General characteristics of bacteria:

• Lack rigid cell wall.
• Lacks nuclear membrane so distinct nucleus and nucleolus absent.
• Most of the bacteria lack chlorophyll called heterotrophic, few bacteria poses chlorophyll called as autotrophic.
• Genetic material represented by histone free DNA.
• Commonly reproduce vegetatively by fission.
• Variations in the genetic character are due to genetic recombination by conjugation, transformation and transduction.

Shape and Size:

a) Spherical: Bacteria shaped like a ball are called cocci, and a single bacterium is a coccus. Examples include the streptococcus group, responsible for “strep throat.”

b) Rod-shaped: These are known as bacilli (singular bacillus). Some rod-shaped bacteria are curved. These are known as vibrio. Examples of rod-shaped bacteria include Bacillus anthracis (B. anthracis), or anthrax.

c) Spiral: These are known as spirilla (singular spirillus). If their coil is very tight they are known as spirochetes. Leptospirosis, Lyme disease, and syphilis are caused by bacteria of this shape.

• Average size: 0.5 to 1 x 2 to 5 micrometer.
• Longest: Bacillus butschlii is 80 micrometer.
• Shortest : Dialister pneumosintes is 0.15 – 0.31 micrometer.