USP 55 Biological Indicators

  • Selection
  • Biological Indicators are typically bacterial spores in a growth medium or inoculated to a spore or paper strip, which are put through the sterilization cycle intended for the drug or medical device, recovered and then checked for survivors. 

    Bacillus stearothermophilus: used to verify steam sterilization cycles.

    Bacillus atrophaeus: used to verify peroxide sterilization cycles.

    Bacillus pumilus:  used to verify gamma irradiation cycles.. 

  • Incubation
  • Incubation conditions will depend upon the organism and the manufacturer’s use instruction.  A typical incubation for Bacillus stearothermophilus is 2 days at 55 – 60°C. 

  • Enumeration
  • The Certificate of Analysis (COA) will indicate the number of spores on a Biological Indicator.  The number is important in verifying the Sterility Assurance Level (SAL) of the validated process.  FOCUS can confirm the spore count via validated methodologies. 

  • Time Kill
  • The Certificate of Analysis (COA) will also indicate the D-value of the Biological Indicator.  D-value is the time required to reduce the population by one log.  Determining D-value requires specialized equipment such as a Bier Vessel.

An alternative to confirming the D-value is to determine the minimum expected survival and maximum expected kill time.

The minimum expected survival time = (Not Less than D Value) x (Log viable spores -2)

The maximum expected kill time = (Not more than D Value) x (Log viable spores +4)

FOCUS can verify these values.