Morphology and Physiology of Microorganisms

Phylogenetic Tree and Domains of Life

Modern classification divides all living organisms into three major domains, each with its own structural and physiological characteristics:

• Bacteria — prokaryotes having a cell wall with peptidoglycan. Include the majority of known pathogenic microbes.
• Archaea — ancient prokaryotes, often inhabiting extreme environments (thermophiles, halophiles). Their cell wall lacks peptidoglycan, and their genetic apparatus has a unique structure. There are no human pathogens among archaea.
• Eukarya — organisms with a defined nucleus and membrane-bound organelles (mitochondria, Golgi apparatus). In microbiology, these include protozoa, fungi, and chromists.

Bacterial morphology

Within the domain Bacteria, microorganisms are classified based on the architecture of their cell envelope. The primary differentiation method is Gram staining:

1. Gram-positive (G+) — have a thick cell wall consisting predominantly of a robust layer of peptidoglycan. Stain blue-purple. Examples: Staphylococcus, Bacillus, Lactobacillus.
2. Gram-negative (G-) — the peptidoglycan layer is thin, but an additional outer membrane of phospholipids and lipopolysaccharides is present on the outside. Stain red. Examples: Salmonella, Escherichia.
3. Cell-wall-deficient bacteria — classified in a separate class Mollicutes (mycoplasmas).

The bacterial cell is unique in its chemical composition. It contains compounds absent in humans: N-acetylmuramic acid, D-isomers of amino acids, and diaminopimelic acid. For this reason, enzymes involved in bacterial cell wall synthesis serve as ideal targets for antibiotics — they destroy the pathogen without damaging host cells.

Several major bacterial phyla are of medical importance: Proteobacteria (Gram-negative, including Helicobacter and Haemophilus), Firmicutes (Gram-positive cocci, bacilli, and clostridia), and Bacteroidetes.

Eukaryotic Microorganisms

The domain Eukarya is represented by cells with complex internal organization. In medical microbiology, special attention is paid to the following kingdoms:

• Protozoa (Protozoa) — unicellular organisms. Their cytoplasm is often divided into two layers: a dense outer ectoplasm and a mobile granular endoplasm containing the nucleus and organelles.
• True Fungi (Eumycota) — heterotrophs lacking chlorophyll, possessing a thick cell wall. Widely distributed in the environment.
• Chromists (Chromista) — a taxon that, following classification revisions, includes certain former protozoa and fungi (e.g., blastocystis).

Physiology of Interactions: Parasitism and Symbiosis

Microorganisms constantly interact with macroorganisms and with each other. In medicine, the following forms of coexistence play a key role:

• Mutualism — mutually beneficial coexistence (e.g., normal intestinal microflora).
• Commensalism — microbes multiply on the skin or mucous membranes without causing benefit or harm under normal conditions (opportunistic flora).
• Parasitism — a form of antagonism in which the microbe uses the host as a habitat and nutrient source, causing harm.

Parasitism can be true (genetically fixed, as in the malaria parasite) or false (accidental entry of a free-living organism into the host body). True parasites possess high reproductive potential and develop molecular mechanisms of defense against host immunity. Such adaptations include encapsulation (creation of a barrier against antibodies), antigenic variation (changing the composition of the glycocalyx), and molecular mimicry (camouflaging as host antigens).

Microbial communities also exhibit antagonism — competition for nutrients, in which one species suppresses another by producing specific antibiotic substances.

Class and exam questions

What else to study on this topic

• Microbiocenosis of Soil, Water, and Air
• Sanitary Indicator Microorganisms
• Methods for determining bacterial enzymatic activity
• Guidelines for Specimen Collection for Bacteriological Examination
• The Role of Microbes in Nutrient Cycling

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