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Microbial Growth – Micro-organisms that Cause Disease

HOW DOES IT WORK?                                        

Time allocation: 1 – 1  Weeks

The experiment looks at 2 concepts that look at disease causing bacteria and other microorganisms.

The first aspect students will be looking at coli-form bacteria (bacteria associated with contamination of feces and soil) in various places and the second will look at the effectiveness of various detergents and whether they do as they claim to.

Experiment 1: Test for Coli-form Bacteria
Time allocation: 1 – 1  Weeks

 The students will grow and observe swab samples taken from surfaces that they use every day and discuss what the findings mean. The presence of coli-form bacteria is commonly used as an indicator of unsanitary food and water. Coli-form bacteria in the family Enterobacteriaceae, including, but not limited to, the genera Escherichia, Klebsiella, Enterobacter and Citrobacter. These bacteria will grow as pink colonies as they digest the content of the agar they produce. They produce metabolic by-products that change the pH of the agar which reacts with the agar and causes the colonies to turn pink. Non-coliform bacteria, but still disease causing bacteria growing on McConkey Agar are not pink (typically colourless), and include members of Enterboacteriaceae such as the genera Proteus, Morganella, Providencia, Edwardsiella, Salmonella, Shigella and Yersenia (plague bacteria).

 

Experiment 2: Testing the effectiveness of “antimicrobial” detergents
Time allocation: 1 – 1  Weeks

In the experiment, investigating how effective the detergents we use to kill microorganisms really are, students will use sterile swabs to wipe a “dirty” surface and run it over the 5 agar plates. They will then use 5 small filter paper circles (one per sample) and soak it in the detergents and water and place it on one agar each. The distilled water acts as a control as distilled water should not inhibit any growth. After a week and half the students can now see the effectiveness of the detergents on inhibiting microbial growth.

Experiment – Test for Coliform Bacteria

AIM:     

    Time allocation: 1 – 1  Weeks

To test the presence of disease causing bacteria on supposedly clean surfaces.

APPARATUS:

  • 10 x prepared MacConkey Agar in petri dishes
  • 10 x Sterile swabs
  • 1 x petri seal tape
  • An incubator or refrigerator
  • Permanent marker

 

METHOD:

  1. Select a surface for the following:
  • Bathroom door handle
  • Kitchen sink
  • Laptop/computer keyboard
  • A person’s hand
  • A cell phone
  1. Label each of the agar plates on the bottom plate using the permanent marker
  2. Using the sterile swabs for sample surfaces a, b, c and e, swipe the surface for about 2-3 minutes to pick up a sufficient amount of bacteria and other contaminants.
  3. Rub the corresponding swabs over the corresponding agar plates.
  4. For the students’ hand select a student with a cold or who is feeling a little sick, they can place their hand directly onto the agar for a few minutes.
  5. Seal the petri dish with insulation tape and incubate for a week upside down for about 1 week.
  6. Observe the difference in growth on the plates.

OBSERVATION:

Below is a description of the expected results of coliform bacteria growing on MacConkey agar. Use this to understand your results:

  • The presence of coliform bacteria is commonly used as an indicator of unsanitary food and water. Coliform bacteria in the family Enterobacteriaceae, including, but not limited to, the genera Escherichia, Klebsiella, Enterobacter, Hafnia and Citrobacter.
  • These bacteria will grow as pink colonies as they digest the content of the agar they produce by-products that change the pH of the agar which reacts with the agar and causes the colonies to turn pink.
  • Non-coliform bacteria, but still disease causing bacteria growing on MAC are not pink (typically colorless), and include members of Enterboacteriaceae such as the genera Proteus, Morganella, Providencia, Edwardsiella, Salmonella, Shigella and Yersenia (plague bacteria).
  • Look at the below pictures to identify what has grown on the agar:

CONCLUSION:

Bacteria and fungi that cause disease are seen growing, on agar that are sampled by swabs of common “contaminating” surfaces. This may cause interest by students as they are exposed to what organisms they are exposed to on a daily basis.

Some bacteria, known as Coli-forms (food borne diseases) can be identified as pink colonies.

 

PRECAUTIONS:

  • Growth of bacteria takes time, allocate 1 and ½ weeks (give or take) to growth of the samples.
  • Students must wear gloves when working with samples.
  • Samples will require refrigeration or incubation when growing.
  • Do not breathe in the air from the petri dish as some of these bacteria produce spores and can be very dangerous.
  • Always wear a lab coat aseptically when dealing with samples