Life Cycle - Human Biology (6B)
Pre Lab 

  • Distinguishing bacteria, viruses, and protozoa.
  • Understanding an infection. 
  • bacteria
  • disease
  • genetic
  • pathogen
  • virus
  • worksheet 
  • culture demonstration (optional)

Students uses a worksheet to compare bacteria, virus, and protozoa.



A disease is the abnormal functioning of the body that prevents the body organs or processes from functioning as they should. There are many causes of disease. Disease can be caused by fungi (i.e. athlete's foot) bacteria (i.e. tuberculosis), virus (i.e. common cold) and protozoa (amoebic dysentery). Not all types of fungi, bacteria, viruses and protozoa and disease-causing agents. The causes of disease are sometimes referred to as germs. The definition of the term germ is sometimes ambiguous. Germs can be carried through food, water, air, animals, and people. "Bad" germs are pathogenic or disease causing organisms and are the ones that make people sick.

Bacteria, viruses, and protozoa cause different infections. The worksheet is just a way for student to look at these "germs" without a microscope. They should look like the following:

Bacteria are extremely small organisms. They are so small that you cannot see them with your naked eye, and need at least 400 times magnification with your microscope to begin seeing them. Identifying bacteria is very difficult since they are very small. Many times scientists have to make a culture of a suspect bacteria to find out what problem the suspect bacteria may have caused. This culture is allowed to grow in a petri dish and the colony of bacteria is then observed. Bacterial diseases include pneumonia, strep throat, trench mouth, boils, blood poisoning, gonorrhea, chancre sores, bubonic plague, cholera, and syphilis.

Viruses are composed of a protein overcoat enclosing a core of either DNA or RNA. Once a virus is attached to a susceptible cell it dissolves a hole through the membrane and uses the cell's processes to reproduce. Diseases that are caused by viruses include the common cold, influenza, herpes, yellow fever, shingles, rabies, polio, and mononucleosis.

Protozoa are single celled organisms. Most protozoa do not cause infections, but a few do. Malaria, dysentery, African sleeping sickness are caused by different species of protozoa.

  1. You may want to prepare agar solutions and grow bacteria cultures to show your students. We do not recommend that the students do this, the germs could get out of control!  It is necessary to stress that probably all of the bacteria grown will be harmless, but that the students shouldn't touch the cultures with their hands.  Make sure that they wash their hands if they touch the cultures
  2. Below are materials and instructions that can be used to make your own cultures.  You can also purchase ready-made agar petri dishes.  You can also use a thin layer of pectin (usually in the grocery store with “Canning” supplies.)

MATERIALS:  agar, water, glassware, hot plates, petri dishes, tape, marking pens

  1. Make sure that all the glass and petri dishes that you use are clean.  If not, wash with soap  and water and let stand to dry upside down.  Do not dry with a cloth or paper towel.
  2. Weigh about 2.5 grams of agar from the bottle labeled NUTRIENT AGAR.
  3. Fill a beaker with 100 ml of water and add agar to water.
  4. Boil the solution and be sure to stir frequently.  Keep your eyes and face away from the top of the beaker.
  5. Let the solution stand at 100 degrees centigrade for 15-20 minutes.
  6. Pour the solutions into the clean petri dishes and tape the dish around the edges where the two tops meet.
  7. Label the dishes and store them until the next lab period.  


  1. You can then prepare your bacteria colonies by inoculating the agar.  Remove the tape from the petri dish and take the top cover off.  With a clean toothpick make two lines so that there are 4 sections in the dish.  With a clean toothpick scrape the inside of your mouth.  Do not poke your skin, just scrape your inside of the cheek.  Rub the toothpick on the agar, very gently.  DO NOT poke or scratch the agar, just rub it.  On the other section, place a piece of hair, spit, scab, or anything else to see if bacteria will grow.   
  2. After you have inoculated all 4 sections tape the petri dish.  Store in a place that is warm (not hot).  After you see growth, we recommend that you use a Microbiology book if you want to identify your colony of bacteria.  Discard the germs with care under sanitary conditions.   
  3. Use the worksheet and have the students create a picture of each of the different types of diseases.  You may also want to do an Internet search to find out more information on diseases. 

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