Prokaryotes and Eukaryotes

Microbial Classification

The world of microbes is broadly divided into non-cellular and cellular forms. Non-cellular forms include viruses, viroids, and prions—they lack cellular and organismal structure, which is why they cannot be called microorganisms (they are only microbes). Cellular life forms have a full-fledged cellular organization. They can be referred to as both microbes and microorganisms. They are divided into two major types: prokaryotes and eukaryotes.

Prokaryotes: Domains Bacteria and Archaea

Prokaryotes lack a true nucleus and membrane-bound organelles (mitochondria, Golgi apparatus). Their cytoplasm contains 70S-type ribosomes. This group includes two domains:

• Domain Bacteria (true bacteria). Their main feature is the presence of a peptidoglycan layer in the cell wall containing unique N-muramic acid. Based on envelope structure, bacteria are divided into Gram-positive (thick peptidoglycan, blue-violet color in Gram staining) and Gram-negative (thin peptidoglycan plus outer membrane, red color). Bacteria lacking a cell wall are also distinguished (class Mollicutes — mycoplasmas). Major phyla within the domain include: Proteobacteria (Gram-negative), Firmicutes (Gram-positive classes Bacilli and Clostridia), and Bacteroidetes.
• Domain Archaea (archaea). These are ancient microorganisms often living in extreme conditions (halophiles, thermophiles, acidophiles) or acting as methanogens. Their key difference from true bacteria is the absence of peptidoglycan and a specific structure of the genetic apparatus (specific ribosomes and rRNA). It is important to remember that there are no human pathogens among archaea.

Eukaryotes: Domain Eukarya

Eukaryotes are distinguished by a complex architecture: they possess a true nucleus with a membrane and nucleolus, and the cytoplasm is filled with highly organized membrane-bound organelles (endoplasmic reticulum, lysosomes). In medical microbiology, three kingdoms of this domain are of greatest importance:

• Protozoa (Protozoa): unicellular organisms bounded by a semipermeable membrane. Their cytoplasm is often differentiated into a dense, transparent ectoplasm and a granular, motile endoplasm. Many lead a parasitic lifestyle.
• Fungi (Eumycota): heterotrophs lacking chlorophyll with a thick cell wall containing chitin.
• Chromists (Chromista): a new taxonomic group formed during reclassification. Organisms previously considered protozoa or fungi (e.g., blastocysts and oomycetes) have been moved here.

Medical Significance of Structural Differences

Knowledge of cytological features is critically important for medicine. The bacterial cell contains components that are entirely absent in humans: peptidoglycan, D-amino acid isomers, diaminopimelic acid. The enzymes synthesizing these substances serve as an ideal target for antibiotics. These drugs are selectively toxic to bacteria and do not damage host tissues.

In addition, some Gram-positive bacteria are capable of forming endospores — resting forms for survival under nutritional deficiency, desiccation, or UV irradiation. The spore forms inside the mother cell. Unlike fungi, this is not a method of reproduction in bacteria: exactly one spore is formed from a single cell. In representatives of the genus Bacillus, the spore size does not exceed the diameter of the vegetative cell.

Class and exam questions

What else to study on this topic

• Cell wall structure of Gram-positive and Gram-negative bacteria
• Methods of staining bacterial structures (Gram stain)
• Morphology and structural elements of bacteria (cocci, bacilli, spiral forms)
• Classification of bacteria by growth requirements (relation to oxygen and temperature)
• Nutritional mechanisms and growth phases of a bacterial population

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.