Bacteria and Virus: Introduction, Differences, and Keynotes

Introduction of Bacteria and Virus

Bacteria are member of a large group of living unicellular microbes having size of cocci 0.5-1.0 µm while bacilli 1-10 µm×3-10µm which have cell walls but lacking an organized nucleus, including some that can cause disease like pneumonia, tuberculosis, food poisoning, etc. They are visible under light microscope. Some example of Gram positive bacteria (Staphylococcus, Pneumococcus, Streptococcus pyogenes, Streptococcus agalactiae, Enterococcus species) and Gram negative bacteria (E. coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus, Citrobacter, Enterobacter, Salmonella, and Shigella)

Staphylococcus aureus growth on blood agar of clinical sample, pus
Fig. Staphylococcus aureus growth on blood agar of clinical sample, pus

Virus is an infective agent that typically consists of a nucleic acid (either DNA or RNA) molecule in a protein coat, is too tiny to be seen by light microscopy, and is able to multiply only within the living cells of a host (obligate intracellular parasite). It is visible only under electron microscope and having size of smallest-parvovirus-20 nm while largest poxvirus-300 nm. Few viral diseases are influenza, AIDS, measles, polio, and COVID-19. Some example of viruses- DNA virus (Hepatitis B virus, human papillomavirus ) and RNA virus (HIV, SARS-CoV-2, Hepatitis C virus and Dengue virus)

Dengue virus NS1 antigen positive, IgM and IgG-negative
Fig. Dengue virus NS1 antigen positive, IgM and IgG-negative

Differences between Bacteria and Virus

The differences between bacteria and viruses are as follows-

S. No.CharactersBacteriaVirus
1Plasma membraneYesNo
2Intracellular parasiteNo (except Rickettsia and Chlamydia)Yes
4Possess both DNA and RNAYesNo
5ATP-generating metabolismYes (except Chlamydia)No
6Binary fissionYesNo
7Pass through bacteriological filtersYes (except Chlamydia)No
8Sensitive to antibioticsYesNo
9Sensitive to interferonNoYes
10Living organismsYesNo
11Quorum sensingYesNo
12MicroscopyCompound microscopeElectron microscope
13Size of MicrobesCocci 0.5-1.0 µm while bacilli 1-10 µm×3-10µmSmallest-Parvo virus-20 nm while largest –Pox virus-300 nm
14Disease causedPneumonia, tuberculosis, food poisoning, and tetanusInfluenza, AIDS, measles, polio, and COVID-19
Table: Bacteria Vs Virus

Keynotes on Bacteria and Virus

  • Rickettsia and Chlamydia of bacteria are intracelluar parasites.
  • Virus is acellular organism due to lacking typical cell.
  • Causative agent of COVID-19 is severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2).


  1. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  2. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  3.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.

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