Biological Indicators And Sterilization Biology

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A biological indicator is a characterized preparation of a Specific microorganism that provides a defined and stable resistance to a Specific sterilization process. It can be used to assist in the performance qualification of the sterilization equipment and in the development and establishment of a validated sterilization process. There are many types of biological indicators. Each type of indicator incorporates a known species of a microorganism of known sterilization resistance to the sterilization mode. Some biological indicators may also contain two different species and concentrations of microorganism. Microorganisms widely recognized as suitable biological indicators are spore forming bacteria. This is because these microorganisms are more resistant than micro flora except towards ionizing radiation process.


Sterilization refers to any process that effectively kills or eliminates transmissible agents (such as fungi, bacteria, viruses, spore forms, etc.) from a surface, equipment, article of food or medication, or biological culture medium. Biological indicators (BIs) are the most accepted means of monitoring the sterilization process because they directly determine whether the most resistant microorganisms (e.g., Geobacillus or Bacillus species) are present rather than merely determine whether the physical and chemical conditions necessary for sterilization are met. Because spores used in BIs are more resistant and present in greater numbers than are the common microbial contaminants found on patient care equipment, an inactivated BI indicates other potential pathogens in the load have also been killed.


To assist the physical operation of a sterilizer.

In order to establish a validated sterilization process for an article

Sterilization of equipments and packaging components used in aseptic process.

To keep a check on the sterilization cycle

Recheck the established sterilization cycles and revalidation if necessary

Factors governing the choice of biological indicators

The strain should be stable

It should be non-pathogenic

The resistance of the test strain should be maximum towards the sterilization process when compared to other species of microbes.

The test strain should be easily recovered and reproducible.


The strain of test organism.

Total viable spore count.

D -value (Decimal reduction value) is defined as ionizing radiation dose required to kill 90% of micro organisms present in sample. This value is a measure of resistance of particular type of sterilization process.

Z-value is defined as temperature required to kill micro organisms to shift by one log scale In case of heat sterilization Z-value denotes resistance of micro organisms with changes in temperature.

Expiry date.





Culture media

Self contained ampoules


Type of sterilization

Biological indicator

Spores count

D -value

Moist heat sterilization

Bacillus stearothermophilus or clostridium sporogenes


0.8 min

Dry heat sterilization

Bacillus subtilis var. niger


5-10 min

Radiation sterilization

Bacillus pumilus



Ethylene oxide sterilization

Bacillus subtilis var. niger


Correct functioning of sterilization cycles should be verified for each sterilizer by the periodic (at least weekly) use of Biological indicators. Users should follow the manufacturer's directions concerning the appropriate placement of the BI in the sterilizer. A control Biological indicator (not processed through the sterilizer) from the same lot as the test indicator should be incubated with the test Biological indicator. The control Biological indicator should yield positive results for bacterial growth. In addition to conducting routine biological monitoring, equipment users should perform biological monitoring.

Whenever a new type of packaging material or tray is used.

After training new sterilization personnel.

After a sterilizer has been repaired.

After any change in the sterilizer loading procedures.


Spore ampoules are used in monitoring the efficacy of steam sterilization processes. Spore ampoules are ideal for use in validation of liquid sterilization cycles. Ampoules are also suitable for monitoring non-liquid loads. No activation is required; simply expose spore ampoule to sterilization process and incubate.

Spore ampoules are manufactured using hermetically sealed Type I borosilicate glass containing a modified Soybean Casein Digest Broth with bromo cresol purple pH indicator and the specified population of Geobacillus stearothermophilus spores. Incubate ampoules at 55 - 60°C for 48 hours. Growth is evident by either turbidity or color shift from a purple to a yellow color. Spore ampoules are labeled with 18 months of shelf life from date of manu­facture. Store under refrigerated conditions (5 ± 3°C). Do not freeze spore ampoules.Spore ampoules are available in several configurations including population levels of 104 and higher

Table 2: SPORE AMPOULES-POPULATIONNAMSA CodePopulationProduct DescriptionPackaging



