What is the primary characteristic of a biological indicator used for sterilization?

The primary characteristic of a biological indicator used for sterilization is that it contains highly resistant bacterial spores. Biological indicators are designed to test the effectiveness of sterilization processes by using microorganisms that are more resistant to sterilization methods than the target pathogens. The presence of these spores, often of species such as Bacillus or Geobacillus, allows for the assessment of whether the sterilization conditions (like heat, steam, or chemical agents) were effective in killing the spores.

By challenging the sterilization process with these highly resistant spores, practitioners can confirm that the sterilization cycle was successful. If the spores are killed, it indicates that the sterilization parameters were met; if they survive, it suggests that the process was inadequate, and further action may be necessary.

The other options do not accurately reflect the function of a biological indicator. Live viruses are not typically used in this context for safety reasons. Measuring temperature levels is more aligned with physical indicators rather than biological ones, and the release of chemicals upon exposure pertains more to chemical indicators than to biological indicators. Thus, the presence of resistant bacterial spores is the defining characteristic that ensures the reliability of sterilization methods.