Passage V

Strains of bacteria carrying a genetic mutation that prevents them from synthesizing the amino acid histidine are called His^-. These strains of bacteria must absorb histidine from their environment in order to sustain their growth. Exposing His^- strains of bacteria to mutagens (substances that induce DNA mutations) can cause new mutations that restore the ability of some bacteria to synthesize histidine. Any bacterium that regains the ability to synthesize histidine becomes His⁺ and is known as a His⁺ revertant.

The number of His⁺ revertants in a population of bacteria can indicate the potential of a substance to be mutagenic in humans. Scientists tested 4 substances, each suspected to be a mutagen, on a His^- strain of the bacteria Salmonella typhimurium.

Study

A sterile petri dish (Dish 1) containing a nutrient agar lacking histidine was prepared. Then, 1 × 10⁸ cells of His^-S. typhimurium were added to Dish 1 and evenly spread over the surface of the nutrient agar. These procedures were repeated for 4 more nutrient agar dishes (Dishes 2-5) except that the bacteria were mixed with 1 of the 4 suspected mutagens before being spread over the surface of the nutrient agar. Table 1 lists, for each of Dishes 2-5, the substance that was mixed with the bacteria before they were added to the dish.

The 5 dishes were incubated at 37°C for 2 days. At the end of the incubation period, the number of colonies growing on the nutrient agar in each dish was determined (see Table 2).