In 2016, two biologists from Indiana University estimated that there are about a trillion species of bacteria on earth.¹ With so many choices, how can a teacher select the best strains for her microbiology class? Below are a few tips for choosing and using microorganism strains.

Search for BSL-1 Strains

The national culture collections in the United States rate microorganism strains from BSL-1 to BSL-4. The lower the BSL (biosafety level), the lower the risk of a microorganism causing disease.

BSL-1 strains pose no, or low risk, to individuals and communities. They may, however, cause disease in an individual with a suppressed or compromised immune system. In addition, certain fungal species may emit spores that can cause allergic reactions in susceptible individuals.

The Centers for Disease Control and Prevention (CDC), in collaboration with the U.S. National Institutes of Health (NIH), have written guidelines to help laboratories handle microorganisms safely. See the Microbiologics document, Microorganism Biosafety Level 1 and 2 Standard Microbiological Practices, for a summary of the guidelines.

Obtain Microorganism Strains from a Reputable Source

The supplier should provide you with a certificate confirming the identity and purity of the strains. Follow the supplier’s directions for storing and maintaining the strains.

Find Microorganisms Relevant to the Class  

Are your students interested in strains found in food, water, soil, or on the human body? The Microbiologics document, Microorganisms for Education, lists over twenty BSL-1 strains of bacteria and fungus that are commonly found in the environment. The ecology, the colony and microscopic appearance, and instructions for growing the culture are listed for each strain. A few of the species listed include:

• Bacillus cereus: The strain’s endospores are widespread in soil, milk, and other foods.
• Escherichia coli: Found in the lower intestine of humans and other mammals. Used as a control by clinical, food, pharmaceutical, and water laboratories.
• Lactobacillus lactis subsp. lactis; Isolated from raw milk, udders, and dairy products.
• Pseudomonas fluorescens: Produces fluorescein pigment.
• Staphylococcus epidermidis: Isolated from moist areas such as the interior nares and toe webs.
• Penicillium chrysogenum: A mold found in various food products.
• Saccharomyces cerevisiae: Used in baking and brewing.

Conduct an Interesting Experiment

Dozens of experiments can be found inthe online manual, Practical Microbiology for Secondary Schools, A Resource for Key Stages 3, 4 & Post–16, and the Equivalent Scottish Qualifications and Basic Practical Microbiology – A Manual.

An experiment your students may find interesting is comparing how Escherichia coli, Staphylococcus epidermidis, and Bacillus subtilis react to an antibiotic such as ampicillin.  Download instructions for conducting this experiment.

If you think your class would like to test tap water, enumerate the number of microorganisms in a food sample, or perform an antimicrobial susceptibility test, you can find instructions for these tests online in government laboratory manuals.  See more resources at the end of the post.

Use Safety Precautions in the Lab

• Students should wear gloves and lab coats.
• If there is risk of splashing, use appropriate personal protective equipment such as goggles.
• No drinking or eating should be allowed in the lab.
• All participants should wash their hands after the lab session.

Decontaminate All Infectious Waste

• Use a disinfectant such as 70% alcohol, to clean counters after each lab session.
• Sharp objects such as glass tubes should be disposed of in a sharps disposal container.
• Autoclave contaminated material or use a biohazard disposal service. See your community guidelines.

Prepare in Advance

Before your class meets you will need to:

• Grow the microorganisms to be used in class. Bacteria usually need 24-72 hours to grow at 35 degrees Celsius. More time is usually needed for fungal cultures. Instructions for growing specific strains can be found in the Microbiologics documents, Microorganisms for Education and Recommended Culture Methods.
• Prepare agar if you are making it in-house.
• Purchase lab supplies needed for the class. Basic supplies include:
• Stock microorganism strains
• Agar – It may be purchased ready-made or it can be made in-house. If it is made in-house, equipment such as a water-bath, sterile petri dishes, a hot plate, a large flask will also be needed.
• Markers for labeling plates
• Platinum, nichrome or plastic inoculating loops
• An incinerator for sterilizing loops
• An incubator (the microorganisms can be grown at room temperature if you don’t have access to an incubator)
• Microscope
• Vortex mixer
• Biohazard bags and sharp containers

Need More Information?

You can find technical information, blogs, and hundreds of microorganism strains in easy-to-use formats like KWIK-STIK™ and LYFO DISK™ at microbiologics.com. Contact our Technical Support team at 1.320.229.7045 or techsupport@microbiologics.com for help choosing the right strain and format for your class.

Microbiologics Educational Resources

Utilize the following illustrated instructions, technical documents, and videos in your classroom:

• How to Perform Serial Dilutions in Microbiology
• Instructions for Use: LYFO DISK™, KWIK-STIK™, KWIK-STIK™ Plus
• Maintenance of Quality Control Strains
• Microorganism Biosafety Level 1 and 2, Standard Microbiological Practices
• Microbiologics Dilution Guide
• Pour Plate Method Best Practices
• Streak Plate Method for Colony Isolation Illustrated Instructions

Online Resources

Advice for Teachers, Suggestions for Experiments

• Microbiology Society. Basic Practical Microbiology – A Manual. Eds. Burdass, D., Grainger, J., Hurst, J. Charles Darwin House, London, UK. 2016
• Microbiology Society. Practical Microbiology for Secondary Schools. Eds. Grainger, J., Hurst, J. Charles Darwin House, London, UK. 2016

Clinical Testing

• European Committee on Antimicrobial Susceptibility Testing (EUCAST). Explains how to perform susceptibility testing and provides QC tables.

Food Testing

• FDA Bacteriological Analytical Manual
• United States Department of Agriculture (USDA). Microbiology Laboratory Guidebook.

Mold

• MSG ERC Doctor Fungus, an online reference to all things mycological.

Safety

• U.S. Department of Health and Human Services. Biosafety in Microbiological and Biomedical Laboratories. 5th Edition. 2009.

Water Testing

• United States Environmental Protection Agency. Manual for the Certification of Laboratories Analyzing Drinking Water

References

¹Nicholas Bakalar. Earth May Be Home to a Trillion Species of Microbes. New York Times. May 23, 2016.

Read Next – 7 Streak Plate Method Best Practices