What are Bacteria and how are they Classified

Bacteria and their Classification




Leeuwenhoek discovered bacteria in 1697 for the first time.


Size and Shape:


Their size range is from lum to 1oum in length and 0.2um to lum in width. On the basis of shape bacteria has four types:


  1. i) Rounded Cocci


  1. ii) Rod like Bacilli


iii) Spiral Shaped Spirilla


  1. iv) Comma shaped Vibrios




Bacteria live both singly and in colonies. Some bacteria occur in groups and some form long beads.




  1. i) Bacteria have a cell wall made up of carbohydrates and amino acids.


  1. ii) Some bacteria has slime capsule around cell wall, which protect them.


iii) Cell membrane is present inside the cell wall and other organelles are lying in cytoplasm.


  1. iv) Nucleus is absent while a single large circular molecule of DNA is present.


  1. v) Small circular molecules of DNA are also present called plasmids.




Bacteria move with the help of flagella. They are called motile bacteria.


Nutrition: – (Types of bacteria on the basis of Nutrition)


  1. i) Photosynthetic bacteria manufacture their own food.


  1. ii) Chemosynthetic bacteria get energy by oxidizing compounds of sulphur and nitrogen.


iii) Heterotrophic bacteria cannot prepare their food and obtain their food from their hosts. Some bacteria are parasites which obtain their food from living things. Some bacteria are saprotrophic which obtain their food from dead remain of living things.




Bacteria reproduce by asexually by simple divisions but some bacteria reproduce sexually.




  1. i) Through droplets.


  1. ii) Through touch.


iii) Through contaminated food and water.


  1. iv) Through insects and other animals.


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