Scientists wanted to know whether having fewer bacteria in their gut microbiome would make a mouse more likely to become infected with Salmonella, a type of harmful bacteria. They ran an experiment with 40 laboratory mice. At the start of the experiment, 20 mice had normal gut microbiomes and 20 mice had low-bacteria gut microbiomes. Both groups were healthy at the start of the experiment. Then, each group of mice ingested (ate) Salmonella bacteria.