July 14th – Geobacillus stearothermophilus (basonym Bacillus stearothermophilus) is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes. The bacterium is a thermophile and is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It will grow within a temperature range of 30-75 degrees Celsius. Some strains are capable of oxidizing carbon monoxide aerobically. It is commonly used as a challenge organism for sterilization validation studies and periodic check of sterilization cycles. The biological indicator contains spores of the organism on filter paper inside a vial. After sterilizing, the cap is closed, an ampule of growth medium inside of the vial is crushed and the whole vial is incubated. A color and/or turbidity change indicates the results of the sterilization process; no change indicates that the sterilization conditions were achieved, otherwise the growth of the spores indicates that the sterilization process has not been met. Recently a fluorescent-tagged strain, Rapid Readout(tm), is being used for verifying sterilization, since the visible blue fluorescence appears in about one-tenth the time needed for pH-indicator color change, and an inexpensive light sensor can detect the growing colonies.