Microorganisms are microscopic organisms that are found in almost all habitats.
History
When it comes to "microorganisms," many people immediately think of pathogens. Less known is that most bacteria are not only harmless but also play an important role in life. In ancient times, infectious diseases were extensively described, and it was long believed that diseases like "malaria" (Latin for bad air) were caused by foul-smelling vapors or decayed bodily fluids.
A decisive breakthrough in microbiology was the development of the microscope. In 1683, van Leeuwenhoek observed bacteria and single-celled organisms under his microscope that he had isolated from hay infusions or dental plaque. However, he could not draw any further conclusions from his observations. The theory of physician Fracastorius, that a living substance could cause a contagious disease, initially received little recognition.
It wasn't until the late 18th century that Lazzaro Spallanzani proved that small organisms in hay infusions did not arise by spontaneous generation but from prior contamination. In 1857, Louis Pasteur finally refuted the theory of spontaneous generation with his experiments on fermentation, making a significant contribution to the development of microbiology.
It was only at the end of the 19th century that the link between certain pathogens and infectious diseases was definitively proven. In 1876, Robert Koch was able to identify the bacterial pathogen as the specific cause of anthrax.
Basics
The researchers Henle and Koch formulated the postulates named after them, which must be fulfilled to identify a particular germ as a pathogen. These are:
- The same pathogen must be present in every case of an infectious disease with a comparable disease progression and stage.
- The pathogen must not occur as a random, non-pathogenic germ in other diseases.
- Cultures of pure pathogens must produce the same disease in humans and a similar disease in animals.
Although the Henle-Koch postulates are clearly formulated, they lose universal significance with newer findings. Particularly in viral diseases, not all postulates must be fulfilled, yet they remain infectious diseases.
Despite the improvement of light microscope resolution and the introduction of staining techniques by Ehrlich (1882) and Gram (1884), the exact structure of the bacterial cell remained unknown for a long time. It was not until the introduction of the electron microscope in 1938 that a more detailed insight into bacterial structure was possible.
Classical microbiological methods remain important, but for diagnostics and systematics, bacterial genetics and biochemistry are of particular significance. In 1944, Avery, MacLeod, and McCarty were the first to prove that deoxyribonucleic acid (DNA) carries genetic information. In 1953, Watson and Crick deciphered the structure of DNA through X-ray crystallography.
In 1892, Ivanowsky discovered the existence of viruses based on the mosaic disease of tobacco plants. However, it was not until 1949 that the introduction of monolayer cell cultures with antibiotic additives by Enders made it possible to study the effect of viruses on cells.
To classify pathogens that cause infectious diseases in humans and animals, a variety of criteria are used, with the size of the pathogens being particularly striking.
Objects smaller than a cell include prions (< 5 nm), viroids (< 5 nm), and viruses (20-200 nm). Single-celled organisms classified as prokaryotes have a size of 200-2000 nm and include chlamydia, rickettsia, mycoplasmas, and bacteria. Multicellular organisms classified as eukaryotes are larger than 2000 nm and include fungi and protozoa. Multicellular organisms also include helminths (worms) and arthropods (jointed-legged animals).
