LEARNING OBJECTIVES:
Use a microscope to identify the types and quantities of bacteria present in probiotic yogurts and plain yogurt.
INTRODUCTION:
The students prepare slides of different types of yogurt and examine them under the microscope to see if there is a difference between the quantity and strains of bacteria present.
USEFUL PRIOR WORK:
The students should know how to use a microscope and how to prepare a microscope slide.
BACKGROUND SCIENCE:
Yogurt is made by fermenting milk using bacteria. The bacteria feed on the sugar (lactose) present in the milk and produce lactic acid as a waste product. This acid is what gives yogurt its characteristic sharp taste. Probiotic yogurts are a specific type of yogurt that contain live active cultures of bacteria. These cultures have typically been added to the yogurt for their potential health benefits. The most common bacteria found in probiotic yogurts are Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria are thought to be beneficial to the human digestive system. This is because the digestive system contains bacteria that help to aid digestion, therefore by building up these colonies of ‘good’ bacteria it reduces the chance of harmful bacteria being able to establish themselves in the digestive system. All yogurts contain ‘good’ bacteria but probiotic yogurts are marketed as containing more of these ‘good bacteria’ than regular yogurts and subsequently are typically priced higher than regular yogurts.
NATIONAL CURRICULUM LINKS:
Cells and organisation
• cells as the fundamental unit of living organisms, including how to observe, interpret and record cell structure using a light microscope
• the structural adaptations of some unicellular organisms.
Nutrition and digestion
• the importance of bacteria in the human digestive system.
MATERIALS NEEDED:
Microscopes, slides, coverslips, swabs, dropper, water, plain yogurt – three different varieties, probiotic yogurt – three different varieties.
SAFETY AND TECHNICAL NOTES:
• You may want to use yogurts from a range of price points so the students can compare, for example, expensive plain yogurt with cheaper probiotic yogurt.
• Emphasise the importance of having a thin layer of yogurt on the slide so the bacteria will be more visible under the microscope.
• Remind the students not to consume any of the yogurt.
• Be aware of any allergies.
METHOD:
To be done in advance by the teacher
Identify the different strains of bacteria present in the yogurts that you will be using for the experiment. They will usually be found on the label or the ingredients list of the yogurt. Prepare a handout or slide that shows the shapes of the different bacteria and their names for the students to use in their identifications. Include some bacteria that are not present in the yogurts.
STUDENTS:
1 Choose the first yogurt you will be testing.
2 Use a swab to add a small amount of the yogurt to a microscope slide. Use the swab to spread the yogurt into a thin layer on the microscope slide.
3 Use the dropper to add one drop of water onto the layer of yogurt.
4 Add a coverslip to the microscope slide so that the layer of yogurt is covered.
5 Place the slide under the microscope and use the lowest power magnification to find a section of yogurt with bacteria present.
6 When you have located the bacteria, use a higher power magnification to examine the bacteria.
7 Draw what you can see and try to count how many individual bacteria are present.
8 Repeat the investigation for the other yogurts.
9 When you have examined all the yogurts try to identify which types of bacteria are present in the different yogurts using the handout you have been given.
DATA COLLECTION IDEAS:
Students can draw what they observe under the microscope and count the number of bacteria present.
They can also identify the type of bacteria present in each yogurt.
DIFFERENTIATION:
• Decrease the challenge: Students could perform a simple streak test on an agar plate for each yogurt and grow the bacteria. They could then compare which yogurt produced the most bacteria.
• Increase the challenge: Students could perform a more accurate count of the number of bacteria present by using a counting chamber slide if this is available.
USEFUL QUESTIONS TO ASK THE STUDENTS:
1 Which yogurt do you think is the best one for your digestive system? Why do you think this?
2 Do you think probiotic yogurts are worth the extra cost? Why do you think this?
3 Was this the most accurate way of determining the quantity and types of bacteria present? How could we improve the investigation?
HOMEWORK:
The students can prepare an advertising poster for a probiotic yogurt including details of the bacteria present and why it is good for your digestive system.
