Dear Dr. Gene E. Tics:

I have been testing your Mutagens Only Hypothesis and based on my results, I have come to the following conclusions:

Mutagens do cause mutations in bacterial DNA, but mutation may also be spontaneous occurring without the presence of a mutagen. This was clearly shown in two separate mutant isolation experiments. In the first experiment, ampicillin-resistant E. coli mutants were isolated by positive selection. Positive selection entails growing the culture on a medium that will allow for growth of only mutant colonies. In this case, only the cells resistant to ampicillin grew on a medium containing ampicillin and we could identify them directly.

In the second experiment, results were obtained from replica-plating which is an example of negative selection isolation. Negative selection is used to identify mutants that have lost the ability tp perform a certain funtion that their parents had. The replica-plating experiments showed cells that lost the ability to synthesize His, Trp or both amino acids and could not survive with their absence from the environment. Bacteria that have lost the ability to synthesize an essential nutrient are called auxotrophic mutants.

A spontaneous mutation may occur for several reasons including occasional mistakes made during replication, repair, or recombination. These processes, although extremely acurate, are certainly not perfect. A disadvantageous mistake, like losing the ability to synthesize an essential nutrient, can be lethal. Some mutations, like acquiring drug resistance, on the other hand, may be beneficial.

Dr. Gene E. Tics received your letter and thanks you for pointing out the errors in his work. For all your achievements you have been awarded with the Microbiology Medal Of The Year....Congratulations!