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The Lab Exercises have been created over many years as a team effort by the Molecular Biology and Microbiology faculty.

1. Date and Time:

Fall 2004, 2 hours

2. Throat flora exercise

Conclude study of throat flora. Prepare a brief report listing the bacteria identified and the test results supporting your conclusions. (Lab Results, last page)

3. Isolation of Staphylococcus

3.1. Special Materials:

Examples of S. aureus and S. epidermidis plates with antibiotic discs, rulers.

3.2. Procedures:

Read antibiotic sensitivity plates. Use the rulers to measure the diameter of the zone of growth inhibition. Use the following table to determine if your isolates are susceptible to a particular antibiotic.

Zone Diameter Interpretive Table
 Diameter of Inhibition zone (mm)
Antimicrobial Agent Disc contents Resistant Intermediate or Indeterminant Moderately Susceptible Susceptible
Ampicillina (AM) 10µg â?¤13 - 14-16 â?¥17
Cefoxitin (FOX) 30µg â?¤14 - 15-17 â?¥18
Clindamycin (CC) 2µg â?¤14 15-20 - â?¥21
Erythromycin (E) 15µg â?¤13 14-22 - â?¥23
Gentamicin (GM) 10µg â?¤12 13-14 - â?¥15
Penicillin Gb 10 U â?¤28 - - â?¥29
Penicillin Gc 10 U â?¤14 - â?¥15 -
Vancomycin (VA) 30µg â?¤9 11-Oct - â?¥12
aGram-negative enteric organisms bStaphylococci cEnterococci

4. Fecal flora

4.1. Special Materials:

Nutrient agar plate, antibiotic discs, forceps, soft agar tubes with glucose, lactose and maltose, example of E. coli and Salmonella on MacConkey lactose plates

4.2. Procedure:

  1. Inspect MacConkey plates for growth and colony color. Also, note the relative number of colonies present.
  2. Score plates for tetracycline and ampicillin resistance.
  3. Gram stain colonies.
  4. Determine antibiotic sensitivity profile of a Lac+, Tetr or Lac+, Ampr clone. Use the same method you did for Staphylococcus aureus (fourth lab).
  5. The use of sugar fermentation tests facilitates identification of bacteria. Sets of tubes containing soft agar, a pH indicator, and various sugars are provided. The sugars are glucose, lactose, and maltose. Inoculate one tube of each sugar with a Lac+ colony. These tubes will be incubated for one or two days at 37°C.