Caption

Scanning electron micrograph (SEM) of Geobacillus stearothermophilus (formerly known as Bacillus stearothermophilus), Gram-positive, rod, spore-forming prokaryote. G. stearothermophilus is a thermophile and is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It is commonly used as a challenge organism for sterilization validation studies and periodic checks of sterilization cycles. First described in 1920 as Bacillus stearothermophilus, it was reclassified in 2001 and is now officially a member of the genus Geobacillus. Spores of bacteria allow the bacteria to survive harsh conditions until the time when the bacterium can grow and reproduce. Magnification: x1,865 when shortest axis printed at 25 millimetres.