by Stuart Fieldhouse – Science Teacher, Outside Education

Safety Notice

General Safety for all experiments:

  • Experiments should be done in with the guidance and help of an adult.
  • A first aid kit should be readily available.
  • The Experimental surface needs to kept clear of unnecessary items.
  • Flammable liquids and materials need to be kept clear of the experimental surface
  • Long hair should be tied back.
  • Sharp instruments and equipment should be used in accompaniment with an adult.
  • Hot materials or liquids should be used with aid and guidance from an adult.
  • Naked flame should only be used with an adult present.
  • Used Biological samples should be carefully disposed of in general waste unless specified otherwise in the experimental method.
  • After completing an experiment hands should be thoroughly washed with water and soap.
  • All waste materials should be disposed of according to the local rules on recycling and waste disposal.

Experiment Specific Safety:

  • Once the samples have been incubated. Do no touch the samples directly.
  • Wash hands after handling the samples
  • Do not open the sample more than the required time to photograph or inspect.
  • Disposal Non-recyclable:
    • If the container you have used is disposable place the unopened containers with contents into a fresh plastic bag and seal the bag and place in the general waste bin for collection.
  • Disposal Recyclable:
    • Using household bleach carefully open and pour enough liquid into the container to adequately cover the potato slice.
    • Wear appropriate protection, clothing and rubber gloves as you would when normally handling bleach solution.
    • Reseal the container and gently agitate the liquid.
    • Open and pour the contents of the container into an outside drain that connects to the main drainage system.
    • The containers can now be rinsed using water and then cleaned in the appropriate manner.
    • Ensure the drain that has been used is flushed with a suitable volume of water to remove bleach from the surface area.

Experiment: Bio-Hazard Sampling

Preparation

Preparing the Substrate:

  1. Take a potato of about 7cm to 10cm in length and cut it into slices of 0.5cm to 1cm. Leave the skin on to help with keeping it in one piece.
  2. Place the slices of the potato into a tub that has a sealable lid. Pour boiling water over the slices replace the lid and allow the water and potato slices to cool down slowly. Do not accelerate the cooling process.
  3. Prepare several jam jars, plastic tubs with lids or similar. Clean out the containers with boiling water and replace the lids.
     
  4. Once the potato slices and water have cooled to room temperature, using tongs or wearing sterile gloves place one or more slices of the potato in the jars/tubs each slices must be separate from the others will one to two centimetres between them. Replace the lid on the tubs.

Method

House sampling:

  1. Rinse a cup with boiling water and then pour about 100ml of boiling water into the cup. Cover the cup and allow the water to cool naturally.
  2. Using “ear bud” cotton wool buds and wearing sterile gloves. Lightly dip one end of the cotton bud into the water and shake of any excess.
  3. Use the wet end of the bud and rub it on the surface that you want to sample. Carefully without putting the cotton bud down a tub with a potato slice in it.  Using a zig-zag/ wavey pattern rub the “sample” end of the cotton bud over the surface of the potato.
  4. Replace the lid, however it is important to leave a small air gap, sufficient for air to diffuse into the container slowly. If it is not possible or you have forgotten to do so this will not overly affect the growth of the microorganisms though it may favour anaerobic organisms.
  5. Label the container and dispose of the cotton bud in the bin.
  6. Repeat steps one to five for as many potato slices that you have except for the last one.
  7. Place the last slice of potato in a container and leave as a control with no sample on it.

Incubation:

  1. Place the containers in a dark and warm/house temperature environment. The growth of the microorganisms will take between 24-72 hours depending on the temperature.
  2. Avoid moving the containers for at least 48hrs as water droplets could wash any microorganisms off of the potato surface. Do not open the containers in this period as this could let in contamination.

Viewing:

If you can see through the lid of the container without then do so to observe the growth. If you have to remove the lid to observe the contents do as follows.

  1. Wear disposable gloves and a mask or face shield.
  2. Open the container as slowly as possible as not to disturb the contents.
  3. Preferably do this outside or underneath an active cooker extractor fan.
  4. Do not “stick your nose” in the container.
  5. Use a mobile phone to photo graph the contents.
  6. Close the container and place to one side for disposal.

Disposal

Non-recyclable:

If the container you have used is disposable place the unopened containers with contents into a fresh plastic bag and seal the bag and place in the general waste bin for collection.

