Difference Between Bacteria and Virus

Main Difference – Bacteria vs Virus

Bacteria and viruses are microscopic microbes. Bacteria are prokaryotes. They are living cells which can be either beneficial or harmful to other organisms. But, viruses considered to be particles that are somewhere between living and non-living cells. Viruses have to invade the body of a host organism in order to replicate their particles. Therefore, most viruses are pathogenic. The main difference between bacteria and virus is that bacteria are living cells, reproducing independently and viruses are non-living particles, requiring a host cell for their replication.

This article explains,

1. What are Bacteria
– Classification, Cellular Structure, Metabolism
2. What is a Virus
– Structure, Classification
3. What is the difference between Bacteria and Virus

What are Bacteria

Bacteria are prokaryotes found in most habitats on the Earth. They are unicellular microorganisms. Bacteria can grow in harsh conditions like acidic hot springs, radioactive waste and deep portions of Earth’s crust. Bacteria form dense aggregations by attaching to surfaces. These aggregations are mat-like structures called biofilms.

Classification of Bacteria

Bacteria can be categorized depending on their morphology. Cocci are the spherical-shaped bacteria. Bacillus are the rod-shaped bacteria. Comma-shaped bacteria are called as vibrio and spiral-shaped bacteria are spirilla and tightly coiled ones are called as spirochaetes. Some bacteria live as single cells. But, some of them live in pairs and are known as diploids. Streptococcus are the bacterial chains. Staphylococcus form ‘bunch of grapes’ like clusters. Filaments are the elongated bacteria like Actinobacteria. Some are branched filaments such as Nocardia.

Figure 1: Cocci

Cellular Structure of Bacteria

Bacterial cells are surrounded by a cell membrane. The membrane-enclosed cytoplasm contains nutrients, proteins, DNA and other essential components of the cell. Bacteria are prokaryotes and lack membrane-bound organelles. Protein localization is carried out by their cytoskeleton. A single, circular chromosome is found in the nucleoid. This simple arrangement of bacteria is referred to as ‘bacterial hyperstructures’.

Murein forms a cell wall outside of the bacterial cell membrane. The thicker cell wall is classified as gram-positive, and the thinner cell wall is classified as gram-negative in the gram staining of bacteria. Flagella are used for the mobility. Fimbriae are the attachment pili. They are used in the sexual reproduction of bacteria, which is known as conjugation. The entire cell is covered by glycocalyx which forms the capsule.

Some genera of gram-positive bacteria form a resistant, dormant structures called endospores. Endospores contain little cytoplasm, DNA and ribosomes, covered by a cortex. They are resistant to radiation, detergents, disinfectants, heat, freezing, pressure and desiccation.

Metabolism of Bacteria

Depending on the carbon source, bacteria can be divided into two groups: heterotrophs and autotrophs. The carbon source is organic compounds in heterotrophs whereas the carbon source is carbon dioxide in autotrophs. Depending on the energy source, bacteria can be divided into three groups: phototrophs, lithotrophs or organotrophs.

What is a Virus

A virus is a particle considered as non-living form. Viruses show neither respiration nor metabolism. A virus consists of its genetic material, either DNA or RNA, covered by a protein core. Usually, viruses are infectious agents, requiring a host for their replication. They infect all life forms including animals, plants, bacteria and archaea. Viruses can be found in almost every ecosystem on the Earth. Thus, they are the most abundant biological entity type. The study of viruses is called virology. Viruses can be visualized by negative staining.

Structure of Viruses

The complete virus particle is referred to as the virion. Virion consists of genetic material enclosed by a protective protein coat called as the capsid. The capsid is formed by identical protein units called capsomeres. The capsid proteins are encoded by the viral genome. The virion consists of a cell membrane derived from the host cell called the lipid envelope. Viral nucleic acid is associated with nucleoproteins. Viral capsid proteins and nucleoproteins are collectively called nucleocapsid.

An enormous structural diversity in the genome is found in viruses when compared to the diversity of plants or animals. A virus may contain either DNA or RNA genome. Hence, two groups of viruses can be identified: DNA viruses and RNA viruses. Most viruses contain RNA genomes. Single-stranded RNA genomes can be found in plant viruses. Double-stranded DNA genomes can be found in bacteriophages.

