Bacterial Growth Experiment

Today we are going to see how bacteria (germs) grow by using Petri dishes. These are special dishes that have a tannish colored medium, called agar, in them to help the bacteria grow.

Some things to remember when collecting your sample:

Choose an area that you would touch with your hands - give examples (primary students will use their fingers placing them on the agar to collect the bacteria).
Do not touch the agar.
Use a clean Q-Tip to collect the bacteria by rolling the Q-Tip on the surface and turning it (demonstrate). The bacteria is now on the Q-Tip; do not touch the Q-Tip with your hands.
Streak the agar plate with the Q-Tip in a tic-tac-toe pattern (demonstrate).
Do not poke the Q-Tip into the agar; it will make a hole which makes it harder to see the bacteria grow.
Put the lid on the agar plate and tape the sides shut so that the bacteria does not get out (demonstrate).
Mark the tape with your grade, teacher, and what surface you used to collect the bacteria.
With the help of a teacher or parent, place the petri dish in the grow lab.
* The temperature in the lab should be around 37º C (98º F)

Here you explain that scientists use C not F and how to convert from F to C (C X 2 - 10% +32 = F).
Bacteria doesn't grow when it is too cold (less than 40º F), that is why we use refrigerators to keep our food fresh.
Bacteria doesn't grow when it is too hot (over 140º F); surgeons sterilize their tools to kill any bacteria or bacteria would be on our skin and it would get infected.
Bacteria grow well when it is close to our body temperature of 98º F so we try to keep the lab at around 37º C.
*** In the morning you will look at the petri dishes and record what you see (form).
Reminder - DO NOT TOUCH THE PETRI DISHES
There may be condensation, water droplets, collected on the dishes - explain condensation.

*** Here's what we are going to do:
Break into groups of 4 or 5.
Each group decides on a place in the school to collect the bacteria - find a place where our hands would be that might have germs.
Again demonstrate and tell how to collect the bacteria and where it is to be put.
It may take 2 or 3 days for the bacteria to grow on the petri dishes.
*** You will be doing a hypothesis - an educated guess- about the bacteria you collected.

*** Demonstrate again how bacteria is spread - someone sneezes into their hands, they touch the doorknob, someone else touches the doorknob, the germs are now on that person's hand.
Henry the Hand ©

Definitions

GERMS
   Germs are microorganisms, tiny living things that can only be seen with the help of a microscope. Billions of microorganisms live on your skin or in your body. Most of them are harmless; some actually help keep you healthy. Disease germs (which scientists call pathogens) are the exception. They are specific microorganisms which are capable of causing contagious diseases.
   Scientists have classified pathogens into six categories: viruses, bacteria, rickettsiae, fungi, protozoans and parasitic worms. Viruses and bacteria are the disease-causing agents of most childhood infections and the ones most relevant to primary grade school children.  

VIRUSES

   Viruses are smaller than any known cell. They are composed primarily of protein and nucleic acid, the genetic material responsible for determining the growth of cells in living organisms. Many of the viruses that have been identified are classified as pathogens, that is, capable of causing illness. Viruses can only multiply when they invade cells. Once inside a cell, a virus can cause disease by changing the normal chemical activity of the cell. The virus directs the cell to reproduce viruses. Once full of viruses, the cell bursts, spreading the new viruses to other cells where the process begins all over again. Viruses are responsible for more diseases than any other type of pathogen. Each different virus causes a specific disease.
   Examples include the common cold, influenza and most contagious childhood infections such as chicken pox, measles, mumps, cold sores and fever blisters. Scientists also suspect that viruses may be responsible for certain forms of cancer.  

BACTERIA

   Bacteria are single-celled organisms that have one of three basic shapes: rods (bacilli), round (cocci) or spiral (spirilla). Bacteria can grow and reproduce quickly by doubling in size and splitting in half. Bacteria can be found throughout the environment. You come into contact with them on almost everything you touch. Millions of bacteria are also normally present on and in the body. Most don’t cause problems and many help with the normal functioning of the body. For example, in the intestines, some bacteria help process body wastes for elimination. Other bacteria, however, are pathogenic. Pathogenic bacteria that live inside the body obtain food for the substance found in human tissue. They can cause disease when they produce specific products such as tetanus or other toxins that poison the body.
   Examples of diseases caused by bacteria include strep throat, pneumonia, diphtheria and tetanus. Pathogenic bacteria can also multiply in food that is improperly stored and can cause food poisoning.