3.4 Microbiology Study Guide - WJEC A Level Biology

Bacterial Classification and Cell Wall Structure

Bacteria come in three main shapes that you'll need to recognise: bacillus (rod-shaped), coccus (spherical), and spiral (corkscrew-like). Think of them as nature's building blocks with different architectural designs.

The cell wall is what gives bacteria their strength and shape, whilst protecting them from bursting when water enters by osmosis. It's like a protective jacket that keeps everything intact.

Gram staining is a brilliant technique that sorts bacteria into two groups based on their cell wall structure. The process involves four steps: crystal violet stains both types purple, iodine locks in the stain, alcohol removes colour from some bacteria, and safranin gives a red counter-stain.

Gram-positive bacteria have thick peptidoglycan walls and stay purple after staining. They're vulnerable to penicillin, which weakens their cell walls, and lysozyme, which breaks down their peptidoglycan. Gram-negative bacteria turn red because they have an extra lipopolysaccharide layer that protects them from penicillin, requiring different antibiotics that target protein synthesis instead.

Quick Tip: Remember "Positive = Purple" to nail gram staining questions in your exams!

Microbial Growth Conditions and Aseptic Techniques

Microorganisms are surprisingly picky about their living conditions. They need nutrients like glucose for energy and nitrogen for making amino acids, plus growth factors such as vitamins. Temperature matters too - most prefer 25-45°C, with disease-causing bacteria loving our body temperature of 37°C.

When it comes to oxygen, bacteria fall into three camps: obligate aerobes must have oxygen, facultative anaerobes prefer oxygen but can survive without it, and obligate anaerobes actually die in oxygen's presence.

Aseptic techniques are your best friend in the lab - they prevent contamination and keep everyone safe. Key practices include sterilising work surfaces, flaming bottle necks, never putting lids on surfaces, and only lifting Petri dish lids slightly. Always incubate student cultures at 25°C, not 37°C, to avoid growing dangerous pathogens.

Sterilisation happens in an autoclave - essentially a high-pressure steam cooker that heats equipment to 121°C for 15 minutes. This kills absolutely everything, making your apparatus completely sterile.

You can measure bacterial populations directly by counting colonies (viable counts) or using a haemocytometer (total counts), or indirectly by measuring how cloudy the culture looks using light absorption.

Lab Safety: Never seal Petri dishes completely - partial sealing prevents anaerobic conditions that could encourage pathogenic growth!

Culture Media and Population Counting Techniques

Different culture media serve specific purposes in microbiology. Defined media contain only known ingredients, whilst undefined media include components you can't fully identify. Selective media only allow certain bacteria to grow, and complete media provide everything needed for healthy growth.

Serial dilution is your go-to method for counting bacterial populations accurately. You create a series of 10-fold dilutions (10⁻¹, 10⁻², 10⁻³, etc.) until you get countable colonies on plates. The sweet spot is usually plates with 30-300 colonies - fewer isn't statistically reliable, and more becomes impossible to count.

A haemocytometer gives more accurate counts than colony counting and uses a special microscope slide to count both living and dead cells. It's brilliant for getting precise total counts.

Turbidimeters measure how cloudy your bacterial culture looks using a colorimeter. The cloudier it is, the more bacteria you have. You compare your reading to a standard graph plotting light absorbance against bacterial numbers. Like haemocytometers, this gives total counts since dead cells also contribute to cloudiness.

The population size equation is straightforward: Number of colonies × Dilution factor ÷ Volume of sample. Master this formula and you'll breeze through practical calculations.

Counting Tip: Always choose plates where you can count distinct, separate colonies - merged colonies or overcrowded plates will ruin your accuracy!