Classification of Viruses

The ICTV (International Committee on Taxonomy of Viruses) classification is the current classification system used for viruses. The general taxonomy structure consists of order, family, subfamily, genus, and the species. Caudovirales, Herpesvirales, Ligamenvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales are the currently established seven orders in viruses. Moreover, viruses are classified depending on the mechanism used to produce their mRNA. This classification system is called Baltimore classification. According to this classification, seven groups of viruses can be identified: dsDNA viruses, ssDNA viruses, dsRNA viruses, dsRNA viruses, (+)ss RNA viruses, (-)ss RNA viruses, ssRNA-RT viruses, and dsDNA-Rt viruses.

On the contrary, four groups of viruses can be identified depending on the morphology: helical, icosahedral, prolate and envelope. The capsid forms a helical structure around the central axis in helical viruses. Icosahedral viruses sometimes consist of a chiral icosahedral symmetry. In prolate, the icosahedron is elongated into a five-fold axis as in bacteriophages. In some viruses, the cell membrane forms a modified form called the envelope. These type of viruses are referred to as envelope viruses. An icosahedral-shaped, Simian virus is shown in figure 2.

Figure 2: Simian Virus

Dependence on the Host for Reproduction

Bacteria: Bacteria do not need a host organism for reproduction.

Virus: Viruses replicate only inside the host.

Living Attributes

Bacteria: Bacteria are living organisms.

Virus: Viruses are considered as organic structures which interact with living organisms, rather than a living organism.

Size

Bacteria: Bacteria are larger, about 1000 nm in size. They are visible under light microscope.

Virus: Viruses are smaller, about 20-400 nm in size. They are visible under the electron microscope.

Cell Wall

Bacteria: Bacteria contain a Peptidoglycan/ Lipopolysaccharide cell wall.

Virus: Viruses don’t have a cell wall. A protein coat is present instead.

Number of Cells

Bacteria: Bacteria are unicellular.

Virus: Viruses don’t have cells.

Genetic Material

Bacteria: A single, circular chromosome is present.

Virus: DNA/RNA strand is present.

Ribosomes

Bacteria: Ribosomes are present.

Virus: Ribosomes are absent.

Metabolism

Bacteria: Bacteria show metabolism within the cell.

Virus: There is no metabolism inside the viral particle.

Reproduction

Bacteria: Reproduction happens through binary fission and conjugation.

Virus: Virus invades the host cell, makes copies of genetic material and proteins, and releases new particles by destroying the cell.

Cellular Machinery

Bacteria: Bacteria possess a cellular machinery.

Virus: Virus lack cellular machinery.

Benefits

Bacteria: Bacteria can be either beneficial or harmful.

Virus: Viruses are usually harmful, can be useful in genetic engineering.

Infection

Bacteria: Bacteria cause localized infections.

Virus: Virus cause systemic infection.

Duration of Illness

Bacteria: Diseases caused by bacteria last longer than 10 days.

Virus: Diseases caused by viruses last 2 to 10 days.

Fever

Bacteria: Bacteria cause fever.

Virus: Viruses may or may not cause fever.

Treatments

Bacteria: Bacterial infections can be prevented by Antibiotics.

Virus: Spread of viruses can be prevented by vaccines.

Examples

Bacteria: Staphylococcus aureus, Vibrio cholera, etc. are examples of bacteria.

Virus: HIV, Hepatitis A virus, Rhino Virus, etc. are examples of viruses.

Diseases/ Infections

Bacteria: Food poisoning, gastritis, ulcers, meningitis, pneumonia, etc. are caused by bacteria.

Virus: AIDS, common cold, influenza, chickenpox, etc. are caused by viruses.

Conclusion

Bacteria and viruses are both microscopic microbes. Both of them can cause diseases in plants and animals. Both these types of microbes contain enzymes required for the DNA replication and protein synthesis. But, viruses require a host organism for the production of viral coat proteins. Therefore, they should invade a second organism for their replication. On the other hand, bacteria can reproduce independently by binary fission. Both microbes consist of a huge diversity compared to other life forms. The key difference between bacteria and virus is the consideration of each form as a living or non-living organism.

Reference:
1.“Bacteria”. Wikipedia, the free encyclopedia, 2017. Accessed 01 March 2017
2.“virus”. Wikipedia, the free encyclopedia, 2017. Accessed 01 March 2017

Image Courtesy:
1. “108897” (Public Domain) via Pixabay
2. “Symian virus” By Phoebus87 at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia