Viruses are biological entities that parasitize cells in order to replicate. Bacteria are simple cells that do not have a nucleus. Both are microscopic, while bacteria can be seen with a light microscope, viruses can only be seen with an electron microscope.
Viruses can enter both bacteria and eukaryotic cells. Those that enter the bacteria are called bacteriophages. Viruses are usually specific to the cell they are going to infect, eg plant viruses do not infect animals, and vice versa.
Bacteria can live independently in the environment, such as Escherichia coli or lactobacilli. However, there are some cases of intracellular bacteria such as Mycobacterium lepraeagent that causes leprosy in humans.
Both bacteria and viruses are widely distributed in nature. They can be harmful or harmless.
When they attack humans, the immune system is responsible for the response against these microorganisms. Antibiotics act on bacteria, while they have no effect against viruses.
Viruses Bacteria Definition Biological particles that parasitize cells to multiply Cells with a simple structure, without a nucleus Distribution Global Global Size 20 nanometers to 250 nanometers
(1000 nm= 1 micrometer) Between 1 and 10 micrometers Structure Nucleocapsule:
lipid and protein envelope
bacterial genome
Cytoplasm
plasma membrane
Cellular wall
Genome Between 3000 bases and 1 million base pairs.
They have only one kind of nucleic acid: deoxyribonucleic acid (DNA), or ribonucleic acid (RNA).
600 thousand to 8 million bases forming a circle.
They contain both DNA and RNA
Reproduction Only when parasitizing a cell Asexual Metabolism Non-existent Aerobic, anaerobic, facultative Examples
adenovirus
Rotavirus: producer of diarrhea in children
Morbillivirus: measles virus
influenza viruses
coronavirus
Human immunodeficiency virus: producer of AIDS
Escherichia coli: Gastrointestinal disorders
Salmonella typhi: typhoid fever
Staphylococcus aureus: skin infections
Helicobacter pylori: causes gastritis in humans
Lactobacillus casei: yogurt producer
Virus
Viruses are particles composed of nucleic acids, which form the viral genome, surrounded by a protein cover called capsid. Nucleic acid and proteins make up the nucleocapsule. A single virus particle is called a virion.
It is estimated that there are 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 bacterial viruses. Most viruses infect bacteria and are called bacteriophages.
Viruses do not have a plasma membrane, cytoplasm, ribosomes, mitochondria, or other organelles. They are strict intracellular parasites, they have to infect living cells to take advantage of their metabolic machinery.
structure of a virus
Virus capsids can be icosahedral (polio virus), helical (tobacco mosaic virus), or complex (bacteriophage T4).
The basic structure of a virion consists of:
nucleic acid center: 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.
capsidA 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 the 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 it contains: RNA virus or DNA virus. The Baltimore classification system is a more recent classification that groups viruses into 7 classes, based on the messenger RNA produced in the replication process.
viral cycle
Diseases caused by viruses
Measles: caused by RNA viruses of the genus Morbillivirus, family of paramyxoviruses. It only infects humans.
Rage: caused by RNA viruses of the genus Lyssavirus, family Rhabdoviridae. It infects animals and humans.
Dengue: caused by RNA viruses of the genus Flavivirus, family Flaviviridae. Transmitted to humans by mosquito bites.
Common cold: caused mainly by RNA viruses of the genus enterovirusesfamily Picornaviridae.
Flu or Influenza: caused by RNA viruses with several genera, depending on whether they are influenza A, B, C and D, family Orthomyxoviridae. It infects mammals.
Poliomyelitis: caused by the poliovirus, an RNA virus of the genus enterovirusesfamily Picornaviridae.
ebola: caused by RNA viruses of the genus ebola virusfamily Filoviridae.
COVID-19: produced by the coronavirus SARS-CoV-2, an RNA virus of the genus Betacoronavirus, family Coronaviridae.
AIDS: produced by the human immunodeficiency virus HIV, an RNA virus of the genus lentivirusesfamily Retroviridae.
Hepatitis B: produced by the DNA virus orthohepadnavirusfamily Hepadnaviridae.
bacteria
Bacteria are prokaryotic cells, that is, 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 plasmatic membrane that encloses the cytoplasm, where the genetic material is found. They also have ribosomes in the cytoplasm, which are responsible for producing bacterial proteins.
Some bacteria have mobile 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 duplicates and separates at the poles, to then form a cleft in the center that gives rise to two daughter bacteria.
types of bacteria
Bacteria that have a cell wall can be classified into Gram-positive and Gram-negative bacteria.
Most bacteria have an extracellular structure called the 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 Mycobacterium tuberculosis.
Anger: produced by the vibrio Vibrio cholerae.
Leprosy: produced by Mycobacterium leprae.
Botulism: produced by Clostridium botulinum.
Tetanus: produced by the Clostridium tetani.
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References
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 Pacifico Editions. Chili.
Castañeda P., P. et al. (2007) Biology I Essential Santillana Manual. Santillana del Pacifico Editions. Chili.
Dimmock, DJ, Easton. AJ, Leppard, KN (2016) Introduction to modern virology 7th ed. Wiley Blackwell. UK.