Viruses, bacteria, parasites, fungi, and algae – a layperson's view
Introduction
During this extraordinary period of COVID-19
pandemic, the media use the word virus endlessly every day. Among others, the
media remind the viewers repetitively how long the virus might remain active on
various surfaces. So, the layperson asks: Is a virus a living organism? What is
the size of a virus? How are viruses different from bacteria, or parasites, or
fungi, or algae? These and similar questions cross a layperson's mind but are
not answered by the media.
This article attempts to address some of these
questions. It is not a comprehensive narrative on the subject. Far from it, this
article only touches upon a few of the relevant terminologies and tries to
demystify those for the layperson. The author wishes to disclose here that he
is not a medical doctor, or virologist, or microbiologist. He is a layperson who delved into the subject
to satiate his curiosity. He searched reliable sources and collated simple
answers to some of the questions and thought it might be of interest to other
laypersons.
The naming of pathogens and diseases
Any virus, bacterium, fungus, or another
microorganism that can cause a disease is a pathogen. A pathogen may also be
referred to as an infectious agent, or merely a germ. (However, this germ is
not to be confused with the word germ that is used concerning plants). The term
pathogen came into use in the 1880s. A pathogen becomes pathogenic when it has reached
a cluster size that is large enough to cause disease.
A pathogen and the associated disease do not
necessarily have the same name. Today, the two most used words are COVID-19 and
coronavirus. While COVID-19 is the name of the disease, the virus causing the
disease is not named 'coronavirus'. Because there are many different types of coronaviruses,
many of which cause disease in mammals and birds. The type of coronavirus that
causes COVID-19 is called SARS-CoV-2, which stands for severe acute respiratory
syndrome coronavirus 2. This name is appropriate because the virus is
genetically related to an earlier type of coronavirus that caused the SARS outbreak
of 2003; that virus was then named SARS-CoV.
While these two types of coronaviruses belong to the same family, they
are nonetheless different. Hence, they have two separate names.
There are different processes, and purposes,
for naming viruses and diseases. Viruses are named based on their genetic
structure. This facilitates the development of diagnostic tests, vaccines, and
medicines. Virologists and the broader scientific community do this work. Viruses
are named by the International Committee on Taxonomy of Viruses (ICTV). (Taxonomy is the science of naming,
describing, and classifying organisms and includes all plants, animals, and
microorganisms of the world). Diseases are named to enable discussion on
disease prevention, spread, transmissibility, and treatment. Human disease
preparedness and response is WHO's role. So diseases are named by WHO in the
International Classification of Diseases (ICD)
This article deals with some of the pathogens
that cause diseases only in human beings, and not in plants, animals, birds, or
any non-human living entity.
Viruses
A virus is an infectious agent of small size
and simple composition that can multiply only in living cells of humans, animals,
plants, and bacteria as they do not have their own mechanism to multiply. When
some disease-causing viruses enter host cells, they start making new copies of
themselves very quickly. They often outpace the immune system's production of
protective antibodies. Rapid virus multiplication can result in cell death and the
spread of the virus to nearby cells.
Viruses are only active while intracellular,
i.e., they are inside host cells, taking control of those cells' mechanisms and
stealing their energy. In contrast, bacteria and parasites can live on the
surface of the host's body or inside the host's organs and tissues.
Reproduction is another significant difference between viruses and bacteria and
parasites. Viruses can multiply inside host cells but cannot reproduce while bacteria
and parasites can reproduce. Thus, viruses are not parasites.
Viruses occupy a unique taxonomic position. They
are not plants, animals, or bacteria, and they occupy their separate kingdom. Viruses
should not even be considered organisms in the strictest sense because they are
not free-living. They cannot reproduce and carry on metabolic processes without
a host cell. All viruses contain nucleic acid and protein. The nucleic acid can
be either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). The nucleic
acid encodes the genetic information unique for each virus
Figure 1 SARS-CoV-2, courtesy Encyclopaedia Britannica / Patrick O'Neill Riley
As for their size, most viruses vary in
diameter from 20 nm (nm = nanometre = 10-9 metre, or one billionth
of a metre) to 300 nm. SARS-CoV-2 virus that causes COVID-19 is 60-140 nm in
size
Recently, during COVID-19 attack, in India, the
first laboratory-confirmed infection by SARS-CoV-2 was on January 30, 2020. The
throat swab specimen had tested positive for SARS-CoV-2 nucleic acid by reverse
transmission polymerase chain reaction (RT-PCR) test. The test used
transmission electron microscopy (TEM)
Viral infections and treatments
Some of the common viral diseases are the common
cold, smallpox, rabies, infectious hepatitis, measles, poliomyelitis,
influenza, viral encephalitis, mumps, herpes simplex, viral conjunctivitis,
Ebola, SARS, and the latest COVID-19.
