WJEC GCSE Biology 2.7 Micro-organisms and Their Applications Exam Questions

About WJEC GCSE Biology 2.7 Micro-organisms and Their Applications

This short Unit 2 topic is about growing and using micro-organisms safely. You need to describe aseptic techniques for inoculating, plating and incubating micro-organisms, and relate colony counts on a plate to the number of bacteria in the original sample. You must explain how temperature affects bacterial growth and how this is used in storing food. The topic also covers the conditions needed to grow Penicillium in an industrial fermenter and extract penicillin. Investigating the effect of antibiotics on bacterial growth is the specified practical, which links directly to antibiotics and resistance in 2.4 and 2.8.

What you need to know

• Aseptic technique Sterilise the inoculating loop by flaming until red hot and let it cool; flame the neck of culture bottles; lift the Petri dish lid only slightly; work near a Bunsen flame. These steps prevent contamination by unwanted microbes.
• Incubating safely Tape the lid with two or four strips, without sealing all the way round, so oxygen can enter and harmful anaerobic bacteria are not encouraged. Incubate upside down at no more than 25 °C in school to reduce the risk of growing human pathogens.
• Colony counts Each colony grows from a single bacterium, so counting colonies, multiplied by the dilution factor and adjusted for the volume plated, estimates bacterial numbers in the original sample.
• Temperature and food Bacteria grow faster in warm conditions up to an optimum. Refrigerating food slows growth, freezing stops it, and cooking at high temperatures kills bacteria.
• Penicillin production Penicillium is grown in a fermenter with a nutrient supply, controlled temperature and pH, sterile air for oxygen, and stirring. Penicillin is then extracted from the liquid and purified.
• Antibiotic practical Discs soaked in different antibiotics are placed on a bacterial lawn; the larger the clear zone of inhibition, the more effective the antibiotic. Area of zone = πr^2.

Common mistakes in 2.7 Micro-organisms and Their Applications

• Saying the loop is flamed to clean it. It is flamed to kill any micro-organisms so the culture is not contaminated.
• Sealing the Petri dish lid all the way round with tape. Fully sealing it creates anaerobic conditions that can favour dangerous bacteria; use only a few strips.
• Incubating at 37 °C in school. Body temperature encourages human pathogens, so school cultures are incubated at 25 °C or below.
• Using the diameter instead of the radius in πr^2 when calculating the area of a clear zone. Halve the diameter first.

Try three 2.7 Micro-organisms and Their Applications exam-style questions