Model to show how bacteria grow in plumbing systems. Bacteria in tap water can multiply when a faucet isn't used for a few days, such as when a house is vacant over a week's vacation, a new study by engineers has found. The study suggests a new method to show how microbial communities, including those responsible for illnesses like Legionnaires' disease, may assemble inside the plumbing systems of homes and public buildings. Source 1w.