Bacteria, fungi and viruses: how microorganisms live

Three kinds of microorganism

• Microorganisms are living things, and near-living particles, too small to see one at a time without a microscope. The three groups you need are bacteria, fungi and viruses.
• Bacteria are single cells with a cell wall, a cell membrane, cytoplasm and ribosomes. There is no nucleus: their DNA is one loop lying free in the cytoplasm, and many also have small extra rings called plasmids and a whip-like flagellum for swimming.
• Fungi are made of cells with a nucleus and a cell wall made of chitin. Yeast lives as single oval cells; moulds such as Rhizopus on bread grow as long threads called hyphae, which tangle into a mat called a mycelium.
• Viruses are not cells at all. A virus is genetic material (DNA or RNA) inside a protein coat called a capsid, sometimes wrapped in an outer envelope.
• Sizes differ hugely: a bacterium is usually 1 to 5 micrometres (µm) long and a yeast cell 3 to 10 µm, but most viruses are 20 to 300 nanometres, so you need an electron microscope to see them.

Life processes, and what AS 92020 asks for

• Living things carry out the life processes, often remembered as MRS GREN: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.
• For AS 92020 you describe one life process of one microorganism in real detail, so choose one you can connect to an environment, such as respiration in yeast or nutrition in a mould.
• A good description names the process, says what goes in and what comes out, and says where and how it happens in the microorganism.
• Writing 'bread mould feeds on bread' is too thin. Writing 'Rhizopus hyphae release enzymes onto the bread that break starch into glucose, and the hyphae absorb the glucose' describes the process.

Nutrition: digesting food outside the body

• Fungi and most bacteria cannot make their own food, so they take it from their surroundings.
• Most fungi and many bacteria are saprophytes: they feed on dead or decaying material such as fallen leaves, bread or old fruit.
• A fungus digests its food outside its body. The hyphae release enzymes into the food, the enzymes break large molecules (starch, protein, cellulose) into small soluble ones (glucose, amino acids), and the hyphae absorb them.
• This is extracellular digestion, and it needs moisture: the enzymes and the small molecules move through a film of water.
• Hyphae keep growing and branching through the food, which gives them a huge surface area for releasing enzymes and absorbing nutrients.
• Some microorganisms are parasites instead, taking food from a living host, such as the water mould that causes kauri dieback feeding in kauri roots.

Respiration: aerobic and anaerobic

• Respiration releases energy from glucose inside every living cell, bacteria and fungi included.
• Aerobic respiration uses oxygen and releases the most energy.
• When oxygen runs out, many microorganisms switch to anaerobic respiration, also called fermentation, which releases much less energy.
• Yeast fermentation makes ethanol and carbon dioxide: the gas makes bread rise and the ethanol is the alcohol in beer and wine.
• Lactic acid bacteria such as Lactobacillus ferment sugar into lactic acid, which sours milk and thickens it into yoghurt.

1. glucose + oxygen → carbon dioxide + water (+ a lot of energy)aerobic, in yeast and many bacteria
2. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂Othe same reaction as a balanced symbol equation
3. glucose → ethanol + carbon dioxide (+ a little energy)anaerobic, in yeast
4. glucose → lactic acid (+ a little energy)anaerobic, in lactic acid bacteria

Excretion: getting rid of waste

• Excretion is removing the waste products of the cell's own chemical reactions.
• The wastes of respiration are carbon dioxide and water, or ethanol or lactic acid, depending on the microorganism and whether oxygen is present.
• Bacteria also excrete other substances, such as ammonia when they break down proteins, and some release toxins.
• The wastes pass out through the cell membrane and build up in the surroundings. That is how microorganisms change their environment: lactic acid lowers the pH of milk, and ethanol builds up in a brew.
• A build-up of their own waste can end up slowing the microorganisms themselves, which is part of the story of a growth curve.

Reproduction: fission, spores and budding

• Bacteria reproduce asexually by binary fission: the cell copies its DNA loop, grows, and splits into two identical daughter cells.
• In ideal conditions some bacteria divide every 20 minutes or so, so the numbers double again and again.
• Moulds such as Rhizopus grow upright hyphae topped with sporangia, round cases that each release thousands of tiny spores. The spores drift on air, water or animals and grow into new hyphae where they land on damp food.
• Yeast can also reproduce by budding: a small bud grows out of the parent cell, gets a copy of the nucleus, and pinches off.
• Spores are tough. Many survive drying out, and some bacteria make resting endospores that survive boiling, which is why canned food is heated under pressure.

How a virus replicates

• A virus has no cytoplasm, no ribosomes and no enzymes for respiration, so on its own it does nothing at all.
• To replicate, it attaches to a particular kind of host cell and injects its genetic material, or the whole virus is taken into the cell.
• The viral genes take over the cell, which then makes copies of the viral genetic material and the coat proteins, using its own raw materials and energy.
• The new parts assemble into new viruses, which leave by bursting the cell open or by budding out through its membrane, ready to infect more cells.
• Viruses that infect bacteria are called bacteriophages; influenza and cold viruses infect the cells lining your airways.

Why a virus is not fully living

• A virus is not a cell, and it does not feed, respire, excrete, grow or respond to its surroundings.
• Its one life-like process, replication, happens only inside a host cell, using the host's machinery and energy.
• Outside a host it is an inert particle, which is why antibiotics, which attack bacterial cells, do nothing to a virus.
• It does carry genes and it changes over time by mutation, which is why biologists place it on the border of living, and why AS 92020 lets you study a virus through its replication.

What microorganisms need to grow

• Every life process runs on enzymes, so a microorganism grows fastest when conditions suit its enzymes.
• Temperature: cold slows enzymes down, warmth speeds them up to an optimum, and too much heat denatures them (changes their shape for good), which can kill the cell.
• pH: each species has a pH it grows best at. Most bacteria prefer close to neutral, while many fungi cope with acidic food such as fruit.
• Moisture: enzymes and dissolved nutrients need water, so dried food keeps.
• Nutrients and oxygen: they need a food source for energy and raw materials, and some need oxygen, some are harmed by it, and many, such as yeast, manage either way.