Most microbes are invisible to the naked eye — a single bacterium is far too small to see. So if you want to study bacteria, measure how fast they grow, or test how they respond to antibiotics, you first need to grow huge numbers of them until they form something visible — like a colony you can actually see and count. That process is called culturing.

Think of it like this: one seed is invisible from a distance, but a field of a million identical plants is obvious. Bacteria clone themselves by splitting in two (binary fission), so given the right conditions and enough time, one single bacterium multiplies into a visible blob called a colony — millions of genetically identical cells all descended from that one original.

• Nutrients — carbon, nitrogen and minerals, usually supplied in a nutrient growth medium (plural: media)
• Oxygen — unless the organism is anaerobic, in which case it needs the absence of oxygen
• Optimum pH
• Favourable temperature

Every step of culturing risks contamination — either your culture gets invaded by unwanted wild bacteria from the air, or (more worryingly) a mutation could turn your culture into a pathogenic strain. That's why the whole process is built around sterile technique:

• All equipment is sterilised before use — killing existing microorganisms by heat or antimicrobial chemicals
• Work is done near a Bunsen flame — the rising hot air currents carry airborne microbes away from your open culture
• Media is kept sealed/covered until use, and cultures are covered immediately after inoculation (but not airtight, if the organism is aerobic — it still needs oxygen)
• Cultures are incubated at around 20 °C in a school lab — deliberately not 37 °C, because 37 °C is the temperature most human pathogens thrive at. Hospitals/research labs may use higher temperatures for faster results, but that carries more risk.
• Used cultures are sealed in a plastic bag and sterilised at high temperature and pressure before disposal

Inoculation is the technique of transferring microorganisms into (or onto) a growth medium using a sterilised inoculation loop. You might transfer bacteria from a liquid broth onto solid agar, or vice versa. The loop is flame-sterilised before and after use to prevent cross-contamination between samples.

Often you don't want a mixed soup of species — you want a pure culture: just one isolated type. This matters hugely in medicine, e.g. to correctly diagnose which specific bacterium is causing an infection. Ways to isolate a single type include:

• Growing under aerobic or anaerobic conditions — this alone eliminates any organisms that can't survive in that environment
• Using a selective medium — nutrients tailored so only the desired organism (or a mutant strain, or antibiotic-resistant strain, or a genetically modified organism) can thrive
• Using an indicator medium — contains a substance that changes colour only when acted on by the target microorganism, letting you visually pick out the right colonies from the rest

Once you've got a culture growing, how do you actually put a number on "how much"? Bacteria are too small to count one-by-one under normal conditions, so scientists use four broad strategies. Each has a different trade-off between accuracy, speed, and whether it can distinguish living from dead cells.

A haemocytometer is a special microscope slide with a grid etched onto it, forming a chamber that holds an exact, known volume of liquid: 0.1 mm³. Originally designed to count blood cells (hence "haemo" = blood), it works just as well for counting microorganisms directly under a microscope.

The method:

• Dilute the broth 1:1 with trypan blue — a dye that stains only dead cells blue, so you know to skip them and count only living, colourless cells
• Fill the haemocytometer chamber with the stained broth
• Count living cells in the four corner squares (each made of 16 smaller squares)
• Be consistent about border cells — e.g. always count cells touching the top/right edges, always ignore cells touching the bottom/left edges (otherwise cells sitting on a shared border get double-counted or missed)
• Calculate the mean cell count from the four corner squares

This method exploits a simple fact: every visible colony on an agar plate started from exactly one living cell. So if you count colonies, you're indirectly counting the original living cells that seeded them. The catch: if your broth is too concentrated, all the colonies merge into one big undifferentiated mass and you can't count anything. The fix is to seriously dilute the sample first — usually via a serial dilution (each tube is diluted by the same factor from the last, e.g. ×10 each time) — until individual, countable colonies appear on the plate.

Fungi are awkward for the two methods above because they often don't exist as neat single cells — instead they grow as a tangled mass of thread-like cells called a fungal mycelium (plural: mycelia). You can't easily count "how many fungus" the way you count bacterial colonies. Instead:

• Diameter method: Inoculate agar plates with fungal spores, incubate, then measure the diameter of the resulting mycelium. Bigger mean diameter = more growth. Great for comparing growth rates under different conditions (e.g. different temperatures).
• Dry mass method: Inoculate a liquid broth with fungal spores, take samples at set time intervals, filter or centrifuge out the mycelia, dry them in an oven overnight, then weigh. Higher mass = more fungal growth.

As a population of microorganisms grows, the liquid culture becomes cloudier — this cloudiness is called turbidity. More cells in suspension → more light gets scattered/absorbed → less light passes straight through. This is measured using a colorimeter, a machine that shines a beam of light through a sample (held in a cuvette) and records how much light is transmitted or absorbed.

