Sunday, 30 June 2013

Biological indicators for sterilization process

Biological indicators for sterilization process


Sterilization  is a process, by which living organisms are killed or removed to the extent that they are no longer detected by standard culture media which have previously been found to proliferate. 

Commonly used methods for sterilization are

1. Physical methods

2. Chemical methods

 1.           Physical methods
·      Thermal (heat) methods
a. Dry heat sterilization
b.Moist heat sterilization
·      Radiation method
     (Gamma rays & UV light)
·      Filtration method


2.           Chemical methods

Gaseous  method
(Ex: Formaldehyde, Ethylene oxide)

Heat sterilization
Oxidation and protein denaturation
Most widely used and reliable method of sterilization, involving destruction of enzymes and other essential cell constituents.
Can be applied only to the thermostable products.
Dry heat is applicable for sterilizing glasswares and metal surgical instruments and moist heat is the most dependable method for decontamination of laboratory waste and the sterilization of laboratory glassware, machine parts,media, and reagents.
Gaseous sterilization
Penetrating ability of gases
Gases being alkylating agents are potentially mutagenic and carcinogenic
Ethylene oxide gas has been used widely to process heat-sensitive devices.
Radiation sterilization
Ionization of nucleic acids
It is a useful method for the industrial sterilization of heat sensitive products.
Undesirable changes occur in irradiated products,an example is aqueous solution where radiolysis of water occurs.
Radiation sterilization is generally applied to articles in the dry state; including surgical instruments, sutures, prostheses, unit dose ointments, plastics
Filtration sterilization
Does not destroy but removes the microorganisms
It is used for both the clarification and sterilization of liquids and gases as it is capable of preventing the passage of both viable and non viable particles.
Does not differentiate between viable and non viable particles
This method is Sterilizing grade filters are used in the treatment of heat sensitive injections and ophthalmic solutions, biological products and air and other gases for supply to aseptic areas.


Biological indicators (BI)

Biological indicators are devices (class 2) for monitoring sterilization process, that consists of a  standardized population of micro organisms with a   known resistance to the mode of sterilization, placed in or on a carrier and enclosed in a protective package.

  Microorganisms used to prepare biological indicators are those capable of forming endospores and the microorganism is used in the “spore state”. Because spores are more resistant than microbes,if spores in the BI have been killed then the other microbes should have also been killed.


The most important characteristic of biological indicators is the level of resistance, this is known as decimal reduction value (D value).The D value is the time taken to reduce the population of a known microorganism by 1 log (or 90% of the population).D values will vary according to the resistance of microorganisms and the population challenge.

Uses of Biological indicators 

vBiological indicators are  being used to monitor the effectiveness of a sterilization process (A negative BI result when incubated under a suitable conditions along with a positive control indicates the efficiency of a sterilization process).

They are only process indicators that directly monitor the lethality of a given sterilization process.

v To validate the performance of a sterilization equipment.


Choice of  Biological indicators for different sterilization process

A single species of bacterial endospores are not universally used to monitor all modes of sterilization, because the resistance of different species of endospores varies and depends on the mode of sterilization.


Biological indicators for different sterilization process are tabulated below

Biological indicators for steam sterilization (moist heat)
Geobacillus stearothermophilus (formally known as Bacillus stearothermophilus)
Biological indicators for dry heat sterilization
Bacillus atrophaeus (formally known as
Bacillus subtilis)
Biological indicators for Ethylene oxide sterilization
Bacillus atrophaeus (formally known as
Bacillus subtilis)
Biological indicators for Ionizing radiation
Bacillus pumilus


Biological indicators (BIs) are considered the highest level of sterility assurance because they actually test the sterilizer’s ability to kill specific strains of highly resistant organisms. Commercially available BIs are non-pathogenic. The target population for biological indicator is >106 .Because it is generally accepted that “devices purporting to be sterile” such as an autoclave, are designed to achieve a 10-6 probability (i.e. there is less than one chance in a million that a microorganism would survive the sterilization process).


The most important characteristics of a biological indicator is that sporulation must definitely occur on a defined medium and the survivors form easily countable colonies. Without possessing these characteristics, the biological indicator is off little value. As this is a critical parameter, it is recommended that a positive control be run along with each test of biological indicators.

All biological indicators must come with a certificate of conformity. The certificate should indicate the population,D-value and purity of the organism.


1 comment:

  1. Thank you for the good article.
    I know for sure that some biological research data is stored in so-called virtual data rooms to be safe.