Bacteria are the oldest and most widespread organisms on Earth: they live in water, soil, air, hot springs, and inside and on other organisms. They are cells without a nucleus — prokaryotes. Without bacteria the cycling of matter would stop: breaking down dead organic matter, fixing nitrogen and forming soil are their work. At the same time, many dangerous diseases are caused by bacteria. The non-cellular form of life — viruses — is studied in this topic too. Exams often ask about prokaryotic structure, modes of nutrition, spores, cyanobacteria and pathogens.
§1. The bacterial cell
A bacterial cell is very small — usually 1–10 µm, barely visible in an ordinary light microscope. Bacteria were first seen by A. van Leeuwenhoek (17th century) through a microscope he made himself.
No nucleus: the genetic material is the nucleoid, a single circular DNA molecule lying free in the cytoplasm. There are also small circular DNAs — plasmids; genes for antibiotic resistance are often carried on them.
The cell wall is made of murein (peptidoglycan). Remember: plant walls are cellulose and fungal walls are chitin — exams like this contrast.
Many have a slimy capsule over the wall: it protects the cell from drying out and from the host’s defence cells.
No membrane-bound organelles: no mitochondria, plastids, ER, Golgi apparatus or lysosomes. Respiratory enzymes sit on infoldings of the plasma membrane.
Ribosomes are small — 70S (cytoplasmic ribosomes of eukaryotes are 80S).
Many have a flagellum for movement; some have fine hair-like pili for attachment.
Prokaryote features: no nucleus, nucleoid, murein wall, 70S ribosomes, no membrane-bound organelles.
By shape, bacteria fall into four groups:
cocci — spherical; in pairs — diplococci, in chains — streptococci, in grape-like clusters — staphylococci;
bacilli — rod-shaped (gut bacillus, tuberculosis bacillus, hay bacillus);
vibrios — comma-shaped (the cholera agent);
spirilla — spiral.
§2. Life processes: reproduction, spores, respiration
Reproduction is binary fission. The DNA is copied first, then a cross-wall forms in the middle and two identical cells result. In good conditions (warmth, moisture, enough food) division repeats every 20–30 minutes, so bacterial numbers grow very fast — exponentially. There is no true sexual reproduction, but there is a sexual process — conjugation: a bridge forms between two cells and a piece of DNA (often a plasmid) passes from one to the other. The number of cells does not increase; only genes are exchanged.
A spore is a resting form with a thick, many-layered coat formed inside the cell in bad conditions (drought, high temperature, lack of food). Spores survive boiling, cold and drying and stay alive for years. One cell gives one spore, so numbers do not rise — spore formation is not reproduction but a way of surviving. When conditions improve, the spore grows into an ordinary cell again. That is why canned food is heated for a long time at high temperature.
By respiration:
aerobes breathe oxygen (most soil bacteria, the tuberculosis bacillus);
anaerobes live without oxygen and get energy by fermentation (the agents of tetanus and botulism, lactic acid bacteria). Tetanus develops in deep, dirty wounds where there is little oxygen.
§3. Modes of nutrition
Autotrophs make organic substances from inorganic ones themselves:
photosynthetic — use light energy: cyanobacteria, green and purple bacteria;
chemosynthetic — use the energy released by oxidising inorganic substances: nitrifying bacteria (oxidise ammonia to nitrite and nitrite to nitrate), iron bacteria, sulfur bacteria. Chemosynthesis was discovered in 1887 by S. N. Winogradsky.
Heterotrophs live on ready-made organic matter:
saprotrophs (decomposers) break dead organic matter down to minerals, playing the role of decomposers in an ecosystem;
parasites live at the expense of a living organism and cause disease;
symbionts live in a mutually beneficial partnership. Root-nodule bacteria on legumes (pea, bean, lucerne, soya) fix free atmospheric nitrogen (N₂) into compounds the plant can use. The gut bacillus in the human intestine makes vitamin K and B-group vitamins.
Cyanobacteria (also called blue-green algae): Nostoc, Oscillatoria, Anabaena. Like plants they photosynthesise and release oxygen, but they have no nucleus — so they are prokaryotes, not plants. Some fix nitrogen in special cells — heterocysts. Ancient cyanobacteria filled Earth’s atmosphere with oxygen.
