INTRODUCTORY MICROBIOLOGY
Grades 10 - 12

This lesson begins with the elementary understanding most people have of microbial diseases. Using the video as a starting point, students should expand their understanding of infectious diseases including the means of transmission, treatment, and prevention. Students will grow cultures of Staphylococcus aureus; and by observing the bacteria under the oil emersion lens of the microscope, will estimate and graph the Staphylococcus aureus count for their own bodies and those of the entire class.

READING RAINBOW #34: Germs

Students should be able to:

• Explain the basic concepts of microbial infectious diseases including their transmission, treatment, and prevention. Grow a bacterial culture of Staphylococcus aureus.
• Perform the Gram Staining procedure and then look at the bacteria they have grown using the oil emersion lens on a microscope.
• Count the number of bacteria they have grown and use this data to extrapolate the number of bacteria growing in their body.
• Develop a raw data table of the number of bacteria present in each member of their class.
• Calculate the mean number of bacteria present in their class.
• Graph the number of bacteria present in their class.

Supplies are available from any of the biological supply companies.
I recommend putting students in groups of two. For each pair the following are needed.

• 1 individually wrapped sterile cotton swab
• 1 pair of latex gloves
• 1 Mannitol salt agar plate
• 3 or 4 paper towels
• 1 glass slide
• 1 Bunsen burner
• 1 inoculating loop
• tap water
  
  The following stains (crystal violet, Gram decolorizer, Gram iodine, and safranin O) come in 250 milliliter bottles. It is ideal to have one set of stains per group; but if funding is limited, two sets will serve a class of twenty-five.
• Crystal violet, Gram decolorizer, Gram iodine, safranin O.
• A microscope with an oil emersion lens is needed to see bacteria. The more microscopes the better.
• A bottle of emersion oil (mineral oil works).
• 1 refrigerator to keep the agar plates in before the lab.
• 1 autoclave or 10% bleach/water solution for disposal of the agar plates that have grown bacteria.
• 1 incubator.

Virus - single or double strand of either RNA or DNA. Viruses do not exhibit the characteristics of living organism; therefore, they are not considered to be alive.

Bacteria - the smallest living organisms.

Bacilli - rod-shaped bacteria.

Cocci - round-shaped bacteria.

Spirilli - spiral-shaped bacteria.

Staphylo - prefix for clumps of bacteria.

Strepto - prefix for chains of bacteria.

Pathogenic - disease causing carriers-individuals who transport a bacteria but do not get sick from it.

Nonpathogenic - not disease causing.

Exogenous - bacteria that enter the body.

Fomites - inanimate objects.

Endogenous - bacteria found in the body. Infection-disease caused by the multiplying of bacteria.

Abscess - walled off inflammation resulting in dead tissue.

Inflammation - swelling that is due to puss and fluids surrounded by a walled off clot.

Puss - white blood cells.

Subcutaneous - below the skin.

Toxigenic - bacteria that produce toxin.

Toxin - poison

Intoxication - illness that results from ingesting preformed toxins

Agar - nutrient media that allows organisms to grow on it but does not break down due to the organism's metabolic processes.

Flame the loop - heating an inoculating loop to kill any bacteria present on it.

Heat fixing a slide - heating a microscope slide to make bacteria stick to the slide.

Gram stain - procedure used to stain bacteria so that they can be seen under a microscope.

Gram positive bacteria - bacteria that stain blue.

Gram negative bacteria - bacteria that stain pink.

This is an introductory lesson so the video provides the starting point for the discussion. One needs to have all of the lab materials purchased and ready to for use.

To give a specific responsibility while viewing tell the students, "As you watch the video it is your responsibility to distinguish between viruses and bacteria, determine the shapes of the viruses and bacteria, two methods by which bacteria make you sick, three methods of prevention, and the basic method of growing a bacterial culture."

Note to the teacher: The video is elementary based but has many good concepts that can be expanded on to make this a useful high school level tool.

