1. Classification on the basis of Oxygen requirement
2. Brown classification; on the basis of haemolytic pattern on sheep blood agar
3. Shermann’s classification; on the basis of physiological characteristics
4. Lancifield classification; serological classification
5. Biochemical classification
6. Classification on the basis of 16s rRNA sequence

1. Classification on the basis of Oxygen requirement:

i. Aerobic or facultative anaerobes: eg. Streptococcus spp

ii. Obligate anaerobes: eg. Peptostreptococcus

2. Brown classification: On the basis of haemolytic pattern on sheep blood agar.

i. Αlpha-haemolysis group:

• Form incomplete haemolysis on blood agar
• Shows greenish discoloration around colony and persistence of some unhaemolyszed RBCs.
• Streptococcus pneumoniae, Viridians streptococci

ii. Βeta-haemolysis group:

• Form complete haemolysis on blood agar
• Give 2-3 mm diameter zome of haemolysis
• Streptococcus pyogens

iii. Non-haemolysis group

• Does not cause haemolysis at all
• These are non-haemolytic group
• Streptococcus faecalis

3. Shermann’s classification: on the basis of physiological characteristics

i. Pyogenic streptococci: Streptococcus pyogens

ii. Lactococci

• Found in dairy products
• They are non-haemolytic group
• eg. Lactoccus

iii. Enterococci:

• They are normal flora of human intestine
• Enterococcus

iv. Viridans streptococci

• They are normal flora of upper respiratory tract of human

 4. Lancefield classification: serological classification

• This classification is based on the difference in the structure of cell wall carbohydrate ie. group specific polysaccharide antigen. Most strain of β-haemolytic group and some strain of α- hemolytic and non-haemolytic group are classified on the basis of cell wall polysaccharide.
• Streptococci are classified into 20 lancifield group from A to V except I and J.

Group A; Streptococcus pyogens

Group B; Streptococcus agalactiae

Group C; Streptoccus equi

Group D; Enterococcus

other (group E to V)

• Group A streptococci ie. S.pyogens is further sub divided into approximately 80 serotypes by Griffith according to their specific surface protein (M, T and R). M-protein is the most important one.
• The pathogenicity of Streptococci depends on the presence of hyaluronic acid capsule and surface M-protein.

5. Biochemical classification:

i. On the basis of sugar fermentation test;

• Streptococci ferments most sugar with production of lactic acid but no gas.
• Accumulation of lactic acid in media terminates bacterial growth.
• Streptococcus pyogens do not ferment ribose and they give PYR test positive.

ii. On the basis of production of certain enzymes

iii. On the basis of antibiotic susceptibility test

iv. On the basis of colony characteristics on different media

v. On the basis of haemolysis pattern

6.  Classification on the basis of analysis of 16s rRNA sequence

i. Pyogenic group: eg. Streptococcus pyogens, S. agalactiae

ii. Anginosius group; eg. Streptococcus anginosus, S. intermedius

iii. Mitis group: eg. Streptococcus mitis, S. pneumoniae

iv. Salivarious group: eg. Streptococcus salivarious, S. thermophiles

v. Bovis group: eg. Streptococcus bovis, S. equines

vi. Mutans group: eg. Streptococcus mutans, S. ferus

Classification of Streptococcus

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A. on the basis of mode of nutrition

B. on the basis of optimum temperature requirement for growth

C. on the basis of optimal pH for growth

D. on the basis of salt concentration

E. on the basis of gaseous requirement

F. on the basis of morphology

G. on the basis of gram staining

H. on the basis of flagella

I. on the basis of spore

A] Classification of bacteria on the basis of mode of nutrition

1. Phototrops:

• Those bacteria which gain energy from light
• Phototrops are further divided into two groups on the basis of source of electron.
  • Photolithotrops: these bacteria gain energy from light and uses reduced inorganic compounds such as H2S as electron source. Eg. Chromatium okenii
  • Photoorganotrops: these bacteria gain energy from light and uses organic compounds such as succinate as electron source.

