Laboratory Investigations in Microbiology

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Chapter 30: Lab Skills Test

Basic microbiology lab skills are an essential aspect of this course. You will be tested on six important techniques:

  1. Aseptic technique: broth-to-broth transfer (7 points)
  2. Streak plate technique: separate a mixed culture (8 points)
  3. Preparation of pure cultures: from your streak plate (5 points)
  4. Staining: Gram stain of a mixed culture (7 points)
  5. Microscopy: Find and focus on bacterial cells (7 points)
  6. Pipetting: Accurate transfer of liquids (6 points)

The lab skills test will be held over several lab days as indicated on the lab syllabus. 


Aseptic technique:  You will be given two sterile TSB (broth) tubes.

  1. Label both tubes with your initials and your lab section (LA, LB, LC)
  2. Perform an aseptic transfer of a loopful of broth from the first tube to the second tube. You may narrate out loud is you wish.
  3. Upon completion, you will be informed of any missed steps. The tubes will be incubated to check for contamination.

Streak plate technique: You will be given a sterile TSA plate and a broth mixture of two bacteria (Staphylococcus aureus and Escherichia coli). 

  1. Label your TSA plate with your name, lab section, and "mix"
  2. Perform a streak plate technique using a loopful of the mix broth
  3. Your plate will be incubated for 2-5 days and graded based on the results

Pure culture technique: You will use your streak plate and 2 sterile TSA slants

  1. Label your two slants "SA" and "EC" (plus: name, lab section)
  2. Aseptically transfer a colony of each bacterium to its respective slant
  3. The slants will be incubated and graded based on growth and purity

Gram staining: You will be given a slide and a mixed broth culture of E. coli and S. aureus

  1. Prepare a heat-fixed bacterial smear using the mixed broth
  2. Gram-stain your slide
  3. You will be graded based on how clear and distinct the stained smear appears under the microscope

Miscroscopy: You will use a prepared slide

  1. Bring your specimen into focus on low power.
  2. Bring the specimen into focus on oil immersion
  3. Correctly adjust the microscope controls (focus, light level, diaphragm)
  4. Identify parts of the microscope as instructed

Pipetting: You will be given a beaker of water and a pipette with a pipette pump

  1. Correctly identify the pipette
  2. Accurately pipette a given volume of water (e.g. 8.0 ml) into a plastic dish
  3. Your accuracy will be checked by weighing the dish (1 ml water = 1.00 g) 
  4. Correctly use a micropipettor



© 2003 - 2017 Josť de Ondarza, Ph.D.