As there is no medicine to treat most of the
viral diseases, the first step in treating a viral infection is preventing its
occurrence and spread. Vaccines are available to prevent some common viral
infections, such as chickenpox, shingles, influenza, HPV, hepatitis B,
hepatitis A, measles, and mumps. The feverish race to discover vaccines for
COVID-19 is presently ongoing. Treatment of viral infections is symptomatic and
generally includes rest, increased fluids, good nutrition, and may require
hospitalization and intensive care, mostly if complications occur. Antibiotics
treat bacterial infections and are ineffective for the treatment of viral
infections. Sometimes drugs called antiviral drugs may be used to help treat
certain types of viral infections. Most antiviral drugs can only help to minimize
the severity of a viral infection but cannot cure the infection
Bacteria, singular bacterium, are a group of
microscopic single-celled organisms that occupy a separate kingdom. They live
in enormous numbers in almost every environment on Earth everywhere from
deep-sea vents, to deep below Earth's surface, to the digestive tracts of
humans. Bacterial cells differ from animal cells and plant cells in several
ways. One fundamental difference is that bacterial cells lack intracellular
organelles, such as mitochondria, chloroplasts (present only in plant cells,
not in animal cells), and a nucleus, which are present in both animal cells and
plant cells. Bacteria do not have a nucleus, but, instead, generally have a
single chromosome: a piece of circular, double-stranded DNA located in an area
of the cell called the nucleoid, or more appropriately plasmid (Figure 2).
Figure
2 Bacteria cell, animal cell, and plant cell,
courtesy Encyclopaedia Britannica
Bacteria lack a membrane-bound nucleus and
other internal structures. Therefore, they rank among the unicellular
life-forms called prokaryotes. Prokaryotes are the dominant living creatures on
Earth. They have been present for perhaps three-quarters of Earth history and
have adapted to almost all available ecological habitats. As a group, they
display exceedingly diverse metabolic capabilities. They can use almost any
organic compound, and some inorganic compounds, as a food source.
Some bacteria can cause diseases in humans,
animals, or plants. Still, most are harmless and are beneficial ecological
agents whose metabolic activities sustain higher life-forms. Humans carry more
bacteria in their gut than cells in their body. Some can cause diseases, but
many serve useful functions. For example, good bacteria in humans prevent
infections, provide micronutrients, help in the development of immunity. Other
bacteria are symbionts of plants and invertebrates, where they carry out essential
functions for the host, such as nitrogen fixation and cellulose degradation.
Without prokaryotes, the soil would not be fertile, and dead organic material
would decay much more slowly. Some bacteria are widely used in the preparation
of foods, chemicals, and antibiotics. Studies of the relationships between
different groups of bacteria continue to yield new insights into the origin of
life on Earth and mechanisms of evolution
Individual bacteria can assume one of three
basic shapes: spherical (coccus), rod-like (bacillus), or curved (vibrio,
spirillum, or spirochete).
Bacteria are the smallest living entities. An
average-size bacterium—such as the rod-shaped Ecoli, a typical inhabitant of
the intestinal tract of humans and animals—is about 2 μm (μm = micrometre = 10-6
metre, or one-millionth of a metre) long and 0.5 μm in diameter. The
spherical cells of Staphylococcus aureus are up to 1 μm in diameter. A few
bacterial types are even smaller. Thus, bacteria are many times bigger in size
than viruses.
Antoni van Leeuwenhoek discovered bacteria, in
1676, using a single-lens microscope of his own design. The pathogenic bacteria
cause significant human death and disease. They cause infections such as
tetanus, typhoid fever, diphtheria, syphilis, cholera, foodborne illness,
leprosy, and tuberculosis. Bacterial infections may be treated with antibiotics
Sir Alexander Fleming, a Scottish biologist,
defined new horizons for modern antibiotics with his discovery of the
antibiotic substance penicillin (1928). The discovery of penicillin (from the
fungus Penicillium notatum), and subsequent discoveries of other antibiotics
and anti-bacterial drugs perfected the treatment of bacterial infections such
as syphilis, gangrene, and tuberculosis
Parasites are organisms
that live in or on an organism of another species (its host) and benefits by
deriving nutrients at the other's expense. Parasites vary in size widely.
Around 70 per cent parasites are not visible to the human eye, such as the
malarial parasite. Still, some worm parasites can reach over 30 m in length.
Human parasites include
various protozoa and worms that may infect humans that cause parasitic
diseases. Human parasites are divided into endoparasites, which cause infections
inside the body, and ectoparasites, which cause infection superficially within
the skin. Malaria, kala-azar, amoebiasis, and giardiasis are some of the common
diseases caused by endoparasites. Louse,
tick, flea, mosquito, bed bug, mite, are ectoparasites.
Generally, the
discovery of parasites in ancient humans relies on the study of faeces and
other fossilized material. The earliest known parasite in a human was eggs of
the lung fluke found in fossilized faeces in northern Chile and is estimated to
be from around 5900 BC. The first written records of parasites date from 3000
to 400 BC in Egyptian papyrus records. They identify parasites such as
roundworms, Guinea worms, threadworms, and some tapeworms of unknown varieties.
In ancient Greece, Hippocrates and Aristotle documented several parasites in the
collection of works Corpus Hippocraticus
Malaria has ravaged the
population of many countries and killed millions of humans. Malaria parasites
were discovered in 1880 in France. Seven years later, in India, Sir Ronald Ross
discovered that mosquitoes transmit the malaria parasite. For this discovery,
he received his Nobel Prize in 1902
Fungi and algae
Fungi, singular fungus, are plantlike organisms
lacking chlorophyll, such as mushrooms, moulds, yeasts, and mildews. Modern
biologists tend to place fungi in their own kingdom, not in the plant kingdom
because they get their nutrients from other living things (or from the remains
of living things that have died) rather than from photosynthesis.
Common diseases caused by fungi are: (a) skin
diseases such as ringworm and athletes foot, (b) inflammation of mucous
membranes like mouth, tongue, and vagina (commonly known as thrush), (c) ear
infection like otomycosis, and (d) eye diseases like corneal infection called
fungal keratitis. Fungal infection is killed naturally with home remedies like
antifungal ointments, coconut oil, apple cider vinegar, and borax. Fungi can
also cause serious life-threatening infections in immune-compromised patients.
Algae, singular alga, are in a different group
of organism than fungi and belong to their own kingdom. Algae do not directly infect
humans, but toxins produced by algae can cause diseases—for example, cell fish
poisoning or some drinking water poisoning. Algae may cause the following
diseases in humans: auto-immune diseases such as multiple sclerosis (MS),
lupus, rheumatoid arthritis, and amnesic shellfish poisoning.
Acknowledgement
The author is grateful to his friend and next-door neighbour, Dr Susanta Ghosh, PhD, FRCP, a distinguished haematologist, for reviewing this article.
References
Archivist, London School of Hygiene & Tropical
Medicine. (2015, 09 16). Ross and the Discovery that Mosquitoes Transmit
Malaria Parasites. Retrieved from Centers for Disease Control and
Prevention: https://www.cdc.gov/malaria/about/history/ross.html
Bing.com/images. (n.d.). Images of SARS-CoV-2.
Retrieved from
https://www.bing.com/images/search?q=sars-cov-2+size+microns&qpvt=sars-cov-2+size+microns&form=IGRE&first=1&scenario=ImageBasicHover
CMM International. (2017, 06 23). The Launch of
the World's First Microsphere Nanoscope. Retrieved from CMM International
Magazine:
http://www.cmmmagazine.com/metrology/the-launch-of-the-world-s-first-microsphere-nanoscope/
Diagnosis, R. (2015, 08 13). Treatment of viral
diseases. Retrieved from Right Diagnosis:
https://www.rightdiagnosis.com/v/viral/treatments.htm
Explorable. (2020). History of Antibiotics.
Retrieved from Explorable: https://explorable.com/history-of-antibiotics#:~:text=Sir%20Alexander%20Fleming%2C%20a%20Scottish%20biologist%2C%20defined%20new,bacterial%20infections%20such%20as%2C%20syphilis%2C%20gangrene%20and%20tuberculosis.
Goldsmith, C. S. (2009). Modern Uses of Electron
Microscopy for Detection of Viruses. Retrieved from Clinical Microbiology
Reviews - American Society of Microbiology:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2772359/
Nouri, A. (2011, 07 05). The discovery of
bacteria. Retrieved from AAAS: https://www.aaas.org/discovery-bacteria
Prasad S, P. V. (2020). Transmission electron
microscopy imaging of SARS-CoV-2. Retrieved from IJMR Indian Journal of
Medical Research: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7224615/
Rogers, K. (2019, 06 19). Bacteria. Retrieved
from Encyclopaedia Brtannnica:
https://www.britannica.com/science/bacteria/The-cell-envelope
Wagner, R. R. (2020, 08 18). Virus. Retrieved
from Britannica:
https://www.britannica.com/science/virus/Malignant-transformation
Wikipedia. (2020, 08 26). Bacteria. Retrieved
from Wikipedia: https://en.wikipedia.org/wiki/Bacteria
Wikipedia. (2020, 05 13). Human parasite.
Retrieved from Wikipedia:
https://en.wikipedia.org/wiki/Human_parasite#:~:text=Generally%2C%20the%20discovery%20of%20parasites%20in%20ancient%20humans,is%20estimated%20to%20be%20from%20around%205900%20BC.
World Health Organization. (2020). Naming the
coronavirus disease (COVID-19) and the virus that causes it. Retrieved
from World Health Organization:
https://www.who.int/emergencies/diseases/novel-coronavirus-2019/technical-guidance/naming-the-coronavirus-disease-(covid-2019)-and-the-virus-that-causes-it
Comments
Post a Comment