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Biological Indicator for Dry Heat Sterilization (DHS) Processes

Learn how to ensure for sterilization using Bacillus atrophaeus as biological indicators in Dry heat sterlizer (DHS).

Test organism

Spores of Bacillus atrophaeus (Bacillus subtilis var. niger) (ATCC 9372; NCIMB 8058; CIP77.18) are used for validation and routine monitoring of the performance of sterilizers and sterilizing processes employing dry heat at sterilizing temperatures, typically at 160°. For routine monitoring, the D-value shall not be less than 2.5 minutes.


The biological indicator organism complies substantially with the morphological, cultural and biochemical characteristics of the strain of Bacillus atrophaeus ATCC 9372; under microscopic examination it consists of Gram positive rods of width 0.7 μm to 0.8 μm, and length 2 μm to 3 μm, the endospores are oval and central and the cells are not swollen; when incubated aerobically in appropriate media at 30° to 50° growth occurs within 24 hours and similar inoculated media incubated concomitantly at 55° to 60° show no evidence of growth in the same period; agar colonies have a dull appearance and may be cream to brown to orange-colored; when incubated in nutrient broth it develops a pellicle, and shows little or no turbidity.

Biological Indicator

The indicator used is Biological Indicator for dry-heat sterilization, paper carrier. It is a defined preparation of viable spores made from a culture derived from a specified strain of Bacillus atrophaeus, on a suitable grade of paper carrier, individually packaged in a container readily penetrable by dry heat, and characterized for predictable resistance to dry-heat Sterilization. It has a particular labeled spore count per carrier of not less than 104 and not more than 108.

Related: New Genetically Engineered Biological Indicators

Dry heat at temperatures greater than 220° is frequently used for Sterilization and depyrogenation of glassware and glass containers. In this case, demonstration of a 3-log reduction in heat resistant bacterial endotoxin can be used as a replacement for biological indicators.

Also see: Types of Biological Indicators
Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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