Recyclable:

  1. Using household bleach carefully open and pour enough liquid into the container to adequately cover the potato slice.
  2. Wear appropriate protection, clothing and rubber gloves as you would when normally handling bleach solution.
  3. Reseal the container and gently agitate the liquid.
  4. Open and pour the contents of the container into an outside drain that connects to the main drainage system.
  5. The containers can now be rinsed using water and then cleaned in the appropriate manner.
  6. Ensure the drain that has been used is flushed with a suitable volume of water to remove bleach from the surface area.

The Science and More

The world around is covered in micro-organisms and the fear of them is played upon heavily by the media and companies trying to sell us products to eliminate them.  However many of these organisms are not a threat to us and many are required by our bodies and other organisms to survive.

  • Take for instance Poo. There are more bacterial cells in one cubic centimetre of faeces than there are cells that make up a person.  In the average human there could be more than one thousand the number of bacteria alive in you than cells that make you up.
  • When humans are born bacteria are transferred from the mother to the child. These bacteria are very important for the healthy growth of the baby baby.

We have become paranoid about having every surface clean, adverts bombard us with exaggerated images of uncleanliness and how bad it is for us.  This is a not healthy for the human machine.  A little mud is good but perhaps not too much.

Therefore it is always an interesting experiment to see who and what we are sharing the space we live with.  There are many places in the house where would expect to find microorganisms edges of the shower, the general waste bin, the floor by the door.

However what about the keyboard you type on every day? The surface of your nose? The shelf in the fridge?

Normally you would use an incubator and agar plates in petri-dishes.  It is unlikely the average  household has these items so we can use substitutes instead.

The Potato

The potato is primarily a source of starch otherwise known as carbohydrates.  Carbohydrate themselves are made up of sugar. Sugar molecules linked together in chains for easy storage when needed and this is exactly what the microorganisms use as food.

Unfortunately the potato has a number of defense mechanisms that stop bacteria and fungi from devouring them.  As it is important to remember the cells in the potato in the cupboard are alive.  You can test this by putting a potato in the ground it will grow into a potato plant.  These can be easily defeated by heat treatment. Placing the potato in boiling water has will destroy the defensive systems as well as help break down the cells in the potato to release the starch stored within and it is ready to . to grow microorganisms.

What will grow?

The two main categories we would expect to see are molds and yeasts (fungi) and bacteria.  There are other types of microorganisms such as viruses, Archaebacteria, Protista and amoeba however these are very unlikely to grow and will not be visible.  If there are any amoeba present they will be feasting on the bacteria.

The warmer the better but not too warm.  Microorganisms are complex machines however their growth rate is simply controlled by nutrient level and temperature.

Green Beans in the freezer are coated with bacteria as they should be having been grown in a field.  Before they are frozen they are washed to get rid of the mud and dust but this does not remove the bacteria and they get frozen along with the beans.  Through research it has been shown that when a bacterium in placed in an environment at -190C all biological processes stop.  The bacteria and fungi that would normally cause the beans to rot are stopped, cold.

To get microorganisms growing well and fast we need to warm them, room temperature of 200C is good but above 300C is better however 400C is getting a little too warm and they tend to stop growing. The fastest growth will occur at about 350C.  This should give results in about two days. 200C  will take about at least four days.  Once they microorganisms have grown then then you can place the containers in a cool place.  Outside buildings will be at the current weather temperature and this really goof for slowing down and preserving the samples.

After several days of growth of the microorganism it will be now possible to see which surfaces and source locations have the most number of microorganism and whether the predominant types are bacteria or fungi.

To know what is growing on the potato samples is at the next level of science.  To see microorganisms clearly requires a high power microscope with specialist lenses.  There are other methods to know what is growing but these require a University level lab.

Molds and fungi can be a little easier to discern based upon the morphology of their growth and structure of the fruiting bodies.

Further Science:

If molds and bacteria grow every everywhere a questions that springs to mind is; does anything eat them?

One of the coolest creatures that does is the Tardigrade and this will be the subject of a future experiment.

Outcomes

Key Stage 1

  • observing closely, using simple equipment.
  • performing simple tests.
  • identify and name microhabitats and organisms.

Key Stage 2

  • setting up simple practical enquiries, comparative and fair tests.
  • recognise that living things can be grouped in a variety of ways
  • describe how living things are classified into broad groups according to common observable characteristics and based on similarities and differences, including microorganisms, plants and animals.

Key Stage 3

  • the structural adaptations of some unicellular organisms.
  • apply sampling techniques.

Key Stage 4

  • evaluating methods and suggesting possible improvements and further investigations.
  • carrying out experiments appropriately, having due regard to the correct manipulation of apparatus, the accuracy of measurements and health and safety considerations.