START the video where Levar Burton looks up from the microscope after looking at the amoeba, hydra, rotifer, and water bear. He says, "There are millions of invisible creatures living in our world."

PAUSE the video where Levar asks, "What do you think a germ is?" Ask the students to listen to what the six children's definition of a germ is and have them compare the kids' ideas to their own idea of what a germ is. RESUME the video.

PAUSE the video after the six children have given their definition of a germ. Ask the students how many of them honestly agree with the kid's definition of a germ. Ask the students to give their own definition of a germ. This will give you a pre-test sort of knowledge as to how much your students already know. Since this is a high school lesson, warn the students that the next section is a little story book story about germs. Have fun with it. Look for the differences between bacteria and viruses. RESUME the video.

PAUSE the video when the narrator says, "Germs are tiny living beings." Make sure students focus on the term "living" since this will be an important distinguishing characteristic between bacteria and viruses. RESUME the video.

PAUSE the video where the narrator says, "the two types of germs are bacteria and viruses." Tell the students to make two lists of characteristics. One for bacteria and one for viruses. RESUME the video.

PAUSE the video after the three shapes of bacteria have been presented. Tell the students it is now time to add some information to the video. The ball-shaped bacteria is properly called "cocci." The rod-shaped bacteria is properly called "bacilli." The spiral-shaped bacteria is properly called "spirilli." When bacteria comes in chains, it has the prefix "strepto" attached to the shape name. When bacteria comes in clumps, it has the prefix "staphylo" attached to the shape. For example, staphylococcus means a clump of round-shaped bacteria. Streptococcus means a chain of round-shaped bacteria. Bacteria are so small that the two main defining characteristics of it are its shape (round, rod, or spiral) and whether it is clumped or chained. RESUME the video.

PAUSE the video after the three shapes of viruses. Ask the students what the three viral shapes are. Explain to the students that viruses are simply single or double strands of either DNA or RNA. As a result, they do not have the characteristics of living organisms. Unlike living organisms, viruses are not made up of cells. They do not have a nucleus, complex membranes, organelles, and they contain either DNA or RNA but not both. They do not move, grow, reproduce, or respire. Therefore, viruses are not living. Bacteria on the other hand exhibits the characteristics of life and therefore, bacteria are living organisms. Emphasize that bacteria and viruses are distinct entities and are not interchangeable terms. RESUME the video.

PAUSE the video after the narrator says that germs are kept out of the body by the skin and nose hair. Tell the students that bacteria are also kept out of the body by the mucus in the nose. Damage to the skin, nose hair, mucus or an excess of bacteria in the environment can allow bacteria to enter the body. RESUME the video.

PAUSE the video after the scraped knee (from a bicycle accident) scene. Have the students review methods by which bacteria entered the body. The video shows sneezing, a shared straw, and skin breakage. Have the students determine where the bacteria lived in order to be passed from person to person in these ways. Bacteria live in the nose, mouth, and on the skin as part of the normal flora of the body. They only cause problems when they are present in excess or in deficient amounts. In the cases presented, the bacteria are in excess.

The following information should be added about bacterial habitat and transmission. The lab activity that goes with this lesson deals with growing Staphylococcus bacteria. It is important that students understand some fundamentals.

Staphylococcus aureus is usually a pathogenic (disease causing) bacterium. Many people carry the bacteria as a normal part of their skin's flora. These people are known as carriers (individuals who transport the bacteria but do not get sick from the bacteria). Staphylococcus epidermidis is usually nonpathogenic (not disease causing) bacterium. This bacteria is also carried by many people as a normal part of their skin's flora.

Habitat: 1. Skin - humans and animals carriers.

2. 20 -50% of humans carry Staphylococcus in their nose but are not affected by it. They are carriers.

Transmission:

1. Exogenous (bacteria that enters the body)

A.Moisture droplets that enter traumatized tissues - such as sneezing on an open wound.

