Health, disease and the development of medicines revision notes

Sterilisation of Equipment and Media

To prevent contamination, all equipment and growth media must be sterile before use. An autoclave uses high-pressure steam at temperatures around 121°C for about 15-20 minutes, effectively killing bacteria, spores, and viruses. This method is crucial for sterilising things like nutrient agar, broth, and empty Petri dishes. Without proper sterilisation, unwanted microorganisms would grow alongside the desired culture, rendering experimental results unreliable and potentially creating biohazards. Other methods include dry heat for glassware or filtration for heat-sensitive liquids.

Transferring Microorganisms Aseptically

When transferring microorganisms, sterile inoculating loops are essential. These loops, made of wire, are typically sterilised by passing them through a Bunsen flame until red hot, immediately before and after each transfer. This process incinerates any microbes present on the loop. The cooled, sterile loop is then used to pick up and transfer microorganisms to new growth media. It's crucial to cool the loop to avoid killing the culture or melting the agar. Minimising exposure of cultures and media to the air is also vital; this often means working quickly and close to a flame to utilise the sterile updraft it creates.

Maintaining Culture Purity

Keeping Petri dishes and culture vials covered at all times, except for the brief moment of inoculation or sampling, is a fundamental aseptic practice. The lids act as physical barriers against airborne contaminants like dust and spores. When opening a culture vessel or dish, it should be done quickly and close to a sterile zone, such as near a Bunsen flame, to minimise the time the sterile contents are exposed to non-sterile air. The base should be lifted off the lid, rather than the lid off the base, to reduce the chance of airborne particles falling in.

Using the Circle Area Formula in Microbiology

The area of circular zones—whether representing bacterial colonies or regions where bacteria cannot grow due to antimicrobials—can be calculated with A = πr². This formula provides a direct and reliable method for determining the size of these zones for comparisons between different experiments.

Measuring for Circle Area Calculations

Often, a ruler or calliper is used to measure the full width (diameter) of the zone. Since the radius is half the diameter, always divide by two before substituting into the formula. Measurements must use consistent units and be as accurate as possible, as a small error in radius can make a large difference in area.

Bacterial and Fungal Infections

Some common infections are caused by bacteria or fungi. Cholera is a bacterial infection that primarily affects the small intestine, leading to severe diarrhoea and dehydration. Tuberculosis (TB), another bacterial disease, mainly impacts the lungs, causing persistent coughs and damage. Helicobacter pylori is a bacterium known to cause stomach ulcers by eroding the stomach lining. Chalara ash dieback is a fungal disease affecting ash trees, causing lesions on the bark and leaves, ultimately leading to leaf loss and potentially tree death.

Viral and Protist Infections

Viruses and protists also cause significant diseases. HIV is a virus that targets and destroys white blood cells, which are crucial for the immune system. This destruction severely compromises the body's ability to fight off other infections, leading to AIDS over time. Ebola, another viral infection, causes haemorrhagic fever, characterised by internal and external bleeding, and is often fatal. Malaria is caused by a protist (Plasmodium) transmitted through mosquito bites. This protist multiplies in the liver and then infects red blood cells, causing symptoms like fever, chills, and anaemia.

Variables, Controls, and Safe Method

The independent variable is the antimicrobial agent (type or concentration); the dependent variable is the diameter of the inhibition zone. Control variables include strain of bacteria, agar thickness, incubation temperature (never above 25°C), disc size, and exposure time. Use aseptic technique: disinfect surfaces, wear gloves and goggles, sterilise tools, only open plates in brief supervised periods. During setup, inoculate the plate evenly, place the discs using sterilised forceps, and include a control disc. Seal with tape and label. After incubation, DO NOT open the plate; measure inhibition zones through the closed lid. Always work under supervision.

Graphical Analysis, Results and Microbiology Safety

After incubation, measure inhibition-zone diameters through the closed lid in millimetres. Repeat measurements and calculate average values for reliability. Suitable graphs: bar graphs (for different agents/types) or line graphs (for increasing concentrations), plotting type/concentration on the x-axis and mean diameter on the y-axis. State uncertainty (e.g., ±1 mm ruler accuracy) and calculate percentage error as (uncertainty / mean measured value) x 100%. All used plates and materials must be sterilised and safely disposed of after use according to school/lab protocols. Never open plates after incubation or incubate at body temperature.