Viruses are biological entities that parasitize cells in order to replicate. Bacteria are simple cells, which do not have a nucleus. Both are microscopic, while bacteria can be seen with an optical microscope, viruses can only be seen with an electron microscope.
Viruses can enter both bacteria and eukaryotic cells. Those that enter bacteria are called bacteriophages. Viruses are usually specific to the cell they will infect, for example, plant viruses do not infect animals, and vice versa.
Bacteria can live independently in the environment, like the Escherichia coli or lactobacilli. However, there are some cases of intracellular bacteria such as Mycobacterium lepraeagent producing leprosy in humans.
Both bacteria and viruses are widely distributed in nature. They can be harmful or harmless.
When they attack the human being, the immune system is responsible for the response against these microorganisms. Antibiotics act on bacteria, while they have no effect against viruses.
|definition||Biological particles that parasitize cells to multiply||Cells with a simple structure, without a nucleus|
|size||20 nanometers to 250 nanometers
(1000 nm= 1 micrometer)
|Between 1 and 10 micrometers|
Lipid and protein envelope
|Genome||Between 3000 bases and 1 million base pairs.
They have only one class of nucleic acid: deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)
600 thousand to 8 million bases forming a circle.
They have both DNA and RNA
|reproduction||Only when they parasitize a cell||asexual|
|Metabolism||non-existent||Aerobic, anaerobic, facultative|
Escherichia coli: gastrointestinal problems
Salmonella typhi: typhoid fever
Staphylococcus aureus: skin infections
Helicobacter pylori: produces gastritis in humans
Lactobacillus casei: yogurt producer
Viruses are particles composed of nucleic acids, which form the viral genome, surrounded by a protein coat called capsid. Nucleic acid and proteins make up the nucleocapsule. A simple virus particle is called a virion.
There are an estimated 10 quintillion viruses on Earth (1031), for example, up to 5000 viral genomes can be detected in 200 liters of seawater.
Viruses are specific to the organisms they infect: animal viruses, plant viruses and bacteria viruses. Most viruses infect bacteria and are called bacteriophages.
Viruses have no plasma membrane, cytoplasm, ribosomes, mitochondria or other organelles. They are strict intracellular parasites, they must infect living cells to take advantage of their metabolic machinery.
Structure of a virus
The basic structure of a virion consists of:
- Nucleic acid center: it can be single-stranded or double-stranded deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The viral genome codes for the viral proteins that replicate the virus and/or form the capsid.
capsid: a protein coat that surrounds the nucleic acid. It is made up of protein subunits called capsomeres. There are three types according to the arrangement of proteins: icosahedral (20 equal faces), helical and complex.
The proteins protect the nucleic acid, identify the target cell, and introduce the viral genome into the cell.
- Lipid envelope: some viruses surround themselves with a phospholipid membrane that they take from the host cell. For example, the human immunodeficiency virus HIV.
The most basic classification of viruses is based on the type of nucleic acid they contain: RNA viruses or DNA viruses. The Baltimore classification system is a more recent classification that groups viruses into 7 classes, according to how the messenger RNA is produced in the replication process.
Diseases caused by viruses
measles: caused by RNA viruses of the genus Morbillivirus, family of paramyxoviruses. It only infects humans.
anger: caused by RNA viruses of the genus Lyssavirus, family Rhabdoviridae. It infects animals and humans.
dengue fever: caused by RNA viruses of the genus Flavivirus, family Flaviviridae. Transmitted to humans by mosquito bites.
common cold: mainly caused by RNA viruses of the genus enterovirusPicornaviridae family.
Flu or flu: caused by RNA viruses with several genera, such as influenza A, B, C and D, family Orthomyxoviridae. It infects mammals.
Poliomyelitis: caused by poliovirus, an RNA virus of the genus enterovirusPicornaviridae family.
Ebola: caused by RNA viruses of the genus EbolavirusFiloviridae family.
COVID-19: produced by the SARS-CoV-2 coronavirus, an RNA virus of the genus Betacoronavirus, family Coronaviridae.
AIDS: produced by the human immunodeficiency virus HIV, an RNA virus of the genus Lentivirusfamily Retroviridae.
Hepatitis B: produced by the DNA virus Orthohepadnavirusfamily Hepadnaviridae.
Bacteria are prokaryotic cells, that is to say, cells that do not have their genetic material enclosed in a nucleus. They are the most basic and primitive living beings. They are everywhere and can live in extreme conditions.
The structure of the bacterium is simple: a plasma membrane that hides the cytoplasm, where the genetic material is. They also have ribosomes in the cytoplasm, responsible for producing bacterial proteins.
Some bacteria have motile structures in their cell membrane: cilia and flagella. These allow them to move in the environment in which they develop. Outside the bacterial cell there may or may not be a cell wall, a rigid structure that protects the cell.
Bacteria live as independent cells and reproduce asexually by binary fission. The genetic material is duplicated and separated at the poles, to then form a crack in the center that gives rise to two daughter bacteria.
Types of bacteria
Most bacteria have an extracellular structure called a cell wall. Depending on the composition of the cell wall, bacteria are classified into:
- Gram-positive bacteria: bacteria with a thick cell wall formed by a thick layer of peptidoglycan, such as staphylococci.
- Gram-negative bacteria: bacteria with a thin peptidoglycan cell wall and a phospholipid outer membrane, such as meningococci.
Diseases caused by bacteria
syphilis: produced by the spirochete Treponema pallidum.
tuberculosis: produced by Koch’s bacillus, the Mycobacterium tuberculosis.
Cholera: produced by vibrios Vibrio cholerae.
leprosy: produced by Mycobacterium leprae.
botulism: produced by Clostridium botulinum.
tetanus: produced by Clostridium tetani.
You may also be interested in seeing:
Amyes, SGB (2013) Bacteria-A very short introduction. Oxford University Press. UK.
Bravo A, M. et al. (2007) Biology II. Santillana Essential Manual. Santillana del Pacífico Editions. Chile.
Castañeda P., P. et al. (2007) Biology I Essential Santillana Manual. Santillana del Pacífico Editions. Chile.
Dimmock, DJ, Easton. AJ, Leppard, KN (2016) Introduction to modern virology 7th ed. Wiley Blackwell. UK.