B. Fomites (inanimate objects) that touch traumatized tissue - toilet seats, door handles. Staphylococcus aureus is particularly resistant. It can live between 6-14 weeks in dried body fluids.

C. Contaminated food - raw milk. A cow with mastitis (swelling of the udder) passes the bacteria to humans who then get a Staphylococcus infection (not mastitis). Mastitis is the number one economically detrimental agricultural disease in the US. Mastitis results in agricultural losses of $2 billion a year. Note, pasteurization kills Staphylococcus.

D. Staphylococcus food poisoning

2. Endogenous (bacteria found in the body flora of carriers)

A. Compound fractures (bone is exposed through the skin) where the bone comes in contact with bacteria already in the body.

B. Lowered host resistance - this allows bacteria to increase their growth rate. AIDS is a good example of lowered host resistance. The body's lowered immune response allows rapid bacterial reproduction.

C. Excessive antibiotic therapy - changes the makeup of the body's flora and thus changes the field of competition. As a result certain bacteria grow rapidly. Example: penicillin halts the growth of most bacteria; however, it does not halt the growth of Staphylococcus. Staphylococcus has become resistant to penicillin. Without competition from other bacteria; more room, food, and etc. are available for the Staphylococcus. As a result, Staphylococcus reproduction is unchecked.

RESUME the video.

STOP, but do not rewind the video after the statement "germs make you sick by multiplying or giving off poisons." The following information should be added about Staphylococcus induced diseases. The main terms are the same for all bacteria induced diseases, but since the lab focuses on Staphylococcus so do the given example diseases.

Staphylococcus aureus induced diseases:

Nontoxigenic inflammatory infections Toxigenic infections Intoxication (not an infection)

I. Nontoxigenic inflammatory infections: The main characteristic of the nontoxigenic inflammatory infection is the presence of abscesses. An abscess is a walled off inflammation resulting in dead tissue. Inflammation is a swelling that is due to puss (white blood cells) and fluids surrounded by a wall of clot.

A. Infection/inflammation of the skin and subcutaneous (below the skin) tissues:

1. Trauma that results in skin breakage such as being hit with a hammer can result in a Staphylococcus infection. People who get Staphylococcus infections in this manner are often nasal carriers of Staphylococcus.

2. Furuncle (boils) are the most common type of nontoxigenic inflammatory infections. A boil is a superficial infection of hair follicles or sweat glands.

3 Sty is an infection at the base of the eyelash.

4. Carbuncle is a really bad group of boils.

5. Impetigo is where areas of the skin are covered in crusty lesions (infected areas). This is usually a disease that affects children. The lesions are not walled off in an abscess. It is highly communicable.

B. Infection/inflammation of the respiratory tract:

1. Tracheobronchitis is a respiratory Staphylococcus infection of the bronchia or trachea. The bacteria grow and cause inflammation of the bronchial tubes or trachea.

2. Pneumonia is a lower respiratory Staphylococcus infection of the lungs. The bacteria grow and cause an inflammation in the lungs.

C. Infection/inflammation of tissues due to bacteria carried by the blood:

1. Endocarditis is an inflammation of the heart tissue (usually the valves) due to bacterial growth there. The bacteria carried by the blood lodges on the valves of the heart causing an abscess. As a result, the body tries to fight off the infection. If the body is successful, no problem. However, if the body is unsuccessful, the valve tissue dies and so may the infected person. Having one's teeth pulled is one method of getting this disease. When the tooth is pulled, an open wound is made in the mouth. This allows bacteria from the mouth to enter the bloodstream. This is why dentists may administer antibiotics to patients who have their teeth pulled.

