Some microbes can grow only in air.
Others can't grow in air. This page is an introduction to aerobes and anaerobes. Pure Skim Milk Medium Use fresh skim milk or add 100 grams of dry skim milk to water to make a final volume of 1 liter. Experienced bacteriologists sometimes use 10 minutes at 10 pounds pressure or Tyndallization to reduce the hydrolysis of lactose. Tyndallization is simple, place your tubes in a steamer or pressure cooker with a lid but leave the steam ports open.