Unit 12 Microbiology and biotechnology
Key unit competence: Explain the structure, life cycles and roles of microorganisms and how they are grown and used in biotechnology.
Key ideas
- Microbiology studies microorganisms (viruses, bacteria, archaea, fungi, protoctists); biotechnology uses living organisms or their products to make useful goods and services (food, medicines, fuels, waste treatment).
- Viruses: nucleic acid (DNA or RNA) inside a protein capsid, sometimes an envelope; obligate intracellular parasites. A bacteriophage (e.g. T4) has a head, tail sheath and tail fibres. Lytic cycle: attachment → injection of nucleic acid → replication of viral parts using host machinery → assembly → lysis of the host, releasing new phages. Lysogenic cycle: viral DNA joins the host chromosome (prophage) and is copied with it for a long time before entering the lytic cycle.
- Retroviruses (e.g. HIV) contain RNA and the enzyme reverse transcriptase, which copies RNA into DNA; the DNA (provirus) is inserted into the host's DNA by integrase and is copied each time the cell divides, then used to make new virus particles that bud from the cell. HIV attacks CD4 T-helper cells.
- Plant viruses (e.g. cassava mosaic virus, maize streak virus, tobacco mosaic virus) spread by insect vectors (whiteflies, aphids, leafhoppers), infected planting material (cuttings, tubers), mechanical contact and sometimes seeds. Control: disease-free planting material, resistant varieties, removing infected plants, controlling vectors.
- Archaea: prokaryotes without peptidoglycan, with unusual membrane lipids; methanogens (make methane: biogas digesters, cattle rumen, swamps), halophiles (salt lakes), thermoacidophiles (hot acidic springs). Their features suggest that early life arose in extreme, hot, anaerobic conditions and that Archaea are close to the origin of eukaryotes.
- Eubacteria: shapes (cocci, bacilli, spirilla, vibrios); Gram-positive (thick peptidoglycan, purple) and Gram-negative (thin peptidoglycan plus outer membrane, pink). E. coli: rod-shaped, Gram-negative, lives in the gut; reproduces by binary fission (every 20 minutes in ideal conditions) and exchanges DNA by conjugation through a pilus. Its presence in water shows faecal contamination; some strains (e.g. O157:H7) cause food poisoning. Harmless bacteria can become dangerous by gaining genes (toxin genes, antibiotic resistance) through plasmids or viruses.
- Moulds: fungal moulds such as Rhizopus and Mucor (hyphae, sporangia, extracellular digestion), Penicillium (blue-green, brush-like conidiophores; makes the antibiotic penicillin), and yeast (Saccharomyces), unicellular, budding, used in baking and brewing. Non-fungal (slime) moulds are protoctists.
- Pathogenic protozoa: Entamoeba histolytica (amoebic dysentery; cysts in faecal-contaminated water and food), Plasmodium (malaria; asexual stages in human liver and red blood cells, sexual stages in the female Anopheles mosquito), Trypanosoma (sleeping sickness; tsetse fly vector).
- Microbes need nutrients (carbon and nitrogen sources, minerals, vitamins), water, a suitable temperature (often 25–37 °C), pH and oxygen (aerobes) or its absence (anaerobes). Culture media: liquid broth or solid nutrient agar; general, selective and differential media. Aseptic techniques: sterilise media and equipment in an autoclave (121 °C), flame the inoculating loop until red-hot, work near a Bunsen flame, lift lids only slightly, tape plates (not fully sealed), incubate inverted at about 25 °C in school, never open plates of unknown cultures, disinfect and autoclave waste.
- Bacterial growth curve in a closed culture: lag (adapting, making enzymes), exponential (log) (rapid binary fission), stationary (birth = death; nutrients limited, toxins accumulate), death/decline. Uses of microbes: antibiotics (Penicillium), enzymes (pectinase for fruit juice, lactase for lactose-free milk, proteases for detergents), fermented foods (yoghurt, cheese, bread, beer), biogas (hydrolysis, acidogenesis, methanogenesis), sewage treatment, insulin and vaccines from genetically modified bacteria, nitrogen fixation.
Common misconceptions
- "All viruses kill their host cells immediately." No: in the lysogenic cycle viral DNA stays in the host for a long time before the lytic cycle starts.
- "Archaea live only in extreme environments." No: they also live in soil, oceans and the guts of animals (methanogens in the cow rumen).
- "E. coli is always harmful." No: most strains are harmless gut bacteria; only some strains cause disease.
- "Sealing a Petri dish completely with tape is the safest way." No: plates are taped at a few points so that anaerobic pathogens are not encouraged.
Worked examples
Example 1. Explain how HIV reproduces inside a T-helper cell.
Solution
HIV attaches to CD4 receptors; its envelope fuses with the membrane and the RNA and enzymes enter. Reverse transcriptase copies the viral RNA into DNA; integrase inserts this DNA into the host chromosome (provirus). The host cell transcribes the provirus into viral RNA and makes viral proteins; new viruses assemble and bud from the membrane, and the T-cell is eventually destroyed.
