BIOLOGICAL INDICATORS

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Biological indicators1,2 are the standardized preparations of specific micro-organisms that have known stable and high resistance towards one or more sterilization methods.

Micro-organisms widely recognized as suitable biological indicators are spore forming bacteria. This is because these micro-organisms are more resistant than micro flora except towards ionizing radiation process.

WHY ARE BIOLOGICAL INDICATORS USED?

  • To assist the physical operation of a sterilizer.
  • In order to establish a validated sterilization process for an article.
  • Sterilization of equipments, materials and packaging components used in aseptic processes.
  • To keep a check on the sterilization cycle.
  • Recheck the established sterilization cycles and revalidate if necessary.

THE BIOLOGICAL INDICATORS USED ARE THREE TYPES

  • The organisms or its spores are added to a carrier such as filter paper or glass and packed in order to maintain their integrity.
  • In the other the spores are added to the representative lots that are to be sterilized.
  • It is a self contained indicator which is designed such that the primary package used for incubation contains growth medium for recovery. In this the entire system is resistant to the sterilization process

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 the maximum towards the sterilization process when compared to other species of microbes
  • The test strain should be easily recovered and reproducible

THE CHARECTERISTICS OF A BIOLOGICAL INDICATOR:

  • The strain of test organism
  • The total viable spore count
  • D-value (Decimal reduction value).This value is a measure of resistance of particular type of sterilization process
  • Z- Value; In case of heat sterilization Z-value denotes the resistance of micro-organisms with changes in temperature
  • Expiry date

RECOMMENDED BIOLOGICAL INDICATORS FOR VARIOUS STERILIZATION PROCESSES.

MOIST HEAT STERILIZATION:

Spores of Bacillus stearothermophilus or Clostridium sporogenes

The number of viable spores should be more than 10 to the power 5 per carrier

The D-value at 121 degrees is 0.8 minutes for Clostridium sporogenes

DRY HEAT STERILIZATION:

Spores of Bacillus subtilis var. niger

The number of viable spores should not be less than 10 to power 5 per carrier

The D-value at 160 degrees should be about 5 to 10 minutes

RADIATION STERILIZATION:

Spores of Bacillus pumilus

Viable spores per carrier should be about 107-108

D-value should be about 3kGy

CHEMICAL STERILIZATION (ETHYLENE OXIDE):

Spores of Bacillus subtilis var.niger

The stock suspension should contain non-germinating spores held in non-nutritive liquid

REFERENCES:

  1. Indian Pharmacopoeia. Govt. of India, ministry of health and family welfare.1996, appendix-12.4.
  2. United State pharmacopeia 29- National formulary 24. The United State Pharmacopeial convention Inc. 2006:1035.


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About the Author

Amol Malpani's picture
Author: Amol Malpani

Comments

Bhavani Prasad.V's picture

Dear Sir,

Very interesting sir. As we have this in the present academics I could find some detailed useful info from this.
I wanted to know some information about the 'D' & 'Z' values that you have mentioned sir, like about how they are determined?, what is the use? Is there any limit restriction for these values? and all..

Regards,

Bhavani Prasad.V

Amol Malpani's picture

Dear Bhavani, Sorry for delay in answer. D-value can defined as how much ionising radiation dose is required to kill 90% of microorganisms present in sample. where as Z value is defied as how much temp is required to kill microorganisms to shift by one log scale (in simple mannar log2 mean 100 to log 1 means 10 same thing as d-value). Based upon the resistance offered by Biological indicators for sterilization cycles it is determined for different microorganisms. As you know there are different conditions of time and temperature of heat sterilization like for example 121 degree for 30 min. this is fixed up by considering D value and Z value. D value and Z value limits can be fixed up by regulatory authorities like AAMI/ISO/USP etc. Regards,
Siva Mavuduru's picture

Sir Can you explain how biological indicator helps in physical operation of sterilizer and sterilization of equipments?
Amol Malpani's picture

Hi Siva, Please check the answer for Bhavani's comment. Regards,
Padmaja's picture

it is very informative.gud presentation.it is very imp information 2 all.

Regards Padmaja.K

Padmaja's picture

can u explain watz D-value and Z-value???????

Regards Padmaja.K

Amol Malpani's picture

Dear Padmaja, Please refer the comment which I replied for Bhavani's comment. You will get your answer. Regards,
Amol Malpani's picture

Thanks A lot you can ask if you have any other doubt.

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