2. Osteomyelitis is an abscess that forms in the metaphyseal region of the long bones. The abscess on the bone causes the bone tissue to die. The blood in the metaphyseal region of the bone is not under as great of a pressure as the blood in the surrounding regions. As a result the bacteria are not "washed away" thus it is a favorable location for them to grow. Ninety percent of the cases of osteomyelitis are the result of Staphylococcus aureus. Many of these cases are the result of Staphylococcus aureus that was on the skin, entered the bloodstream through a wound, and then settled and grew in the bone.

II. Toxigenic infections:
Some bacteria produce toxins. Bacterial toxins are poisons that harm the animal host. There are two types of toxins - endotoxins and exotoxins. Endotoxins are located in the cell wall of the bacteria. Endotoxins are only active when released from the cell wall such as when the bacteria dies and degrades. Exotoxins are toxins that are produced and secreted by the living bacteria. Staphylococcus produces only exotoxins. The Staphylococcus exotoxin that is responsible for the most important toxigenic infection symptoms is the toxic shock syndrome toxin-1 (TSST-1).

In toxigenic infections, the infection is caused by the bacteria, but the damage to the body is caused by the toxin produced by the bacteria. Since the toxin is a molecule, it is carried away from the bacteria by the bloodstream. Therefore, damage to the body occurs away from the sight of infection.

A. Toxic shock syndrome (menstrual and nonmenstrual)

1. Menstrual - occurs in females who use high absorbency tampons. As a result of toxic shock syndrome, high absorbency tampons are no longer produced and marketed. Staphylococcus is naturally present in the vagina in small numbers. When high absorbency tampons are in place, the number of bacteria increases dramatically, thus increasing the amount of toxin produced. This results in menstrual toxic shock syndrome. Two possible explanations exist with the first being the one thought to be the most plausible by researchers. The vagina is an anaerobic (lacking oxygen) environment. The insertion of the high absorbency tampon allows air into the vagina, thus it becomes an aerobic (oxygen present) environment. This allows rapid propagation of the bacteria. The second explanation is that magnesium ions inhibit Staphylococcus growth and toxin production. The material in the high absorbency tampons binds the magnesium ions so that they no longer inhibit bacterial growth.

2. Nonmenstrual - occurs in males and females who have a toxigenic Staphylococcus infection. Nonmenstrual toxic shock accounts for one third of toxic shock syndrome cases. The toxin that causes the symptoms is usually TSST-1.

3. How TSST-1 Works: TSST-1 is a superantigen which overstimulates white blood cells so that they produce an excessive amount of cytokine molecules. In low concentrations, cytokines are beneficial by helping the immune system fight off infections; but in high concentrations, they cause the symptoms of toxic shock syndrome. The symptoms of toxic shock syndrome include:

a. Fever

b. Rash

c. Desquamation (skin peels off)

d. Hypotension - low blood pressure which results in decreased blood flow to the extremities and thus the person goes into shock. Shock is where the cardiovascular system shuts down. This is the most important symptom because, if death is to occur, this is what kills the person.

III. Intoxications (Staphylococcus food poisoning): Intoxications are illnesses that result from ingesting preformed toxins - usually in food. Intoxications are not infections since the bacteria do not need to be present in the body. The toxin only needs to be present for the poisoning to occur.

The toxin responsible for this disease is called staphylococcal enterotoxin. The toxin stimulates the vagus nerve. The vagus nerve is the abdominal nerve that is connected to the medulla (part of the brain). The stimulation of the vagus nerve results in the stimulation of the part of the medulla that causes vomiting. The person vomits. Staphylococcal enterotoxin is resistant to heat; therefore, cooking the food does not make it safe for consumption. Staphylococcus food poisoning results from eating food with the enterotoxin in it - not the bacteria. Common locations of Staphylococcus in food are pastry, meat, and any food containing mayonnaise.

Symptoms: One to six hours after the toxin is ingested the body responds - vomiting and possibly diarrhea. The body usually eliminates itself of the toxin within 24 hours.

FAST forward the video to the part where the doctor in the story is looking at the child. PLAY the video.

