Looking at your results of the ames test , using bacteriological agar medium plates lacking the amino acid histidine, you find that there are many colonies growing on the agar. your interpretation of this result is:
a. the bacterium has turned its operon genes on, producing proteins necessary for growth
b. the chemical being tested is mutagenic
c. the bacterium does not have any plasmids, so it cannot grow on the medium
d. the drug being tested has no inhibitory effect on the bacterium, therefore it is not a good antibiotic to give for treatment of disease

B

Explanation:

The Ames test is a bacteriological technique commonly used to determine if a particular chemical or substance can cause mutations. To achieve this, researchers use bacteria (commonly Salmonella typhimurium) as this bacteria strain is unable to grow in a medium that does not contain histidine (a very important amino acid in the biosynthesis of proteins). If the studied substance is able to cause mutations, bacteria will mutate, and they will be able to grow on that particular media even if they don’t have histidine for their metabolism. Therefore, if there are colonies growing, the chemical being tested is mutagenic.  

I hope this clarify your inquiry.