Viruses are submicroscopic particles made by complicated assemblies of proteins, nucleic acids, lipids, and carbohydrates. These smallest infectious agents can infect a human, animal, plant, or bacterial cells. Viruses are strict obligate intracellular parasites incapable of replication without living hosts. Due to the lack of independent replication capability, viruses are considered non-living.
Bacteria are unicellular prokaryotes. They are ubiquitous and play both beneficial and harmful roles. Bacteria are used as micro-factories in the industries for synthesizing various important compounds, as a starter culture for making yogurt and cheese, and as probiotics to prevent infections. Pathogenic bacteria can cause diseases from small boils to deadly diseases like meningitis or plague.
There are many clear-cut structural differences between bacteria and viruses such as size, cellular components, nature, and size of nucleic acid. Other differences lie in the method of replication, host cell requirements, and weapons used to kill them. The main question which always intrigued many is, are viruses alive?
Some of the major differences between bacteria and viruses are summarized in the table below;
| Variable | Bacteria | Viruses |
| Size Approximate diameter (um)2 | Typical bacterial size ranges from 1-5 μm. Thiomargarita magnifica, the world’s largest bacteria is up to 2 cm long and is visible to the naked eye· It is 50 times bigger than any other known bacteria. | Most virus particles range from 0.02 μm to 0.2 μm. A giant DNA virus called Mimivirus has a particle size of 0.7 μm in diameter. |
| Life | Bacteria are living organisms. | Scientists are not sure whether viruses are living or non-living. There is a long ongoing debate among scientists with proponents and opponents of it. |
| Structure | Bacteria have a nucleus or nucleoid, which contains DNA; this is surrounded by cytoplasm, where proteins are synthesized and energy is generated. | Viruses have an inner core of genetic material (either DNA or RNA) but no cytoplasm, and so they depend on host cells to provide the machinery for protein synthesis and energy generation. |
| Nature of outer surface | Rigid wall containing peptidoglycan | Protein capsid and lipoprotein envelope |
| Motility | Some of the bacteria are motile. These bacteria move by means of flagella. | Viruses are nonmotile |
| Method of Replication | Bacteria replicate by binary fission during which one parent cell divides to make two progeny cells while retaining its cellular structure. | Viruses disassemble, produce many copies of their nucleic acid and protein, and then reassemble into multiple progeny viruses. |
| Host cell requirements for replication | Viruses are obligate intracellular parasites. They must replicate within host cells because they lack protein-synthesizing and energy-generating systems. | With the exceptions of rickettsiae and chlamydiae, which also require living host cells for growth, bacteria can replicate extracellularly. |
| Nature of the nucleic acid | Bacteria contain both DNA and RNA. | Viruses contain either DNA or RNA but not both. |
| Genome size | Bacterial genome size ranges from 0.6 to 8.0 megabases (Mb) and generally encodes 600–6000 proteins. For example, E. coli strain K12 has a genome size of 4.6 million bases (mb). | Viruses vary dramatically in genome sizes. The size of the DNA viruses is larger than that of RNA viruses. The genome size of viruses ranges from the smallest (0.859 kbp) recorded in Circovirus to the largest one (1.9 million base pairs) in Mimivirus. The flu virus (influenza virus) genome contains only 15,000 nucleotides. |
| Examples | Some of the common examples of bacteria are Lactobacillus spp, E. coli, Salmonella Typhi, Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, Neisseria gonorrhea, etc. | Some popularly known viruses are influenza virus (flu virus), rhinovirus, rotavirus, coronavirus, measles virus, HIV, hepatitis B virus, rabies virus, etc. |
* 1 Mb = 1,000,000 bases
You can visit this blog if you are interested in knowing the difference between bacterial and viral infections.
References and further readings
- Brown N & Bhella D (2016). Are Viruses Alive, Issue: What is Life? Microbiology Society.
- What’s the difference between bacteria and viruses? The University of Queensland Australia. Retrieved on 5th July, 2022 from https://imb.uq.edu.au/article/2020/04/difference-between-bacteria-and-viruses
- Claverie JM, Grzela R, Lartigue A, Bernadac A, Nitsche S, Vacelet J, Ogata H, Abergel C. Mimivirus and Mimiviridae: giant viruses with an increasing number of potential hosts, including corals and sponges. J Invertebr Pathol. 2009 Jul;101(3):172-80. doi: 10.1016/j.jip.2009.03.011. Epub 2009 May 18. PMID: 19457438.
- Volland JM, Gonzalez-Rizzo S, Gros O, Tyml T, Ivanova N, Schulz F, Goudeau D, Elisabeth NH, Nath N, Udwary D, Malmstrom RR, Guidi-Rontani C, Bolte-Kluge S, Davies KM, Jean MR, Mansot JL, Mouncey NJ, Angert ER, Woyke T, Date SV. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles. Science. 2022 Jun 24;376(6600):1453-1458. doi: 10.1126/science.abb3634. Epub 2022 Jun 23. PMID: 35737788.