2. Chemotrops:

• Those bacteria gain energy from chemical compounds
• They cannot carry out photosynthesis
• Chemotrops are further divided into two groups on the basis of source of electron.
  • Chemolithotrops: they gain energy from oxidation of chemical compound and reduces inorganic compounds such as NH3 as electron source. Eg. Nitrosomonas
  • Chemoorganotrops: they gain energy from chemical compounds and uses organic compound such as glucose and amino acids as source of electron. eg. Pseudomonas pseudoflava

3. Autotrops:

• Those bacteria which uses carbondioxide as sole source of carbon to prepare its own food.
• Autotrops are divide into two types on the basis of energy utilized to assimilate carbondioxide. Ie. Photoautotrops and chemoautotrops
  • Photoautotrops: they utilized light to assimilate CO2. They are further divided into two group on the basis of electron sources. Ie. Photolithotropic autotrops and Photoorganotropic autotrops
  • Chemoautotrops: they utilize chemical energy for assimilation of CO2

4. Heterotrops:

• Those bacteria which uses organic compound as carbon source
• They lack the ability to fix CO2
• Most of the human pathogenic bacteria are heterotropic in nature
• Some heterotrops are simple, because they have simple nutritional requirement. However there are some bacteria that require special nutrients for their growth; known as fastidious heterotrops.

B] Classification of bacteria on the basis of optimum temperature of growth

1. 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 fattyacids in their cell membrane which gives fluid nature to the cell membrane even at lower temperature.
• Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata, Psychroflexus

2. Psychrotrops (facultative psychrophiles):

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

3. Mesophiles:

• Those bacteria that can grow best between (25-40)C but optimum temperature for growth is 37C
• Most of the human pathogens are mesophilic in nature
• Examples: E. coli, Salmonella, Klebsiella, Staphulococci

4. 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,

5. 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: Thermodesulfobacterium, Aquifex, Pyrolobus fumari, Thermotoga

C] Classification of bacteria on the basis of optimum pH of growth

1. Acidophiles:

• Those bacteria that grow best at 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:

• Those bacteria that grow best at alkaline pH
• Example: vibrio cholerae: optimum pH of growth is 8.2

3. Neutriphiles:

• Those bacteria that grow best at neutral pH (6.5-7.5)
• Most of the bacteria grow at neutral pH
• Example: E. coli

D] Classification of bacteria on the basis of salt requirement

1. Halophiles:

• Those bacteria that require high concentration of NaCl for growth.
• Cell membrane of halophilic bacteria is made up of glycoprotein with high content of negatively (-Ve) charged glutamic acid and aspartic acids. So high concentration of Na+ ion concentration is required to shield the –ve charge.
• Example: Archeobacteria, Halobacterium, Halococcus

2. Halotolerant:

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

E] Classification of bacteria on the basis of gaseous requirement

1. Obligate aerobes:

• Those bacteria that require oxygen and cannot grow in the absence of O2.
• These bacteria carryout only oxidative type of metabolism.
• Examples; Mycobacterium, Bacillus

2. Facultative anaerobes:

• Those bacteria that do not require O2 but can use it if available.
• Growth of these bacteria become better in presence of O2
• These bacteria carryout both oxidative and fermentative type of metabolism
• Examples: coli, Klebsiella, Salmonella

3. Aerotolerant anaerobes;

• Those bacteria do not require O2 for growth but can tolerate the presence of O2.
• Growth of these bacteria is not affected by the presence of O2.
• These bacteria have only fermentative type of metabolism.
• Example: lactobacillus

4. Microaerophiles:

• Those bacteria that grow on low concentration of Oxygen but At atmospheric level of Oxygen growth of these bacteria is inhibited.
• These bacteria only have oxidative type of metabolism
• Example: Campylobacter

5. Obligate anaerobes:

• Those bacteria that can grow only in absence of Oxygen.
• Oxygen is harmful to obligate anaerobes
• These bacteria have only fermentative type of metabolism
• Examples: Peptococcus, Peptostreptococcus, Slostridium, methanococcus

6. Capnophiles:

• Those bacteria that require carbondioxide for growth.
• They are CO2 loving organism
• Most of the microaerophiles are capnophilic in nature.
• Example: Campylobacter, Helicobacter pylori, Brucella abortus

F] Classification of bacteria on the basis of Morphology

1. Coccus:

• These bacteria are spherical or oval in shape
• On the basis of arrangement, cocci are further classified as-
• i) Diplococcus: coccus in pair. Eg, Neissseria gonorrhoae, Pneumococcus
• ii) Streptococcus: coccus in chain. Eg. Streptococcus salivarius
• iii) Staphylococcus: coccus in bunch. Eg. Staphylococcus aureus
• iv) Tetrad: coccus in group of four.
• v) Sarcina: cocus in cubical arrangement of cell. Eg. Sporosarcina

2. Bacilli:

• These are rod shaped bacteria
• On the basis of arrangement, bacilli are further classified as-
• i) Coccobacilli: Eg. Brucella
• ii) Streptobacilli: chain of rod shape bacteria: Eg. Bacillus subtilis,
• iii) Comma shaped: Eg. Vibrio cholarae
• iv) Chinese letter shaped: Corynebacterium dephtherae