PAUSE the video after the narrator says, "if a bacteria makes you sick the doctor may prescribe drugs, but there are no drugs for viral infections. Shots may be given to prevent viral infections." Ask the students why this is. (Bacteria are living and thus can be killed by drugs. Viruses are not living and thus cannot be killed. You cannot kill something that is not living and remember viruses do not have the properties of living organisms.) RESUME the video.

PAUSE the video where the narrator says, "you can help yourself stay healthy by exercising, eating right, and staying clean." Review this idea with the students. Take special care to note one of the main ways to avoid bacterial transfer is to wash your hand after using the restroom. (Bacteria are so small they may not appear to be present but they are.) RESUME the video.

PAUSE the video where the lab technician calls the red stuff in the bottom of the petri dish "a dish of food." Discuss with the students that this growth media is called "agar." Agar is a nutrient media that allows organisms to grow on it but it does not break down due to the organisms' metabolic processes. There are different types of agar that allow different types of bacteria to grow on it. In the lab you will be doing you will be using Mannitol salt agar. Mannitol salt agar allows Staphylococcus aureus to grow, but inhibits most other bacterial growth. Tell the students to pay attention for the lab technician to say, "a dark warm place." She is referring to an incubator. In the case of the lab you will be doing, warm is 37 degrees Celsius. RESUME the video.

PAUSE the video after the lab technician says, "beta streptococci." Ask the students what it should look like. (It is a chain of circles.) Tell the students to see bacteria they must be stained and looked at under an oil immersion microscope lens. This special lens allows light to pass directly into the lens rather than bend where it enters the lens. Bacteria are so small that every bit of light is needed to see them. The staining procedure you are going to use in the lab is the Gram Stain. RESUME the video.

PAUSE the video at the picture of E. coli. Ask the students what shape it is. (The shape is bacilli.) RESUME the video.

PAUSE the video where the lab technician says, "Staph aureus." Point out to the students that the proper name of the organism is Staphylococcus aureus. Ask the students what it will look like. (It will be clumps of cocci bacteria.) Ask the students where this type of bacteria lives. (It lives on the skin.) FAST FORWARD the video though the moldy food section and RESUME the video at the section where Levar Burton is swabbing his mouth. Provide a focus for viewing by telling the students to watch the procedure carefully. This is the procedure you will follow in the lab to grow a culture of bacteria.

PAUSE the video after the lab technician puts the loop in the fire and spreads the bacteria out over the plate. Point out to the students that the cotton swab was only used to swab the top one-fourth of the agar dish. The lab technician stuck the inoculating loop into the fire in a procedure known as "flaming the loop." Ask the students why she did this. (She did this to kill all bacteria that may be living on the inoculating loop.) The lab technician then spread the bacteria out very thin. Ask the students why. (She did this so that individual colonies could grow and be seen. Otherwise the bacteria are so grouped together no useful information can be derived.) REWIND the video and show this section over again. RESUME the video.

STOP the video after the lab technician reviews the idea of keeping clean. Emphasize to the students it is very important to wash your hands especially after using the toilet.

After viewing and adding in the discussion topics, you may want to go back and show parts of the video with the sound turned off. At the end of the lessons, students will be able to discuss the topic at a higher level than the video. This is an excellent method of review and reinforcement.

Note to the teacher: It is time to do the lab. The streaking the agar plates takes about 15 minutes. It is best to do this at the end of a class period. Incubate the cultures over night and then begin the staining procedure the following day. Observe the bacteria and do the math exercises the day after that.

Isolation of Staphylococcus aureus lab

Copy and distribute Figure 1 to the students.


PROCEDURE

STEP 1

1A. Using a new sterile cotton swab, collect specimens from human skin.

1B. To collect samples from dry surfaces, simply rub the swab on the surface. Sweaty surfaces (such as arm pits) work the best. Areas that are particularly clean (especially if they have been recently washed with antimicrobial soap) do not work well. Rub hard enough to get a good sample but do not rub so hard that you injure the subject. Bacteria are so small you will not see anything on the swab, but the bacteria is present.

