Think of a biological indicator (or BI) as a "challenge test" for your sterilization process. It's a small device containing a high number of specially selected, highly resistant bacterial spores—most commonly Geobacillus stearother mophilus. These spores are like the ultimate survivalists; they are much harder to kill than everyday bacteria and viruses. If the sterilization process can defeat these tough spores, you can be confident it has eliminated all less-resistant contaminants.