3. Mycoplasma

• They are cell wall lacking bacteria
• Also known as PPLO (Pleuropneumonia like organism)
• Mycoplasma pneumoniae

4. Spirochaetes:

• They are spiral shaped bacteria
• Spirochaetes

5. Rickettsiae and Chlamydiae;

• They are obligate intracellular parasites resemble more closely to viruses than bacteria

6. Actinomycetes

• They have filamentous or branching structure
• They resemble more closely to Fungi than bacteria
• Example: Streptomyces

G] Classification of bacteria on the basis of Gran staining

1. Gram positive bacteria:

• cell wall of these bacteria is composed of peptidoglycan layer only.
• Eg. Staphylococcus, Streptococcus, micrococcus

2. Gram negative bacteria:

• cell wall of these bacteria is composed of Peptidoglycan and outer membrane.
• Eg. E. coli, Salmonella

H] Classification of bacteria on the basis of Flagella

1. Monotrichous bacteria:

• bacteria having single flagella in one end of cell.
• eg. Vibrio cholera, Pseudomonas aerogenosa

2. Lophotrichous bacteria:

• bacteria having bundle of flagella in one end of cell.
• eg. Pseudomanas fluroscence

3. Amphitrichous bacteria:

• bacteria having single or cluster of flagella at both end of cell.
• eg. Aquaspirillium

4. Peritrichous bacteria:

• bacteria having flagella all over the cell surface.
• Eg. E.coli, Salmonella, Klebsiella

5. Atrichous bacteria:

• bacteria without flagella.
• Eg. Shigella

I] Classification of bacteria on the basis of Spore

1. Spore forming bacteria:

• Those bacteria that produce spore during unfavorable condition.
• These are further divided into two group

i) Endospore forming bacteria:

• Spore produced within the bacterial cell.
• Bacillus, Clostridium, Sporosarcina etc

ii) Exospore forming bacteria:

• Spore produced outside the cell
• Methylosinus

2. Non sporing bacteria:

• those bacteria which do not produce spore.
• Eg. E. coli, Salmonella

Classification of bacteria

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Bacterial are unicellular prokaryotic organism. Bacterial cell have simpler internal structure. It lacks all membrane bound cell organelles such as mitochondria, lysosome, golgi, endoplasmic reticulum, chloroplast, peroxisome, glyoxysome, and true vacuole. Bacteria also lacks true membrane bound nucleus and nucleolus. The bacterial nucleus is known as nucleoid.

A typical bacterial cell have following structure.

A. Structure Outside cell wall

1. Capsule
2. Flagella
3. Pili
4. Sheath
5. Prostheca
6. Stalks

B. Structure Inside cell wall

1. Cell wall
2. Cytoplasmic membrane
3. Nucleoid
4. Mesosome
5. Ribosome
6. cytoplasm
7. Spore

1. Capsule:

• Capsule is 0.2µm thick viscus layer outer layer to the cell wall.
• Capsule is 98% water and 2% polysaccharide or glycoprotein/ polypeptide or both.
• There are two types of capsule.

i. Macro-capsule: thickness of 0.2µm or more, visible under light microscope

ii. Microcapsule: thickness less than 0.2µm, visible under Electron microscope

• Capsule is very delicate structure. It can be removed by vigorous washing.Capsule is most important virulence factor of bacteria.

Function:

• It helps in attachments as well as it prevent the cell from desiccation and drying.
• Capsule resist phagocytosis by WBCs

 2. Flagella:

• It is 15-20 nm hair like helical structure emerges from cell wall.
• Flagella is not straight but is helical. It is composed of flagellin protein (globular protein) and known as H antigen.
• Flagella has three parts. Basal body, Hook and filament

Function:

• It helps motility of the bacteria

3. Pili or fimbriae:

• Pili are hollow filamentous and non-helical structure.
• They are numerous and shorter than flagella
• Pili is the characteristic feature of gram –ve bacteria.
• Pili is composed of pilin protein.
• Bacteria containing pili: Shigella, Proteus, Neisseria gonorrhoae, E. coli

Function:

• Attachment: pili helps the bacteria to attach the host cell surface. Most of the human pathogens of respiratory tract, urinary tract are attached with the help of pili.
• Pili (fimbrae) possess antigenic property
• Specialized function: some pili are modified for specialized function. Eg. Sex pilus (F-pili) help in transfer of DNA from donor to recipient cell during conjugation.
• F-pili also act as receptor for bacteriophage.

