Top 10 Microorganisms for Educating Students

Microbiology 101 provided my first exposure to live bacterial cultures in college (aside from those my roommates created in our minifridge). The distinct smell of an Escherichia coli culture, the unique green coloration of a fuzzy penicillium colony and the many failed attempts at lighting my Bunsen burner are fond memories of the experiences that hooked me on microbiology.

The fundamental microbiology laboratory skills I learned during basic courses such as aseptic transfer, streaking for isolation, staining techniques, bright-field microscopy, differential testing and execution of biochemical assay have held true and helped me throughout my career.

What strains should educators use in their classes to demonstrate these basic skills, reactions and methods? We have some suggestions. The list below can help educators choose the best strains for teaching their microbiology students. We’ve included a brief description of each strain, a few key reactions or skills they can demonstrate, and products formats we recommend for students.

All strains are a low biosafety level (1 or 2) to ensure safe handling by young microbiologists.

1. Staphylococcus aureus

Biosafety Level 2
Morphological Characteristics Beta-hemolytic on Sheep’s Blood agar plates
Staining Technique Gram stain: Gram-positive cocci occurring singly, in pairs, or in irregular clusters
Biochemical Responses
  • Catalase (3% Hydrogen Peroxide): Positive
  • Coagulase (rabbit plasma-tube): Positive
Example Strain ATCC 25923
Formats Available

2. Staphyloccocus epidermidis

Biosafety Level 1
Morphological Characteristics
  • Gamma-hemolytic (non-hemolytic) on Sheep’s Blood agar plates
  • Two colony types on Sheep’s Blood Agar: both are small, smooth, entire edge and circular – one is white and the other is translucent to gray
Staining Technique Gram stain: Gram-positive usually in pairs and tetrads
Biochemical Responses Coagulase (rabbit plasma-tube): Negative
Example Strain ATCC 12228
Formats Available

3. Escherichia coli

Biosafety Level 1
Morphological Characteristics Deep pink colonies with surrounding pink precipitate (+ Lactose Fermentation) on MacConkey Agar
Staining Technique Gram stain: Gram-negative straight rod
Biochemical Responses
  • Oxidase (Kovacs): Negative
  • Beta-glucuronidase (E.coli broth w/MUG) : Positive = growth with fluorescence
Example Strain ATCC 8739
Formats Available

4. Proteus mirabilis

Biosafety Level 2
Morphological Characteristics
  • Grey colonies with swarming on Sheep’s Blood agar
  • Colonies are colorless (- lactose fermentation) with some swarming on MacConkey agar
Staining Technique Gram stain: Gram-negative straight rod
Biochemical Responses Oxidase (Kovacs): Negative
Example Strain ATCC 29245
Formats Available

5. Mycobacterium smegmatis

Biosafety Level 1
Morphological Characteristics Small to medium, circular to irregular, flat, erose edge, dull and rough, translucent, cream turning yellow/orange with age
Staining Technique
  • Kinyoun Acid Fast: Positive (AFB)
  • Gram Stain: Gram-positive rod, medium to long
Biochemical Responses Catalase (3% Hydrogen Peroxide): Positive
Example Strain ATCC 14468
Formats Available

6. Pseudomonas aeruginosa

Biosafety Level 2
Morphological Characteristics
  • Produces Pyocyanin – colonies turn green after 48 hours on Nutrient and Tryptic Soy Agar
  • Colorless to gray/blue colonies (no lactose fermentation) on MacConkey agar
Staining Technique Gram stain: Gram-negative rod, straight or slightly curved
Biochemical Responses
  • Oxidase (Kovacs): Positive
  • Motility B Medium: Positive
  • Contains Inducible AmpC beta-lactamase
Example Strain ATCC 27853
Formats Available

7. Candida albicans

Biosafety Level 1
Morphological Characteristics Small to medium, white, circular, convex, dull colonies on Potato Dextrose agar
Staining Technique Gram stain: Gram-positive, ovoidal, budding yeast cells
Biochemical Responses
  • Germ Tube Test: Positive
  • Cornmeal Agar: Positive for chlamydospore production
Example Strain ATCC 10231
Formats Available

8. Peniciliium rubens

Biosafety Level 1
Morphological Characteristics Rapid growth of downy white colonies that turn yellow then gray/green and yellow on the underside
Staining Technique Lactophenol blue stain: Hyaline septate mycelia that produce hyaline conidiophores. The conidiophores branch into brush-like penicillus. Spores are borne in long chains from terminal sterigmata.
Biochemical Responses Produces penicillin
Example Strain ATCC 9179
Formats Available

9. Streptococcus pyogenes (Group A)

Biosafety Level 2
Morphological Characteristics Two colony types on Sheep’s Blood agar:

  • One is medium and beta-hemolytic
  • One is small and alpha-hemolytic
Staining Technique Gram stain: Gram-positive cocci
Biochemical Responses
  • Catalase (3% Hydrogen Peroxide): Negative
  • Bacitracin differential: Sensitive
Example Strain ATCC 19615
Formats Available

10. Streptococcus agalactiae (Group B)

Biosafety Level 2
Morphological Characteristics Small, white to gray colonies with beta-hemolysis on Sheep’s Blood Agar
Staining Technique Gram stain: Gram-positive cocci in short to long chains
Biochemical Responses
  • Catalase (3% Hydrogen Peroxide): Negative
  • Bacitracin Differential: Resistant
  • CAMP Positive Control
Example Strain ATCC 12386
Formats Available

Download our Microorganisms for Education document to see more strains you may want to consider using during your microbiology courses.

Microbiologics also offers Parasite Suspensions and QC Microbiology Slides for education. Visit microbiologics.com to find the right strain and format for your classroom.

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