1 ml Spore Ampoule




1 ml Spore Ampoule


PS1-NAMSA code no's


These are pure suspensions of viable spores with known resistance characteristics and popula­tion levels. The convenience of the Spore Suspensions allows for direct inoculation of products, typically verifying sterility of devices where a traditional biological indicator strip cannot be used. They can also be used for a variety of other micro­biological testing including cleaning effectiveness studies. Spore Suspensions are packaged in 10 ml vol­umes and are sold in pharmaceutical grade glass vials with screw cap and septum which allows for withdrawal of the suspension using either a pipette or a needle and syringe assembly. The spores are suspended in Water for Injection (WFI) in a variety of population levels standardized for 0.1 ml inoculums. The Sus­pensions are labeled with an 18 or 24 month shelf life based on the organism and date of fill. Spore Suspensions require storage under refrigerated conditions (2° - 8°C). Each vial of Spore Suspension is accompanied by a Certifi­cate of Analysis detailing the source, assayed population, resistance characteristics on paper carrier and expiry date.


Spore strips consist of inoculated filter paper, 6 mm x 30 mm, packaged in glassine peel pouches or envelopes. The glassine packaging provides protection from environmental contaminants during transport exposure. The spore strips can easily be removed from the glassine pouch by tearing or peeling the pouch open for transfer to culture media or challenge device assembly. Spore strips are available in populations from 102to 108 for each organism. Strips are packaged in shelf packs of 100 strips and labeled with an 18 or 24 month shelf-life based on the organism and date of manufacture. Store strips under room temperature conditions (15 - 30° C). Each pack is accompanied by a certificate of analysis. Spore strips are certified for population, purity, and resistance (D-value, Z-value, survival and kill where applicable.

Table3: SPORE STRIPS POPULATION OF DIFFERENT BIOLOGICAL INDICATORSNAMSA CodeType of sterilizationBiological indicatorSpore Population Per Strip

STN - 04

STN - 05

STN - 06

Ethylene Oxide or Dry Heat

Bacillus atrophaeus




STS - 04

STS - 05

STS - 06


Geobacillus stearothermophilus

104 (10,000)



STP - 06

STP - 07

STP - 08


Bacillus pumilus

106 (1,000,000)

107 (10,000,000)


STN, STS, STP-NAMSA code no's.

Mini Spore Strips & Spore Discs:

Mini Spore Strips (2 mm x 10 mm) and Discs (3 mm and 6 mm diameters) will fit into small areas of a device. Mini spore strips and discs are packaged in bulk or individu­ally in glassine envelopes in shelf packs of 100 strips/discs. These products are manufactured with an 18 or 24 month shelf-life from the date of manufacture. Store under room temperature conditions (15 - 30° C). Each pack is accom­panied by a certificate of analysis. Spore strips and discs are certified for population, purity, and resistance (D-value, Z-value, survival and kill where applicable).


A self contained biological indicator consists of spore population levels of 105 and 106 for steam sterilization and for Ethylene Oxide (EO) processes in a 106 population. Each SCBI unit consists of a plastic vial with a cap, a crushable glass ampoule with recovery media, and a paper disc inoculated with spores. A biological Indicator strip eliminates the need for aseptic transfer of the BI to culture media and offers a shortened incubation period.

The recovery media consists of a modified Soybean Casein Digest Broth with pH indicator. Activate for incubation by de­pressing the cap completely and crushing the media ampoule. Growth is evident by either turbidity and/or a color shift of the media. SCBIs are labeled with the shorter shelf life of the two components, the inoculated disc and media ampoule which have independent expiry periods. Store under room tempera­ture (15°C - 30°C) conditions. SCBIs are offered in boxes of 50 units.

Table4: SELF-CONTAINED BIOLOGICAL INDICATORSNAMSA CodeSterilization ProcessOrganismPopulationReadout





24 hours



Geobacillus stearothermophilus


24 hours



Bacillus atrophaeus


48 hours

NAMSA-NATO Maintenance and Supply Agency ,NATO-North Atlantic Treaty Organization


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