STEP 2

Rub the cotton swab on the collection sight and then run the swab over the top one-fourth of the Mannitol salt agar petri dish. See Figure 1.

DISPOSAL

Used swabs need to be decontaminated by either autoclaving them or placing them in a 10% bleach/water solution. The decontaminated swabs should then be placed in a separate trash bag and incinerated.

1. Light the Bunsen burner. "Flame the loop" by placing the loop part of the inoculating loop in the Bunsen burner flame until the loop turns red.

This kills all bacteria that may be on the loop. Place the loop in a piece of bacteria-free agar to cool it. "Streak" the Mannitol salt agar plate. To streak the plate, using the flamed loop smear some of the sample at about a 90 degree angle from the swabbed area. Streak back and forth until about one-fourth of the plate is covered. See Figure 1. At a 90 degree angle from this streaked area, streak another one-fourth of the plate. See Figure 1. The goal of this is to spread out the sample so that the bacteria are not touching. Therefore when they grow, they will form isolated colonies. Reflame the loop so that you leave it clean.

2. Place the streaked agar plates in an incubator at 37 degrees C for 24 hours.

STEP 3

1. Bacteria will be visible 24 hours later. The Mannitol salt agar should have mostly Staphylococcus growing on it. Staphylococcus aureus ferments the Mannitol and therefore is a yellow color. Staphylococcus epidermidis does not ferment the mannitol and therefore is red.

Copy and distribute Table 1 to the students.

Look at your plates and determine if fermentation has taken place. Record your results in table 1.

The next thing you will need to do to confirm you have grown Staphylococcus is the Gram stain. Staphylococcus are Gram positive cocci in clusters. Gram positive bacteria stain blue and Gram negative bacteria stain pink. Perform the Gram stain as follows and record your results in table 1.

The dyes used in this procedure will stain your clothes. Wear appropriate attire. Wearing latex gloves during the staining part of this procedure will prevent your hands from turning colors.

2a. Light the Bunsen burner. "Flame the inoculating loop" by placing the loop in the Bunsen burner flame. This kills all bacteria that may be on the loop. Place the loop in a piece of bacteria-free agar to cool it then gently touch the loop to some of the bacteria that has fermented the Mannitol.

Remember this is a yellow color. Get a little on the loop.

2b. Put a drop of water on a clean slide. Smear the bacteria into the water and spread it around until it is evenly distributed.

2c. "Heat fix" the slide. This process attaches the bacteria to the slide. Hold the slide with your fingers and pass it back and forth through the Bunsen burner flame until the slide is completely dry. It should never get hotter than what you can comfortably hold. If it does you have cooked the bacteria and they can't be seen. Heat fix the slide until it is dry.

2d. Hold the slide on an angle over the sink and drop several drops of crystal violet on the bacteria. Allow this to set for one minute.

2e. Run enough tap water over the slide to wash the crystal violet off. Do not scrub.

2f. Hold the slide on an angle over the sink and drop several drops of Gram's iodine on the bacteria. Allow this to set for one minute.

2g. Run enough tap water over the slide to wash the Gram's iodine off. Do not scrub.

2h. Hold the slide on an angle over the sink and drop no more than five drops of Gram decolorizer (isopropyl alcohol works) on the bacteria. Go to step 2i immediately or all the stain will be removed - you will have messed up. All of your bacteria will be pink.

2i. Run enough tap water over the slide to wash the Gram decolorizer off. Do not scrub.

2j. Hold the slide on an angle over the sink and drop several drops of Safranin O on the bacteria. Do not scrub.

2k. Run enough tap water over the slide to get the excess Safranin O off. Do not scrub.

2l. Blot the slide dry with a clean paper towel.

