General characteristics of living nature and the concept of the microworld
For convenience of study, the entire living world is conventionally divided into two major categories. This dichotomy includes the macromorld and the microworld. The macromorld includes all living objects that can be clearly seen with the unaided eye — plants, animals, and, of course, humans themselves.
In contrast, the microworld unites living organisms whose sizes are beyond the capabilities of our vision. To study such objects, researchers strictly require special optical instruments, such as microscopes.
The size characteristics of representatives of the microworld have clear boundaries:
- The lower boundary is represented by viruses. Their sizes vary in the range from 10 to 400 nanometers (which corresponds to 0.01–0.4 µm).
- The upper boundary includes microorganisms such as bacteria, fungi, and protozoa. Their sizes start from 10 micrometers and can be significantly larger.
The collective term "microbes" itself, which is actively used today to designate all these invisible-to-the-eye beings (bacteria, fungi, protozoa, and viruses), was first introduced into scientific circulation in the late 19th century by the French scientist Sedillot.
Microbial taxonomy by cell organization type
Depending on how the cell is structured (or its absence), all microorganisms are classified into several fundamentally different groups.
- Eukaryotes. The name of this group comes from the Greek word karyon, which translates as "nut" or "nucleus". The main distinguishing feature of eukaryotes is the presence of a clearly defined cell nucleus. In microbiology, this group includes fungi and protozoa. It is worth noting that eukaryotic microorganisms can have either single-celled or multicellular structures.
- Prokaryotes. Unlike the previous group, prokaryotic cells do not have a defined nucleus; their genetic material is located directly in the cytoplasm. Typical and main representatives of prokaryotes are bacteria.
- Acellular forms (non-cellular organisms). This is a unique group of agents that have no cellular structure at all. This includes:
- Viruses — complex complexes consisting of nucleic acids and protective proteins.
- Infectious nucleic acids — free DNA and RNA molecules not covered by a protein coat capable of causing an infectious process.
- Prions — specific infectious protein particles that do not contain nucleic acids, yet are capable of causing severe diseases.
Genetic organization and structural complexity
One of the fundamental rules of microbiology is that the overall structural and functional complexity of any microorganism directly depends on the size of its genome. The more genes an agent contains, the more complex structures it is able to form and the more diverse its metabolism.
The quantitative characteristic of genes clearly demonstrates this pattern:
- Viruses have the simplest organization. Their genetic apparatus is minimal and contains up to 100 genes.
- Bacteria and fungi demonstrate an intermediate level of genetic complexity. Their genome can contain up to 5,000 different genes.
- Protozoa act as the most complex representatives of the microworld. Their genetic apparatus is the largest among microbes and includes from 5,000 to 10,000 genes.
Ecology, distribution, and significance for humans
Microorganisms are distinguished by an astonishing ability to survive, so their habitat is absolutely ubiquitous. Microbes successfully colonize soil, fresh and salt water bodies, the atmosphere, and are even able to survive in open space conditions. In addition, the most important habitat for them is macroorganisms — plants, animals, and humans.
The ecological scale of the microworld is hard to overestimate: the total biomass of all existing microbes on Earth repeatedly exceeds the total biomass of all plants and animals combined.
Microbes are of colossal importance to humans. The body of every human is a natural, permanent habitat for the so-called normal microflora, which performs many vital functions. The scale of this coexistence is striking: the quantitative ratio is such that up to $10^{14}$ microbes permanently inhabit the body of one healthy person.
Briefly: answers to topical questions
What chemical substances make up the virion envelope?
Virion envelopes are composed of proteins, and in complex viruses, also lipids and polysaccharides.
- Simple viruses — consist of a nucleic acid and a protein coat (capsid); capsid proteins are called capsomeres.
- Complex viruses — have a nucleic acid, a capsid, and an outer lipoprotein envelope (supercapsid), and may include lipids and polysaccharides.
Envelope proteins are divided into capsid proteins (forming the protein coat) and supercapsid proteins (making up the outer envelope in complex viruses).
What functions do representatives of the normal human microflora perform?
Representatives of the normal human microflora perform protective, metabolic, and immunomodulatory functions.
- Colonization resistance — suppression of pathogenic and opportunistic flora through the production of organic acids, antibiotics, and bacteriocins.
- Participation in digestion — utilization of substrates, activation of membrane digestion, synthesis of amino acids.
- Vitamin-synthesizing function — synthesis of vitamin K and B vitamins.
- Regulation of absorption — enhancement of absorption of iron, calcium, phosphates, and vitamin D.
- Immunostimulatory function — stimulation of the immune system by antigens, regulation of immunity, protection of secretory IgA.
- Detoxification — reduction of histamine formation and anti-allergic action.
To memorize the relationship between gene count and microbial complexity, use the simple ladder "Vi-Ba-Fu-P": Viruses (up to 100) — Bacteria and Fungi (up to 5,000) — Protozoa (up to 10,000).
Class and exam questions
Who first coined the term "microbes" and when?
This term was proposed by the French scientist Charles-Emmanuel Sedillot in the late 19th century to encompass bacteria, fungi, protozoa, and viruses.
What is the main difference between eukaryotes and prokaryotes?
Eukaryotes (from Greek karyon — nucleus) have a clearly defined cell nucleus, whereas prokaryotes, which include bacteria, lack a defined nucleus.
What belongs to acellular forms of life?
Acellular or non-cellular forms include viruses (a complex of nucleic acids and proteins), infectious nucleic acids (free DNA and RNA), and prions (infectious proteins).
How does genome size affect microorganism complexity?
There is a direct correlation: the more genes, the more complex the organization. Viruses have up to 100 genes, bacteria and fungi up to 5,000, and protozoa up to 10,000.
What else to study on this topic
- Features of the cell wall and cytoplasm of a prokaryotic cell
- Mechanisms of infectious action of prions and free nucleic acids
- Principles of operation of optical instruments for studying objects 10–400 nm in size
- Composition and functions of normal microflora in the human body (up to 10^14 microbes)
- Life cycle of viruses as acellular forms of life
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