Potential Mutagens Essay, Research Paper
“A mutagen is a chemical that mutates or alters DNA.” Many times, mutagens may be carcinogens as they may induce cancer. Dr. Bruce Ames developed a method by using specific mutant strains of the bacteria Salmonella typhimurium to detect mutagens and possible carcinogens. This lab will test various potential mutagens using Dr. Bruce Ames procedure and methods.
Salmonella typhimurium is a prototrophic bacterium, which simply means that it can survive on its own as long as it has a source of energy, a carbon source, and inorganic salts. For the experiment, the auxotrophic form of salmonella is used. This mutated version cannot synthesize the amino acid histidine. Growth of this type of bacteria will only grow if hidtidine is added to the median or if the mutation occurs which corrects the original mutation. The latter is known as a back mutation or a reversion, where the mutation causes the bacteria to return to its wild type state. This type of mutation is either brought about by normal replication errors, or by mutagenesis, due to the action of a mutagen. This experiment will use four different strains of the bacteria salmonella (see table 1). Each strain has a unique mutation, but they all lack the ability to produce histidine (His-). This test is applied to measure the ability of chemicals, which may be potential mutagens, that convert the His- bacteria to His+, hence producing twice the number of colonies in a His- environment as compared to the number that occur spontaneously.
Several petri dishes were prepared each containing 1ml of melted soft agar, 60 ml of a certain type of bacteria, and 20 ml of a potential mutagen. The dishes were incubated for 72 hours and finally observed. Table 2 gives the overall results of the potential mutagens. From these results, it was determined that the cough drop in bacteria TA98 (see table 1) had produced more than two times the number of colonies formed when compared to both the negative (water) and positive (sodium azide) controls. Therefore, the cough drop was used to further test this chemical as a potential mutagen. Ten more petri dishes were set up each one with a different concentration of potential mutagen (cough drop) to water. They were set up using TA98 from of salmonella. Two petir dishes were prepared containing 100% of the cough drop, another two containing a water, another two containing a mixture of 50/50 cough drop to water, another two containing a mixture of 40/60 cough drop to water, and another containing 70/30 cough drop to water respectively. The results are observed in table 3, which shows a breakdown of how the colonies developed. Ironically, the 100% cough drop containing gel did not produce far more colonies than did the negative control as would have been expected when comparing these results with table 2. The same is observed in the different concentrations of cough drop to water. The positive and negative controls help to compare and contrast in order to determine whether a certain chemical is a possible mutagen or not. If medians, which contained histidine, were used, colonies would have grown for all chemicals tested including the negative control and would appear as a positive test as a mutagen.
Overall, the Ames test does help to determine whether chemicals are mutagens or not. The first part of the experiment, yielding results for table 2 gave a good indication on what a potential mutagen may be. Yet, the further evaluation of that particular chemical, which results can be found on table 3, did not produce results, which would establish that particular chemical as a mutagen.
Table 1: Mutant types of Salmonella used for testing different chemicals
Mutated Salmonella Type Description of Mutation
TA97a Frameshift mutation (deletion) in the hisC gene
TA98 Frameshift mutation (insertion) in the hisD gene
TA100 Base substitution in the hisG gene
TA102 Nonsense mutation in the hisG gene
Table 2: Results for testing possible mutagens
Potential Mutagen TA97a TA98 TA100 TA102
Sodium azide 3 300+ 25 53
Water 37 10 20 35
Dye 38 3 20 17
Sunny Delight 6 1 25 14
Sugar free cookies 32 45 40 48
Cough drops 36 400+ 25 62
Table 3:Results for testing the cough drop as a potential mutagen
Petri Dishes Containing Mutagen Number of colonies
1 (100% cough drop) 98
6 (100% cough drop) 105
2 (water) 78
7 (water) 65
3 (50/50 cough drop/water) 80
8 (50/50 cough drop/water) 60
4 (40/60 cough drop/water) 74
9 (40/60 cough drop/water) 50
5 (70/30 cough drop/water) 43
10 (70/30 cough drop/water) 50