4. Sheath:

• Some bacteria forming chain or trichome are enclosed by a hollow tube like structure known as Sheath.
• Aquatic bacteria mostly form sheath
• Some sheathed bacteria are; Sphaerotilus, leptothrix, clonothrix etc

Function:

• Mechanical support
• Sometime sheath is impregnated with ferric or manganese hydroxide which provide strength to sheath.

5. Prosthecae:

• Prosthecae are semi-rigid extension of cell wall and cell membrane
• One bacteria may contains one or many prosthecae.
• Some prosthecae develop bud at the tip and hence helps in reproduction.
• Some prosthecate bacteria are: Caulibacter, Stella, Prosthecobacter, Hyphomicrobium

Function:

• Prosthecae increase surface area for nutrition absorption. It is usually formed in bacteria living in very dilute environment where concentration of nutrition is low.
• Helps in adhesion
• Asexual reproduction by budding

6. Stalk:

• It is non-living ribbon like tubular structure.
• It is formed by excretory product of bacteria.
• Some stalked bacteria are: Gallionella, Planctomyces

Function:

• Helps in attachment to solid surface.

7. Cell wall:

• It is an important structure of a bacteria.
• It give shape to the organism.
• On the basis of cell wall composition, bacteria are classified into two major group ie. Gram Positive and gram negative.

Gram positive cell wall

Cell wall composition of gram positive bacteria.

1. Peptidoglycan
2. Lipid

• Teichoic acid

Gram negative cell wall

Cell wall composition of gram negative bacteria

1. Peptidoglycan
2. Outermembrane:
   • Lipid
   • Protein
   • Lipopopysaccharide (LPS)

Peptidoglycan:

• It consists of glycan backbone formed by repeated unit of NAG (n-acetyl Glucosamine) and NAM (N-acetyl muramic acid) and the glycan backbone is cross linked by peptide bond.
• Peptidoglycan layer is present in cell wall of both gram positive as well as gram negative bacteria. However, gram positive have thick layer of peptidoglycan.

Teichoic acid:

• Teichoic acid is water soluble polymer of glycerol or ribitol phosphate present in gram positive bacteria.
• It constitutes about 50% of dry weight of cell wall.
• It is the major surface antigen of gram positive bacteria

Outer membrane:

• It is an additional layer present in gram negative bacteria.
• It is composed of lipid bilayer, protein and lipo-polysaccharide(LPS)

LPS:

• LPS is composed of lipid-A and polysaccharide.
• Lipid-A: it is phosphorylated glucosamine disaccharide. It is antigenic
• Polysaccharide: it consists of core-polysaccharide and O-polysaccharide.

8. Cell membrane:

• Cell membrane is the inner layer that lies inside the cell wall and encloses the cytoplasm.
• It is also known as cytoplasmic membrane or plasma membrane.
• It is about 80nm thick.
• Cell membrane of bacteria is composed of phospholipid and proteins.

Function:

• It is selectively permeable as it allows to pass selective substances such as sugar, aminoacids across it.

9. Nucleus:

• Nucleus is the most important part of the cell.
• It controls and directs all the cellular activities and stores hereditary information of cell
• Bacterial nucleus is known as nucleoid; it lacks nuclear membrane, nuceloplasm and nucleolus.
• Bacterial DNA is naked (lacked histone protein)

Function:

• It contains and stores hereditary information of the cell.
• It controls all cell activities.

10. Ribosome:

• Bacterial ribosome is of 70s type.
• Ribosomes are rounded granules found freely floating in the cytoplasm
• Ribosomes are known as universal cell organelle because it is found in both bacterial cell and eukaryotic cell.
• Chemically the ribosomes are made up of nucleic acids (particularly RNA and proteins).

 Function:

• It helps in protein synthesis

 11. Mesosome:

• Mesosome is a spherical or round sac like structure found commonly in gram positive bacteria.
• Function: It is the site for respiration in bacterial cell

12. Cytoplasm:

• It is colorless, viscus fluid present inside cell membrane.
• All the cell organelles and inclusions are found floating in cytoplasmic fluid.
• It contains proteins, lipid, minerals, nucleic acids, glycogen, water etc.

Function:

• It helps to distribute water, oxygen as other substances throughout the cell.
• Literally, all the cellular content including nucleus, and other cell organelle are floating in cytoplasm.

13. Spores (endospore):

• Spore is metabolically dormant structure produced during unfavourable condition by the process called sporulation
• Sporulation occur during late log phase or early stationary phase
• Under favourable condition spores germinate to give vegetative cell.

To visualize these internal structure of bacteria, we use electron microscope 

Bacterial cell Structure and Function

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