Theory:

Bacteria are single-celled, microscopic organisms that live in a variety of environments. These are the simplest unicellular organisms with rigid cell walls that resemble plant cells. These species occur in soil, the ocean, and even the human intestine. There are numerous beneficial bacteria that aid in the digestion of both humans and animals. The diseases typhoid and tuberculosis are caused by bacteria.
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Bacterial morphology
Introduction:
  • The study of bacteria is called Bacteriology.
  • Bacteria are classified under the kingdom called Monera.
  • Bacteria are present in nearly every habitat on earth, including humans. Some species can live under extreme conditions of temperature and pressure.
  • Bacteria are microscopic, single-celled, prokaryotic organisms because they do not have a true nucleus and other membrane-bound cell organelles.
  • Bacteria play an essential role in human survival—they break down nutrients in the digestive system into simpler forms.
  • The majority of bacterial species exist as single-celled forms. Some types of bacteria appear to be filaments of loosely joined cells, e.g. Actinomycetes.
The size ranges from less than \(1\) to \(10\)μm in length and \(0.2\) to \(1\)μm in width.
 
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Different types of bacteria's and their size
  • Bacteria may be motile (able to migrate independently) or non-motile (unable to move on their own). For motility, they have unique structures called flagella that facilitate locomotion found on the cell surfaces.
  • Flagella are long, helical shaped structures, composed of subunits of proteins named flagellin. The base of the flagellum looks slightly broader, which is called as a hook. Hook joins filament to the motor portion of the flagellum called a basal body. The basal body is attached to the cytoplasmic membrane and cell wall. The location of flagella varies with bacterial species.
Classifications of bacteria:
1. Based on the arrangement of flagella:
 
Depending on the flagellum, bacteria are classified into four types.
 
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The types bacteria's based on the flagellum
  
Monotrichous: Having a single flagellum at one pole.
   
E.g. Vibrio cholerae, Pseudomonas aeruginosa
Lophotrichous: A cluster of flagella located at one end only.
 
E.g. Pseudomonas fluorescens, Spirillum
Amphitrichous: Either a single or cluster of flagellum present at both cell poles.
 
E.g. Aquaspirillum serpens, Alcaligenes faecalis
Peritrichous: Flagella arranged along the sides of the bacteria.
  
E.g. Salmonella typhi, Escherichia coli
  
2. Shape of the bacteria:
 
Depending on the shapes, bacteria are classified into three types.
 
Type of bacteria
Shape
Example
Cocci - Spherical shaped (Cocci - Plural; Coccus - Singular)These are unicellular, spherical or elliptical shaped.Streptococcus pneumoniae
Bacilli -Rod-shaped (Bacilli - Plural; Bacillus - Singular)Rod-shaped or cylindrical bacteria that either remain single or in pairs.Mycobacterium tuberculosis
Spirillum -Spiral shaped (Spirilla - Plural; Spirillum - Singular)These are spiral or spring-like with multiple curvature and terminal flagella.Treponema pallidum
Vibrio -Comma shaped (Vibrios - Plural; Vibrio - Singular)These are curved and have the appearance of a comma.Vibrio cholerae
 
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The different shapes of bacteria
i. Cocci - Spherical shaped bacteria: (Cocci - plural; Coccus - singular):
This type of bacteria is unicellular, spherical or elliptical shaped.
Example:
Streptococcus pneumoniae
Types of coccus bacteria include,
  • Monococcus
  • Diplococcus
  • Streptococcus
  • Tetracoccus
  • Sarcina
  • Staphylococcus
They are named based on how their bacterial cells are arranged.
 
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Types of coccus bacteria
ii. Bacilli - Rod-shaped bacteria:  (Bacilli - plural; Bacillus - singular):
These are rod-shaped or cylindrical bacteria that either remain singly or in pairs.
Example:
Mycobacterium tuberculosis
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Types of bacilli bacteria
iii. Spirilla - Spiral shaped bacteria:  (Spirilla - plural; Spirillum - singular):
These bacteria are spiral or spring-like with multiple curvature and terminal flagella.
Example:
Treponema pallidum
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Types of spirilla bacteria
 
Important!
Atrichous bacteria have no flagella.
Reference:
https://commons.wikimedia.org/wiki/File:Bacterial_morphology_diagram.png#/media/File:Bacterial_morphology_diagram.svg
https://upload.wikimedia.org/wikipedia/commons/8/8c/Shapes_of_bacteria_and_size_comparisons.jpg