Topic G: Microorganisms and Humans (Elective)

This elective focuses on the smallest organisms and their large impact on human life. You study the structure and growth of bacteria, viruses and fungi, how microbes are cultured safely, how they are harnessed in food and industry from bread to antibiotics, and how some cause disease while our immune system and medical measures keep them in check. DSE questions often combine practical technique, such as aseptic culturing and reading growth data, with applied understanding of how microbes are exploited or controlled.

The biology of microorganisms

Microorganisms are organisms too small to see without a microscope and include bacteria, viruses and fungi such as yeast. A bacterial cell has a cell wall, cell membrane, cytoplasm and a single loop of DNA, but no true nucleus, and some have a protective capsule or flagella. Viruses are much smaller, consist only of genetic material in a protein coat, and can reproduce only inside a living host cell, which is why they are not considered fully living. Yeast is a single-celled fungus. Bacteria reproduce rapidly by dividing in two when conditions of warmth, moisture and nutrients are suitable. You should be able to compare the structures of bacteria, viruses and fungi and explain why viruses depend on host cells to multiply.

Growth and culturing of microorganisms

Microorganisms are grown, or cultured, on nutrient media that supply the food, water and conditions they need. In a closed culture, the number of cells follows a growth curve: a lag phase while the cells adjust, a log phase of rapid exponential increase, a stationary phase as nutrients run out and wastes build up, and finally a death phase. To culture microbes safely, aseptic technique is used to prevent contamination and to protect the worker: equipment is sterilised, the inoculating loop is flamed, and plates are sealed and incubated at a controlled temperature, usually below body temperature to discourage human pathogens. You should be able to describe and explain each step of aseptic technique and interpret a microbial growth curve.

Microorganisms in food production

Humans have long used microorganisms to make and preserve food. Yeast respires anaerobically in fermentation, producing carbon dioxide that makes bread rise and ethanol used in brewing wine and beer. Bacteria are used to make yoghurt and cheese, where they convert lactose into lactic acid that thickens milk and gives a sour taste, the acid also helping to preserve the food. Microbes are also used to make foods such as soy sauce. At the same time, controlling unwanted microbes preserves food: methods such as refrigeration, heating, drying, salting and adding sugar slow or stop microbial growth. You should be able to name the microbe and the chemical change in a given food process and explain how a preservation method prevents spoilage.

Microorganisms in industry and medicine

Microorganisms are grown on a large scale in fermenters to make useful products. Fungi and bacteria are cultured to produce antibiotics such as penicillin, which treat bacterial infections, and microbes are used to make enzymes for use in food processing, washing powders and other industries. Genetically modified bacteria can be made to produce human proteins such as insulin. In a fermenter, conditions of temperature, pH, oxygen and nutrient supply are carefully controlled to maximise the yield, and the product is then extracted and purified. You should be able to describe why industrial fermenters need controlled conditions, name useful products made by microbes, and explain the advantages of using microorganisms rather than chemical methods for these products.

Microorganisms and disease

Some microorganisms are pathogens that cause disease in humans. Bacteria can cause diseases such as tuberculosis, cholera and food poisoning, sometimes by releasing toxins, while viruses cause diseases such as influenza, AIDS and the common cold by invading and destroying cells. Diseases spread by routes including air, food and water, contact and body fluids, so understanding the route allows prevention. Food poisoning, for example, is reduced by proper cooking, refrigeration and hygiene that prevent bacteria multiplying in food. You should be able to link a named disease to its causative microbe and transmission route and suggest matching control measures, distinguishing diseases caused by bacteria, which antibiotics can treat, from those caused by viruses, which they cannot.

Defence, immunity and treatment

The body resists microbial disease through its defences and with medical help. Non-specific barriers such as skin, mucus and stomach acid keep many microbes out, while white blood cells provide the specific immune response: phagocytes engulf microbes and lymphocytes make antibodies against their antigens, leaving memory cells for lasting immunity. Immunisation uses vaccines to build this immunity in advance against diseases such as those caused by bacteria and viruses. Bacterial infections can be treated with antibiotics, but their overuse selects for resistant bacteria, so they must be used responsibly. You should be able to outline how the immune system deals with a microbe, explain how vaccination and antibiotics help, and discuss why antibiotic resistance is an increasing problem.

Beneficial roles and biotechnology links

Not all microbes are harmful; many are essential and useful. Decomposer bacteria and fungi break down dead matter and recycle nutrients in ecosystems, and bacteria in the human gut help digestion and produce some vitamins. In sewage treatment, microorganisms break down organic waste so that water can be safely returned to the environment, and microbes are used to break down oil spills and other pollutants in bioremediation. These roles connect microbiology to ecology and to biotechnology, where genetically modified microbes make medicines and other products. You should be able to give examples of beneficial microbes, explain their role in recycling and waste treatment, and appreciate that the same group of organisms includes both helpful species and dangerous pathogens.