2m.Place a drop of immersion oil on the bacteria and place the slide under the oil immersion lens of the microscope.

2n. Focus. If you have little blue dots in clumps, you are looking at Staphylococcus aureus.

Health professionals determine the amount of bacteria in your body by using a procedure similar to the one you used in table 1. How much bacteria in this case Staphylococcus aureus is in your body? This exercise is at best a guess. Count the number of bacteria on the slide. Measure the length and the width of the slide. Multiply length times width. This gives you the surface area of the slide. Write this number down with the number of bacteria present. Measure how tall you are and call this number "h." Measure one-half of your body width and call this number "r." Plug these numbers into the formula for calculating the surface area of a cylinder. The formula is (2)(")(r)(h) + (2)(")(r2) = your surface area. Take this number and divide by the surface area of the slide. Finally multiply this number times the number of the bacteria on the slide. The number should be huge (billions, trillions or greater). This gives you an approximation of how many Staphylococcus aureus bacteria are on your skin. Make sure all of your measuring units are the same.

2o. Clean up. Wash the slides and your hands with an antimicrobial soap.

DISPOSAL -- Used petri dishes need to be decontaminated by either autoclaving them or placing them in a 10% bleach/water solution. The decontaminated petri dishes should then be placed in a separate trash bag and incinerated.

Make a raw data table on the chalk board. Have each class member fill in how many Staphylococcus bacteria are present on their skin. Upon completion of the raw data table, have the students calculate the mean number of bacteria present. Use of scientific notation will be a benefit. Finally have the students graph the data. Discuss what the graphs look like and interpret the results.

Have students contact health officials (doctors, dentists, hospitals, clinics, etc.) and find out which bacterial infections occur most often. This data should be presented to the class orally supported by a graph of the raw data. If computers are available, all graphs should be computer generated. Students should draw some conclusions about the frequency and location of bacterial infections.

Have a doctor or other health professional come to the school as a guest speaker. Have the speaker discuss growing bacterial cultures and interpreting the results. Have the speaker discuss how a method for treatment is determined.

Via the Internet, have students send messages all over the world to health organizations to determine the prevalent bacterial infections and method of treatment. Students should determine if certain infections are more prevalent in rural, urban, developed, or third world areas. Data should be presented orally to the class and include computer generated graphs.

Future topics of discussion and lab activities include:

• The body's natural defense system (antibodies, white blood cells, etc.).
• Antibiotics, antimicrobial agents, and vaccines.
• Using the bacteria you have already grown on the Mannitol salt agar, you can grow pure cultures of Staphylococcus aureus in brain/heart infusion broth. Using a flamed and cooled inoculating loop, place a colony of Staphylococcus aureus in the broth and incubate it for twenty- four hours at thirty-seven degrees Celsius. This culture can then be swabbed onto the entire surface of a 100 millimeter Mueller-Hinton plain agar plate. Place one Difco Dispens-O-Disc Susceptibility Test Disc of ampicillin 10 mcg, erythromycin 15 mcg, penicillin G 10 units, and tetracycline 30 mcg on the agar. Incubate the culture for twenty-four hours at thirty-seven degrees Celsius. Measure the zones where no bacteria grew and compare it to the chart that comes with the susceptibility test discs. Use this information to determine which antibiotic is most effective at killing Staphylococcus aureus. One set of test discs is enough for a class of twenty-five.

VIDEO AVAILABLE FROM

• Taping it off of the air. See your local PBS schedule.
• Idaho State Library.
• Reading Rainbow programs or teacher guides-contact GPN 1-800-228- 4630

Lesson developed by Master Teacher John Lindley, Marsing High School, Marsing,Idaho.

Technical microbiological assistance provided by Dr. Greg Bohach, Department of Microbiology, University of Idaho, Moscow, Idaho. Mathematical assistance provided by Jody Hoff, Homedale High School, Homedale, Idaho.

Click here to view the worksheet associated with this lesson.