Compounds containing nitrite ions (NO₂⁻) are present in many foods eaten by humans. Scientists tested the effect of different concentrations of nitrite ions on the population growth of 5 species of bacterium. All 5 species are pathogenic and can infect the human digestive system. For each species of bacterium, the scientists placed suspensions of the bacteria in a microwell plate. A microwell plate is a plastic plate containing 96 wells, which are similar to small test-tubes. In each well, the scientists added: • a suspension of the bacterial species • a solution containing nitrite ions • nutrient broth. The scientists incubated the microwell plate at 37°C for 24 hours. After 24 hours, the scientists estimated the number of bacteria in each well by measuring optical density, using a microwell plate reader. For each species of bacterium, the scientists repeated the experiment with several different concentrations of solution containing nitrite ions. The scientists determined the lowest nitrite concentration at which no bacterial population growth had occurred after 24 hours.