Answer:
gain; presence
Explanation:
The Ames test is a lab methodology used to determine whether a particular chemical substance can trigger mutations in an organism, which can be used to indicate whether exposure to the target chemical compound is associated with a greater risk of developing cancer. The Ames test can use a strain of Salmonella typhimurium carrying mutations in a key gene that encodes a protein involved in histidine biosynthesis. In consequence, this S. typhimurium strain is considered to be a 'histidine auxotroph' which is unable to synthesize histidine residues. After exposure to the target chemical compound, bacteria are then cultured in a medium lacking histidine. Subsequently, bacteria that regain the ability to synthesize histidine proliferate to form colonies. Finally, the number of these colonies is then compared with the number of mutants in absence of the potential mutagen, enabling thus the determination of the mutagenic potential of the target chemical compound.
