Results

You decide to do experiments on the well-studied bacterial species E. coli. You obtain a sample of E. coli cells that have been isolated from harmful radiation and chemical mutagens. You inoculate two Petri plates with a 1:10000 dilution of the cells. The medium in Plate A was enriched with the amino acids tryptophan (Trp) and histidine (His).The culture that grows on Plate A is the master culture. The medium in Plate B was enriched with Trp, His, and the antibiotic ampicillin. The plates after incubation for 24 hours at 37°C, are shown in Figures 1 and 2.

FIG. 1: The Master Culture.
E. coli grown on medium enriched with Trp and His for 24h at 37°C. (1:10000 dilution)
FIG. 2: E. coli grown on medium enriched with ampicillin, Trp and His for 24h at 37°C. (1:10000 dilution)

You decide to use the replica-plating technique to grow the master culture on different mediums. In this technique, you press a sterile piece of velvet over the master culture. Some cells from each colony adhere to the velvet. You then press the velvet onto four different sterile plates:

  1. A Control Plate: medium enriched with His and Trp.
  2. A Trp Minimal Plate: medium lacking Trp but containing His.
  3. A His Minimal Plate: medium lacking His but containing Trp.
  4. A His and Trp Minimal Plate: medium lacking both His and Trp.

The results of growth of these plates after incubation at 37°C for 24 hours are shown in Figures 3, 4, 5 and 6.
FIG. 3: E. coli grown on control plate for 24h at 37°C. FIG. 4: E. coli grown on Trp minimal plate for 24h at 37°C.
FIG. 5: E. coli grown on His minimal plate for 24h at 37°C. FIG. 6: E. coli grown on His and Trp minimal plate for 24h at 37°C.

After examining this data, you started to think that there was something wrong with Dr. Gen E. Tics' Mutagens Only Hypothesis. What do you think is wrong? What parts of this data conflict with his claims? (Click on Discuss in the left margin after you come up with some ideas.)