§4. Importance in nature and the economy
Cycling of matter: decomposers break down dead remains, nitrogen bacteria drive the nitrogen cycle, soil humus is formed.
Food industry: lactic acid bacteria make yoghurt, kefir, cheese and sour cream and are used for pickling cabbage and cucumbers; acetic acid bacteria make vinegar.
Medicine and biotechnology: antibiotics (for example streptomycin comes from actinomycetes), vitamins; by genetic engineering a human gene is put into bacteria to produce insulin and other proteins.
Agriculture: legumes enrich the soil with nitrogen in crop rotation; making silage.
The harmful side: food spoilage (rotting, souring) and disease.
Ways to protect food from bacteria: cooling, drying, salting, sugaring, boiling, pasteurisation, sterilisation.
Pasteurisation, proposed by L. Pasteur, means heating a product to 60–70 °C to kill active bacteria (spores may survive). Sterilisation is a harsher treatment that destroys spores too.
§5. Bacterial diseases
Cholera — a vibrio; an intestinal disease spread by dirty water.
Tuberculosis — Koch’s bacillus (identified by R. Koch in 1882); mainly attacks the lungs, airborne droplets.
Tetanus — an anaerobic bacterium entering through a wound contaminated with soil.
Anthrax, plague, brucellosis — passed to humans from animals.
Prevention: personal hygiene, boiling water, proper food storage, vaccination, isolating patients. Treatment uses antibiotics; taking them without a doctor’s advice leads to resistant bacteria.
§6. Viruses
Viruses are a non-cellular form of life. In 1892 D. I. Ivanovsky, studying tobacco mosaic disease, discovered viruses.
Structure: a nucleic acid (DNA or RNA — never both) and a protein coat around it — the capsid. Some also have an outer lipid envelope (influenza, HIV).
No metabolism of their own; they multiply only inside a host cell: the viral nucleic acid enters the cell and new viruses are assembled using the cell’s ribosomes and materials.
A bacteriophage is a virus that infects bacteria: it attaches by its tail, injects its DNA, and after new phages are assembled the cell bursts and dies.
Viral diseases: influenza, measles, chickenpox, hepatitis, polio, HIV infection (AIDS), COVID-19; in plants — tobacco mosaic.
Antibiotics act on bacteria, not on viruses. The main defence against viruses is vaccination.
Worked example
Why does wheat sown after legumes give a good harvest?
Root-nodule bacteria on the legumes fix atmospheric N₂.
When the plant remains decay, nitrogen compounds build up in the soil.
Wheat uses this nitrogen to make proteins and nucleic acids.
Answer: legumes enrich the soil with nitrogen — this is the basis of crop rotation.
Worked example
One bacterium divides every 30 minutes. How many bacteria will there be after 3 hours (if all survive)?
3 hours = 180 minutes; number of divisions: 180 / 30 = 6.
Each division doubles the number: N = 2ⁿ.
N = 2⁶ = 64.
Answer: 64 bacteria.
Worked example
Home-canned food was boiled too briefly; months later the lid of the jar bulged. Which bacterium is dangerous and why did it survive?
There is no air (oxygen) inside the jar — anaerobic conditions.
Spores of the anaerobic botulism agent are not killed by short boiling.
The spores germinate, the bacterium multiplies, releasing gas and a powerful toxin — the lid bulges.
Answer: the botulism agent; it survived thanks to its spores and its anaerobic life, and such food must not be eaten.
Check yourself
What is the region containing a bacterium’s circular DNA called?
What substance makes up the bacterial cell wall?
Is spore formation a form of reproduction?
Who discovered chemosynthesis?
What shape is the cholera agent?
Which group does Nostoc belong to?
How many cells come from one bacterium in 2 hours if it divides every 20 minutes?
Who discovered viruses?
Common mistakes
Writing that bacteria have mitochondria or a nucleus — they have no membrane-bound organelles at all.
Treating spore formation as reproduction.
Calling Nostoc and other cyanobacteria plants (true algae) — they are prokaryotes.
Thinking antibiotics also work against viruses.
Calling conjugation reproduction — the number of cells does not change, only genes are exchanged.
Saying the bacterial wall is made of cellulose or chitin.