Bacteria as prokaryotic organisms
Bacteria belong to prokaryotes, which are cells lacking a distinct nucleus. Their genetic material is represented by a circular DNA molecule located directly in the cytoplasm as a nucleoid. Unlike eukaryotic chromosomes, the nucleoid consists of two DNA strands having a fibrillar appearance and is not associated with histones.
The bacterial cell contains only non-membrane-bound organelles, primarily ribosomes. Additionally, it may contain plasmids (small circular DNA molecules carrying extra genetic information) and various inclusions (nutrient reserves, lipid droplets, etc.) that support cell viability.
A special structural element is mesosomes, which are invaginations of the cytoplasmic membrane. They increase the surface area of the membrane, which is important given the lack of a developed system of membrane-bound organelles. The science that studies bacteria is called bacteriology.
Cell wall and Gram staining
The bacterial cell wall is built of murein (peptidoglycan), which consists of polysaccharide chains cross-linked by short peptide bridges into a continuous molecular network. The wall provides protection against mechanical and chemical impacts and maintains cell shape.
Based on the structure of the cell wall, Gram-positive and Gram-negative bacteria are distinguished:
- Gram-positive: thick peptidoglycan layer (up to 90–95%), teichoic and lipoteichoic acids present, intensely stained by the dye.
- Gram-negative: thin peptidoglycan layer (5–10%), abundant lipopolysaccharides and proteins, presence of a periplasmic space containing hydrolytic enzymes, weakly stained.
Reproduction and resting forms
Bacteria reproduce by binary fission — simple division into two. This division is not equivalent to mitosis, since prokaryotes lack a nucleus and there is no involvement of the nuclear envelope and chromosomes. In laboratory conditions, a single cell forms colonies — an aggregation of microorganisms of the same culture.
Under unfavorable conditions, bacteria form resting forms:
- Endospores are formed inside the mother cell through uneven division and budding off of the protoplast; they are surrounded by two layers of cell wall and are extremely resistant to temperature, radiation, and desiccation.
- Exospores are formed externally by budding in methane-metabolizing bacteria; they are resistant to desiccation, but not to high temperatures.
- Cysts: the entire vegetative cell transforms into a cyst state.
- Akinetes are resting forms of cyanobacteria with a thickened wall and reserves of nutrients and pigments.
When favorable conditions return, resting forms undergo hydration, swell, and germinate, giving rise to active vegetative cells.
Bacterial taxonomy
Bacteria are classified according to several features.
By cell shape:
- Cocci — spherical: staphylococci (clusters), diplococci (pairs), streptococci (chains).
- Bacilli are rod-shaped.
- Vibrios are comma-shaped.
- Spirilla are spirally twisted rods.
- Spirochetes are heavily twisted, up to 10–15 turns.
By the number and arrangement of flagella: monotrichous (one flagellum), lophotrichous (a tuft at one end), amphitrichous (tufts at both ends), peritrichous (flagella over the entire surface). In addition to flagella, bacteria possess short appendages called pili, which ensure attachment to the substrate and cell-to-cell contact.
By type of nutrition:
- Autotrophs: photosynthetic (cyanobacteria) and chemosynthetic (sulfur bacteria, iron bacteria, hydrogen bacteria, nitrifying bacteria).
- Heterotrophs: saprotrophs (fermentation and putrefactive bacteria) and parasites (cholera vibrio, tetanus bacillus).
By oxygen requirement: aerobes require oxygen, while anaerobes exist without it.
Genetic material transfer
Three methods of genetic information exchange are distinguished in bacteria:
- Conjugation is the unidirectional transfer of a portion of genetic material through direct contact between two bacterial cells.
- Transduction is the transfer of DNA from one cell to another using a bacteriophage.
- Transformation is the uptake of a free DNA molecule from the environment by a cell.
Significance of bacteria and related concepts
Bacteria play the role of decomposers in food webs: they break down dead organic remains and excrement into mineral substances available to producers. Bacteria also participate in molecular nitrogen fixation and fermentation processes, serve as a source of antibiotics, and are used in biotechnology.
It is important to distinguish between probiotics and prebiotics:
- Probiotics are live microorganisms of the normal intestinal microflora (symbionts) that help digest food.
- Prebiotics are substances that create optimal conditions for the growth of beneficial microflora; they are prescribed, among other things, after a course of antibiotics.
Many bacteria are pathogens of dangerous diseases: tuberculosis, cholera, dysentery, syphilis, tetanus, botulism, typhoid fever. They affect the respiratory tract, gastrointestinal tract, genital organs, bones, and joints. For the USE, it is important to distinguish bacterial diseases from viral ones: the bacterial nature is often indicated by the name of the pathogen itself (e.g., Escherichia coli, tetanus bacillus, cholera vibrio).
Key points
- Bacteria are prokaryotes: no nucleus, possess circular DNA as a nucleoid, ribosomes, plasmids, and mesosomes.
- The cell wall consists of murein (peptidoglycan); based on its structure, Gram-positive and Gram-negative bacteria are distinguished.
- Reproduction is by binary fission, which is not equivalent to mitosis due to the absence of a nucleus.
- Resting forms: endospores, exospores, cysts, akinetes ensure survival under unfavorable conditions.
- Classification by shape (cocci, bacilli, vibrios, spirilla, spirochetes), by flagella (mono-, loph-, amphi-, peritrichous), by nutrition and oxygen requirement.
- Methods of DNA transfer: conjugation, transduction, transformation.
- Decomposer bacteria, nitrogen fixers, human symbionts (probiotics); prebiotics are substances, not organisms.
- Bacterial diseases: tuberculosis, cholera, dysentery, syphilis, tetanus, botulism, typhoid fever.