Turbidity alone doesn't directly tell you "how many cells" — it's an indirect measure. To turn it into an actual number, you build a calibration curve: measure the turbidity of several known-concentration cultures (counted separately with a haemocytometer), plot turbidity against cell count, then use that curve to estimate the cell count of any future unknown sample just from its turbidity reading.

Bacteria reproduce asexually through a process called binary fission: one cell splits into two identical daughter cells. The steps are:

• The single, circular DNA molecule (chromosome) replicates
• Any plasmids present also replicate
• The parent cell divides in two, with the cytoplasm roughly split between the two new cells
• Each daughter cell ends up with one copy of the circular DNA and a variable number of plasmids

If you plot the population size of a growing bacterial culture over time, you get a very characteristic S-shaped-then-declining curve with four distinct phases:

Here's a problem: during exponential phase, a bacterial population can leap from a few hundred cells to several million within hours. If you tried to plot that on a normal (linear) graph, the early data points (a handful of cells) would be squashed flat against the x-axis, invisible next to the millions later on. You simply can't fit "10" and "10,000,000" sensibly onto the same linear axis.

The fix is a logarithmic scale. Instead of the y-axis increasing by equal amounts (0, 10, 20, 30...), it increases by equal multiples — usually powers of 10 (1, 10, 100, 1000, 10,000...). This means the huge range of values — from single figures to millions — can all be shown clearly on one graph, and you can actually see the shape of the early growth as well as the later growth.

During exponential phase, you can actually predict population size mathematically using the formula below.

This practical measures yeast growth over time using turbidity (via a colorimeter) as an indirect proxy for cell number — building directly on the optical methods you met in Section 2.

This experiment can be extended by changing the temperature or concentration of the glucose solution and repeating, to investigate the effect of these variables on yeast growth rate — a classic exam extension question.

Because turbidity can be thrown off by dead cells and other suspended particles (as noted earlier), you can cross-check your turbidity data using a haemocytometer, or by using the microscope's field of view together with graph paper photocopied onto acetate to calculate an exact viewing area, then estimating cell density from there.

• Wear eye protection
• Take care around Bunsen burners
• Use aseptic technique when transferring microorganisms
• Incubate at a safe school-lab temperature — 20°C, not 37°C
• Thoroughly disinfect work surfaces and hands afterward
• Safely destroy cultures at the end of the experiment (seal, sterilise, dispose)

Bacteria are prokaryotes — much smaller and structurally simpler than eukaryotic cells. Key differences from eukaryotic cells:

• Cytoplasm lacks membrane-bound organelles (no mitochondria, no nucleus, etc.)
• Smaller ribosomes — 70S, compared to 80S in eukaryotic cells
• No nucleus — instead a single circular bacterial chromosome, free in the cytoplasm, not associated with proteins (unlike eukaryotic DNA wrapped around histones)
• A cell wall containing the glycoprotein murein (also called peptidoglycan)

Many prokaryotic cells also have extra structures:

• Plasmids — small loops of DNA, separate from the main chromosome
• Capsule (slime capsule) — protects the bacterium from drying out and from attack by the host's immune cells
• Flagella (singular flagellum) — long, rotating tail-like structures for movement; some bacteria have more than one
• Pili (singular pilus) — thread-like structures that let bacteria attach to other cells/surfaces, and are involved in gene transfer during sexual reproduction (conjugation)
• Mesosomes — infolded regions of the cell membrane that can be sites of respiration

Viruses are fundamentally different — they're not cells at all. They're non-cellular, meaning they have no cytoplasm, no ribosomes, and no plasma membrane in the way a cell does. Structurally, a virus is just two (or three) essential parts:

• A nucleic acid core — either DNA or RNA, which can be single- or double-stranded
• A protein coat (capsid) — made of repeating protein units called capsomeres
• An envelope (sometimes) — an outer layer made from the host cell's own membrane phospholipids, picked up as the virus leaves the cell. Because it's derived from a host cell membrane, very few plant viruses have envelopes (plant cell membranes and exit mechanisms differ).

Some viruses also carry proteins inside the capsid to help them hijack a host cell — for example, HIV carries the enzyme reverse transcriptase, which converts its RNA into DNA once inside a cell. Viruses also have attachment proteins (virus attachment particles) sticking out from the capsid or envelope, which let the virus latch onto a specific host cell.

Viruses are grouped by the genetic material they contain and how they replicate:

• Bacteriophages inject their genetic material directly into bacteria (the protein capsid stays outside)
• Some animal viruses enter via endocytosis, fusing their viral envelope with the host cell's surface membrane
• Plant viruses often need a vector (e.g. an insect) to physically breach the tough cell wall

Once inside a host cell, a virus doesn't necessarily cause disease straight away. It follows one of two pathways: