21. Pathogens
Contents of the chapter
21.1 Introduction
Most disease-causing organisms or pathogens are small, and it is not possible to detect them with the naked eye. Such pathogens are called microbes. Bacteria, viruses and microscopic fungi are all examples of microbes.
Humans can also have parasites that grow to large sizes. For example, the tapeworm is a parasite that lives in the human gut and can grow to over a meter in length. The tapeworm causes weight loss and fever, among other things. Such parasites often end up in humans in the form of small eggs or larvae, but later grow larger.
The most effective way to protect yourself against pathogens is through good hygiene. This means eating food that contains the least possible microbes, prepared with clean hands and in clean rooms. Microbes cannot be avoided completely, nor do they have to be, because some of them are hugely beneficial to us.
Washing your hands refularly is one of the central features of good hygiene!
21.2 Human external defense
The human external defense system consists of the skin and the mucous membranes. The tear fluid secreted by the eyes also contains antibacterial ingredients.
Skin consists of an epidermis and a dermis. Your epidermis is very dense and rapidly regenerating. Only fat-soluble substances are absorbed through it. The top layer of the epidermis is scaly, and it is called the stratum corneum. Melanin-secreting cells (melanocytes), in turn, are located in the lower parts of the epidermis. Melanin causes a person's skin to tan, preventing UV radiation from affecting the deeper layers of the skin.
Skin hairs grow from the dermis and open to the sebaceous (=oil) glands. The sebum secreted by the sebaceous glands makes the skin supple and impermeable to water. If the sebaceous gland becomes blocked, the sebum secreted by it accumulates at the of the sebaceous gland. Bacteria can multiply easily in such a sebum layer, causing an inflammation. This inflamed sebaceous gland becomes a zit or a spot.
The dermis also contains the sweat glands and the skin's sensory organs. Temperature and pressure can be sensed on the skin. Blood vessels run right in the lower parts of the dermis. They bring oxygen, nutrients and heat to the skin.
The skin helps to regulate body temperature. In cold weather, the blood vessels in the skin constrict and heat loss is reduced. In warm weather or during exercise, the blood vessels in the skin dilate and release heat. In addition, heat is removed from the body through the sweat produced by the sweat glands.
The mucous membranes protect the digestive system and the genitals. Like the skin, the mucous membrane is very dense, and its surface can be strongly acidic. In addition, the mucus it secretes collects pathogens that travel with the mucus out of the body. The cells in the stomach wall produce hydrochloric acid, which effectively destroys microbes due to its high acidity.
21.3 Human internal defense
The human internal defense system destroys pathogens that have bypassed the body's external defense system. The human internal defense system includes white blood cells and the inflammatory response.
White blood cells move along blood and lymph, but they are also able to penetrate other cells into other tissues in the body.
White blood cells are especially abundant in the liver and in lymph nodes. Some white blood cells are phagocytes.
Some white blood cells, in turn, produce antibodies. Antibodies are used to label harmful targets on eating cells that can destroy them.
Some of the white blood cells that produce antibodies remain as memory cells after the attack of the pathogens. The next time the memory cells recognize the pathogen and quickly produce an antibody. In this case, the person does not even have time to get the symptoms of the disease caused by the bacterium but heals before the onset of the disease (immunity).
In the drawing on the right, a the stick has pierced the skin, making bacteria enter the body. White blood cells migrate from the capillary to the tissue (1 and 2) and destroy bacteria (3 and 4).
21.4 Vaccination
The effectivity of vaccines is based on memory cells. Vaccines contain attenuated pathogens or parts of pathogens. They do not harm the person, but instead cause the formation of memory cells.
When the pathogens are passed on to the person, the memory cells recognize them and immediately produce enough antibodies to fight them.
Vaccines can be given to prevent both bacterial and viral diseases. However, their production is challenging due to the constant transformation of viruses and bacteria.
21.5 Bacteria as pathogens
All microorganisms that we cannot see with the naked eye are considered microbes. Viruses also belong to the group of microbes, although they are not otherwise considered living organisms. Most microbes are completely harmless to humans, but some can act as pathogens.
Bacteria are extremely small microbes: their size ranges from about 0.0005 to 0.7 millimeters in diameter. Although they are the smallest cells in the living world, they are very important to life on Earth. Some bacteria are even able to photosynthesize (e.g. cyanobacteria). Some, in turn, decompose other organisms for food. This also benefits plants that receive some of the nutrients broken down by bacteria. Some bacteria live in symbiosis with other organisms. A couple of kilograms worth of bacteria live in the human intestines and skin. These bacteria are not harmful to humans.
The anatomy of a bacterial cell.
The bacterial cell is surrounded by a cell wall and a cell membrane. The cell is filled with cytoplasm, different membraneous structures and a single chromosome. In addition to the chromosome, small circular plasmids also contain some of the bacterium's DNA. Some bacteria still have a slimy capsule around them and may also have pili (singular pilus) and a flagellum. The pili and the flagellum help the bacterium swim in a moist environment.
Bacteria multiply asexually by dividing in two. The fastest bacterial cells can divide once every 20 minutes. This is why the number of bacteria grows rapidly in favorable conditions. When under unfavorable conditions, bacteria do not multiply at all. However, they can survive challenging conditions, such as boiling water or extreme drought, by "going into sleep". When this occurs, the bacteria transforms its cell structure into a durable resting form.
Bacteria can be controlled with antibiotics. Antibiotics are substances that prevent bacteria from dividing. When the number of bacteria remains low, they do not cause symptoms, and the body’s white blood cells are better able to fight them off. In order for antibiotics to work, their concentration must be kept steadily high in the body. Because of this, a course of antibiotics prescribed by a doctor must be eaten to its very end, even if the symptoms caused by the bacterium disappear before the end of the course. Bacteria can develop a resistance to antibiotics, after which the use of that antibiotic is futile. Bacteria that are resistant to many of the most common antibiotics are called nosocomial bacteria.
Bacteria can be identified by their structure and shape. Today, however, genetic identification methods are more often used, especially in medicine. Some bacteria are also protogenic.
21.6 Viruses
Viruses are not living organisms. On their own, they are capable of nothing until they enter a living cell. Without cells, viruses would not be able to multiply.
Viruses are even smaller than bacteria, and their DNA is protected by a protein coat. The DNA contained in the virus moves inside a living host cell. This causes the cell to make a copy of the virus DNA, producing more viruses. The multiplied viruses are released from the host cell, with the host cell often dying as a result.
Diseases caused by viruses are often troublesome, because the viruses are protected within host cells. Antibiotics have no effect on viruses at all.
The image on the right displays the basic structure of a virus. It is so simple that viruses are not even classified as living organisms. They are always parasitic and need a host to survive. The numbers show various structural features of viruses: 1. surface protein (this allows the virus to recognize its host cell), 2. envelope (derived from the host cell), 3. coat (protein), 4. genetic material.
| Name of the virus | When and where? |
|---|---|
| SARS (Severe Acute Respiratory Syndrome, coronavirus) | A virus that spread across Asia during the year 2003. Approximately 8 000 people were infected, with just under one thousand fatalities. Wikipedia |
| Ebola | An epidemic that affected Africa between the years 2014 and 2016. Approximately 30 000 people were infected, with approximately 13 000 fatalities. Wikipedia. |
| Zika | A virus spread by mosquitoes in South America between the years 2014 and 2016. Millions of people were infected, with no confirmed fatalities. Wikipedia |
| Coronavirus (COVID-19) | Originated in China and spread into a global pandemic in 2019-2020. Wikipedia. |