In the mid-seventeenth century, the microscope was invented by Antoni van Leeuwenhoek. With the help of this instrument he discovered many kinds of ‘small animals’, which he together with the term microbes, later referred to as micro-organisms. Because Louis Pasteur linked them to certain diseases, research began on the large number of different species that had a relationship with disease. Afterwards, more and (much) smaller organisms could be made visible by means of the electron microscope. Microorganisms are in principle everywhere, except in healthy tissue and on man-sterilized materials. Below is a brief description of various types of micro-organisms known to date.
Bacteria are single-cell organisms that multiply by cell division. They usually need a certain temperature and acidity of the environment to be able to divide. Extremely large numbers can be formed in a very short time. Most bacteria die during dehydration or when it gets too cold or too hot. However, there is a group that can form traces in unfavorable conditions. These traces can often survive under harsh conditions for a long time (sometimes even hundreds of years). If the conditions improve afterwards, bacteria will develop from these tracks.
A non-imaginary threat to humanity can arise through the large-scale thawing of the permafrost by global warming. This exposes areas where contact with humans and animals could not take place for a very long time and where ‘old’ micro-organisms are waiting for spreading. As the first ‘returned’ pathogen, the Bacillus anthracis (anthrax), which caused the deadly anthrax, was found on a large scale in reindeer in Finland.
It happens regularly that bacteria undergo changes and create new diseases in a slightly different form. Scientists are, for example, afraid of the arrival of a new form of the bubonic plague bacterium – Yersinia pestis – but other bacteria could also threaten humanity with new and more aggressive forms.
Bacteria can be divided into shape, colonization mode and whether or not they are oxygen-dependent. Below is a summary of the various species. In addition, examples of diseases that they can produce are included.
- spherical (cook), example: meningococcus Neisseria meningitidis, causative agent of meningitis;
- rod-shaped (bacillus), example: Clostridium tetani, causative agent of tetanus;
- spirally (spiril, spirochete), example: Borrelia burgdorferi, cause of Lyme disease
Classification according to colonization structure
- streptococci, lying as a string or strictly next to each other. Example: Streptococcus mutans, involved in the development of caries;
- staphylococci, are clustered together. Example: Staphylococcus aureus, instigator of, among others, boils, hives and the highly contagious ‘currant beard’ (impetigo)
- diplokocks are two by two. Example: gonococcus Neisseria gonorrhoeae causing gonorrhea.
Classification according to oxygen dependence
- living with oxygen, these bacteria are called aerobic;
- live without oxygen, these bacteria are called anaerobic (anaerobic bacteria are found, among other things, in pockets).
Some bacterial species are pathogenic to humans due to the pathogenic toxins (toxins) they secrete during their metabolism. Cooking or baking food does kill the bacteria, but the toxins that are formed remain intact. This can sometimes cause food poisoning. Raw food that is stored in the refrigerator will spoil after some time, because bacterial growth is inhibited but does not stop completely! As long as there are live bacteria in the food, they can also continue to produce toxins. To prevent the formation of toxins, the bacteria must be switched off as quickly as possible. Raw food first cooking or baking and then storing in the refrigerator for several days is therefore safer than first storing for a few days and preparing just before use.
Viruses are many times smaller than bacteria. The existence of viruses has only been discovered after the electron microscope was taken into use. Viruses can not live independently, because they only have one strand of protein with genetic material (DNA or RNA) instead of the usual number for a living cell. They are therefore dependent on host cells for their reproduction. They penetrate and use the DNA of the host cell to multiply their own DNA or RNA in large quantities.
The host cell has thus been converted into a virus factory. The host cell succumbs after some time and the newly formed viruses are released into the body of the host. Furthermore, viruses are not surrounded by a cell wall but by a thin protein capsule. Some viruses settle in the host’s body for the rest of his or her life. An example of this is the herpes simplex virus, which causes acute acute illness in children at a young age. The virus remains very latent (without symptoms of disease) in the body after healing of the acute phase and, with reduced resistance, it stands up again, usually in the form of a so-called cold sore. Viruses can also be (or remain) present in large numbers while the patient has little or no symptoms of disease and therefore does not feel ill at all. These patients are then carriers of a virus and can be a constant source of infection, depending on the viral load.
Fungi are filamentous organisms. They are persistent and therefore difficult to combat. A known, annoying fungus is Candida albicans, the cause of, among other things, swimmer’s eczema, fungal nails, white deposits in the oral cavity and chapped mouths. Fungi, however, also have a positive meaning, because they can deliver many useful products. They are used in the food industry (blue cheese), but are also known in the pharmaceutical industry, including as producers of penicillin.
A prion (proteinaceous infectious particle) is a very small protein structure that contains no genetic material. In a healthy body, many prions occur naturally. These are mainly in the brain and in the central nervous system. Pathogenic prions have a different form than the physiological prions and act as a kind of stamp, so that the form of healthy prions also deviates. This change in body prions causes softening of the brain tissue. Prion diseases are transmissible from human to animal. BSE (bovine spongiform encephalopathy, better known as mad cow disease) is the animal form that causes the dreaded Creutzfeldt-Jakob disease after transmission to humans.
Protozoans are somewhat larger unicellular organisms that are counted among the animal world. They can move through a kind of whipstones (flagella) or because they can form a false foot. An example of this type of organism is the amoeba that occurs in the tropics and can cause severe intestinal inflammation.