Example 2. A culture of bacteria starts with 100 cells and divides every 20 minutes. How many cells are there after 2 hours in the exponential phase?
Solution
2 hours = 120 min = 6 generations. Number = 100 × 2⁶ = 100 × 64 = 6400 cells.
Practice questions
Describe the structure of a bacteriophage.
Show answer
A polyhedral protein head (capsid) containing DNA; a collar; a hollow tail tube with a contractile sheath; a base plate with tail fibres that attach to the bacterium.
Common mistake: Including a nucleus.
Distinguish between the lytic and lysogenic cycles.
Show answer
Lytic: virus replicates immediately, assembles new particles and bursts the host cell. Lysogenic: viral DNA integrates into the host chromosome as a prophage and is copied with it for a long time without killing the cell, later entering the lytic cycle.
Common mistake: Saying the lysogenic cycle destroys cells at once.
Explain how plant viruses are transmitted and controlled.
Show answer
Transmission: insect vectors (whiteflies, aphids), infected cuttings or tubers, mechanical contact, seeds. Control: clean planting material, resistant varieties, removing infected plants, controlling vectors, crop rotation.
Common mistake: Saying plant viruses are controlled with antibiotics.
Describe three groups of Archaea and where they live.
Show answer
Methanogens: anaerobic swamps, rumen, biogas tanks; halophiles: salt lakes; thermoacidophiles: hot acidic springs.
Common mistake: Calling Archaea eukaryotes.
Describe the life cycle of Plasmodium.
Show answer
An infected female Anopheles injects sporozoites; they multiply in liver cells, then merozoites invade red blood cells and multiply, bursting cells (fever cycles); some form gametocytes, which a mosquito takes up; fertilisation and development in the mosquito form new sporozoites in its salivary glands.
Common mistake: Leaving out the mosquito stage.
State five aseptic techniques used when culturing microorganisms.
Show answer
Sterilise media and equipment in an autoclave; flame the loop until red-hot; work near a Bunsen flame; open lids slightly and briefly; tape lids at a few points and incubate inverted at 25 °C; disinfect benches and wash hands; autoclave waste.
Common mistake: Sealing plates completely and incubating at 37 °C.
Describe the four phases of a bacterial growth curve.
Show answer
Lag: adaptation, enzyme synthesis. Exponential: rapid binary fission. Stationary: birth rate equals death rate (nutrients limited, wastes build up). Decline: death rate exceeds birth rate.
Common mistake: Saying the lag phase is death.
State five uses of microorganisms in biotechnology.
Show answer
Antibiotics (penicillin); enzymes (pectinase, lactase, proteases); yoghurt, cheese, bread and beer; biogas; sewage treatment; insulin and vaccines from GM bacteria; nitrogen fixation.
Common mistake: Listing only diseases.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Transfer of DNA between bacteria through a pilus is: a. conjugation 2. E. coli reproduces asexually by: b. increase juice yield from fruit 3. Lactase is used to make: c. binary fission 4. Pectinase is used in the food industry to: d. lactose-free milk Show answer
1 → a (conjugation) · 2 → c (binary fission) · 3 → d (lactose-free milk) · 4 → b (increase juice yield from fruit)
- It spreads plasmid genes.
- One cell divides into two.
- It breaks lactose into glucose and galactose.
- It breaks down pectin in cell walls.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
halophiles · Entamoeba histolytica · faecal contamination · thermophiles only- The presence of E. coli in drinking water indicates .
- Amoebic dysentery is caused by .
- Archaea that live in very salty lakes are .
Show answer
1: faecal contamination · 2: Entamoeba histolytica · 3: halophiles
- It lives in the gut.
- Spread by cysts in contaminated water.
- Halo means salt.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. An inoculating loop is sterilised by: a. heating it until red-hot in a flame 2. In the lytic cycle, the host cell finally: b. Penicillium 3. The antibiotic penicillin is made by the mould: c. bursts, releasing new viruses 4. Cassava mosaic virus is spread mainly by: d. whiteflies and infected cuttings Show answer
1 → a (heating it until red-hot in a flame) · 2 → c (bursts, releasing new viruses) · 3 → b (Penicillium) · 4 → d (whiteflies and infected cuttings)
- Then it is cooled before use.
- Bursting is lysis.
- Discovered by Fleming.
- Using clean cuttings controls it.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
methanogens · red blood cells · autoclave · CD4 T-helper cells- Media and glassware are sterilised in an .
- HIV mainly destroys .
- Archaea that produce methane are .
Show answer
1: autoclave · 2: CD4 T-helper cells · 3: methanogens
- Steam at 121 °C under pressure.
- This weakens immunity.
- They live in anaerobic places like biogas tanks.