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Introduction to Microbiology and Immunology

For medical students3 min readUpdated 2026-10-10

Microbiology studies living entities that exist beyond the resolution limit of the human eye. Microbes traditionally include bacteria, viruses, fungi, and protozoa, whose total biomass on Earth significantly exceeds the mass of all plants and animals combined.

VirusesForm the lower limit of the microbial world: their sizes range from 10 to 400 nm.
Bacteria and FungiDetermine the upper limit of microorganism sizes: from 10 µm and above.
GenomeThe complexity of a microbe depends on its genome: from 100 genes in viruses to 10,000 in protozoa.
HabitatMicrobes inhabit everywhere: in soil, water, atmosphere, space, and inside macroorganisms.
MicrofloraUp to 10^14 diverse microorganisms permanently inhabit the human body.

General Characteristics of Living Nature and the Concept of the Microworld

For study purposes, the living world is conventionally divided into two large categories. This dichotomy includes the macrocosm and the microcosm. The macrocosm includes all living objects that can be clearly seen with the naked eye—plants, animals, and humans.

In contrast, the microcosm unites living creatures whose sizes lie beyond the capabilities of our vision. To study such objects, researchers require special optical instruments, such as microscopes.

The size characteristics of representatives of the microcosm have clear boundaries:

The collective term "microbes," which is actively used today to denote all these invisible creatures (bacteria, fungi, protozoa, and viruses), was first introduced into scientific circulation in the late 19th century by the French scientist Charles Sédillot.

Systematics of Microbes by Type of Cellular Organization

Depending on how the cell is arranged (or its absence), all microorganisms are classified into several fundamentally different groups.

  1. 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.
  2. Prokaryotes. Unlike the previous group, prokaryotic cells do not have a defined nucleus; their genetic material is located directly in the cytoplasm. Typical and primary representatives of prokaryotes are bacteria.
  3. Acellular Forms (Non-cellular organisms). This is a unique group of agents that lack a cellular structure altogether. These include:
  4. Viruses — complex complexes consisting of nucleic acids and protective proteins.
  5. Infectious nucleic acids — free DNA and RNA molecules lacking a protein coat capable of causing an infectious process.
  6. 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 complexity of structure and function 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.

Quantitative gene characteristics clearly demonstrate this pattern:

Ecology, Distribution, and Importance to Humans

Microorganisms are characterized by a striking capacity for survival, making their habitats ubiquitous. Microbes successfully colonize soil, fresh and salt water bodies, the atmosphere, and are even capable of surviving in open space. In addition, macroorganisms—plants, animals, and humans—serve as a vital habitat for them.

The ecological scale of the microcosm cannot be overstated: the total biomass of all existing microbes on Earth vastly exceeds the total biomass of all plants and animals combined.

For humans, microbes are of colossal importance. The body of every human serves as 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 constantly inhabit the body of a single healthy human.

Mnemonic

To remember the link between gene count and microbial complexity, use the mnemonic "ViBaFruP": Viruses (up to 100) — Bacteria and Fungi (up to 5,000) — Protozoa (up to 10,000).

Frequently asked questions

Which chemical substances make up the envelope of virions?

Virion envelopes contain proteins, and in complex viruses, lipids and polysaccharides as well.

  • Simple viruses — consist of nucleic acid and a protein coat (capsid); capsid proteins are called capsomeres.
  • Complex viruses — possess nucleic acid, a capsid, and an outer lipoprotein envelope (supercapsid), which may include lipids and polysaccharides.

Envelope proteins are divided into capsid proteins (forming the protein shell) and supercapsid proteins (making up the outer envelope in complex viruses).

What functions do representatives of the human normal microflora perform?

Representatives of the human normal microflora perform protective, metabolic, and immunomodulatory functions.

  • Colonization resistance — suppression of pathogenic and opportunistic flora via 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-group 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, support of secretory IgA.
  • Detoxification — reduction of histamine formation and anti-allergic action.
Who first coined the term "microbes" and when?

This term was proposed by the French scientist Charles Sédillot in the late 19th century to unite 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 forms include viruses (complex of nucleic acids and proteins), infectious nucleic acids (free DNA and RNA), and prions (infectious proteins).

How does genome size affect the complexity of a microorganism?

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.

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