric nitrogen into nitrogenous compounds which are utilized by the plants. In return, the plant provides nutrients and protection to the bacteria.

Classification of Bacteria on the Basis of Temperature Requirement

Psychrophiles:

• Bacteria that can grow at 0°C or below but the optimum temperature of growth is 15 °C or below and maximum temperature is 20°C are called psychrophiles

• Psychrophiles have polyunsaturated fatty acids in their cell membrane which gives fluid nature to the cell membrane even at lower temperature.

• Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata, Psychroflexus.

Psychrotrops (facultative psychrophiles):

• Those bacteria that can grow even at 0°C but optimum temperature for growth is (20-30)°C

Mesophiles:

• Those bacteria that can grow best between (25-40)^o C but optimum temperature for growth is 37C

• Most of the human pathogens are mesophilic in nature.

• Examples: E. coli, Salmonella, Klebsiella, Staphylococci.

Thermophiles:

• Those bacteria that can best grow above 45C.

• Thermophiles capable of growing in mesophilic range are called facultative thermophiles.·

• True thermophiles are called as Stenothermophiles, they are obligate thermophiles,

• Thermophils contains saturated fattyacids in their cell membrane so their cell membrane does not become too fluid even at higher temperature.

• Examples: Streptococcus thermophiles, Bacillus stearothermophilus, Thermus aquaticus.

Hypethermophiles:

• Those bacteria that have optimum temperature of growth above 80C.·

• Mostly Archeobacteria are hyperthermophiles.·

• Monolayer cell membrane of Archeobacteria is more resistant to heat and they adopt to grow in higher remperature.

• Examples: Thermo desulfobacterium, Aquifex, Pyrolobus fumari, Thermotoga.

Classification of Bacteria on the Basis of Oxygen Requirement

Classification of Bacteria on the Basis of pH of Growth

1. Acidophiles: These bacteria grow best at an acidic pH. The cytoplasm of these bacteria are acidic in nature. Some acidopiles are thermophilic in nature, such bacteria are called Thermoacidophiles. Examples: Thiobacillus thioxidans, Thiobacillus, ferroxidans, Thermoplasma, Sulfolobus

2. Alkaliphiles: These bacteria grow best at an alkaline pH. Example: Vibrio cholerae optimum ph of growth is 8.2.

3. Neutrophiles: These bacteria grow best at neutral pH (6.5-7.5). Most of the bacteria grow at neutral pH. Example: E. coli

Classification of Bacteria on the Basis of Osmotic Pressure Requirement

Halotolerant:

• Most of the bacteria do not require NaCl but can tolerate low concentration of NaCl in growth media are called halotolerant

Halophiles:

• Require moderate to large salt concentrations.

• Cell membrane of halophilic bacteria is made up of glycoprotein with high content of negatively charged glutamic acid and aspartic acids. So high concentration of Na+ ion concentration is required to shield the –ve charge.

• Ocean water contains 3.5% salt. Most such bacteria are present in the oceans.

• Archeobacteria, Halobacterium, Halococcus.

Extreme or Obligate Halophiles:

• Require a very high salt concentrations (20 to 30%).

• Bacteria in Dead Sea, brine vats.

Facultative Halophiles:

• Do not require high salt concentrations for growth, but tolerate upto 2% salt or more.

Classification of Bacteria on the Basis of Number of Flagella

On the basis of flagella the bacteria can be classified as:

1. Atrichos: – These bacteria has no flagella. Example: Corynebacterium diptherae.

2. Monotrichous: – One flagellum is attached to one end of the bacteria cell. Example: – Vibro cholerae.

3. Lophotrichous: – Bunch of flagella is attached to one end of the bacteria cell. Example: Pseudomonas.

4. Amphitrichous: – Bunch of flagella arising from both end of the bacteria cell. Example: Rhodospirillum rubrum.

5. Peritrichous: – The flagella are evenly distributed surrounding the entire bacterial cell. Example: Bacillus.

Classification of Bacteria on the basis of Spore Formation

1. Spore forming bacteria: Those bacteria that produce spores during unfavorable conditions. These are further divided into two groups:

i) Endospore forming bacteria: Spore is produced within the bacterial cell. Examples. Bacillus, Clostridium, Sporosarcina etc

ii) Exospore forming bacteria: Spore is produced outside the cell. Example. Methylosinus

2. Non sporing bacteria: Those bacteria which do not produce spores. Eg. E. coli, Salmonella.

What are spores?

Spores are the unicellular and reproductive structure, which are greatly resistant to extreme environmental conditions, like high temperature, pH, salt concentration, etc. Such reproductive structures exist in variable, sizes, shapes and numbers. The phenomena of the formation of spores by the vegetative cells are termed as Sporulation or Sporogenesis.

Based on the positions, these spores are broadly classified into two types:

1. Endospore

2. Exospore

Endospore and Exospore

• Endospores and exospores are two varieties of spores or the reproductive structures generated as resting systems.

• Endospore is the common type of spores produced mainly by bacteria.

• Exospores are produced in the eukaryotic cells of fungi, algae and cyanobacteria.

• Endospores are formed inside the mother cell, whereas, exospores are formed towards the end of the mother cell and released as bud.

Endospore and Exospore – Differences