Bacteria are microscopic, single-celled organisms that thrive in diverse environments. These are the most simple unicellular organisms, with rigid cell walls similar to those found in plant cells.
These organisms can live in soil, the ocean and inside the human gut. Many beneficial bacteria help in the digestion process in human and animals.
Typhoid and tuberculosis are some of the diseases caused by bacteria
Shape of the bacteria:
1. Based on the shape:
Type of bacteria
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
Types of bacteria based on the shape 
2. Based on the arrangement of flagella:
a. Monotrichous (Mono means one): Having a single flagellum at one pole.
E.g. Vibrio cholerae, Pseudomonas aeruginosa.
b. Amphitrichous: Either a single or cluster of flagellum present at both cell poles.
E.g. Aquaspirillum serpens, Alcaligenes faecalis.
c. Lophotrichous: A cluster of flagella located at one end only.
E.g. Pseudomonas fluorescens, Spirillum
d. Peritrichous (flagella in periphery): Flagella arrangedalong the sides of the bacteria.
 E.g. Salmonella typhi and Escherichia coli.
Bacteria types based on flagella
  • The size of the bacteria ranges from \(0.0002 - 0.01mm\).
  • Study of bacteria is called Bacteriology.
  • Bacteria are classified under the kingdom called Monera.
  • Atrichous bacteria have no flagella.
  • Streptococcus mutants is mostly found in teeth (oral cavity) of human beings that causes tooth decay.