10/05/2020

Sterility Indicators.

 

Introduction:

  • Definition: Sterilization is defined as complete removal of microorganisms from an object, surface or a product.

  • It is essential that the process of sterilization must have strict control to ensure absence of viable organisms in the sterile products.

  • There are basically two types of controls,

    • Control on the process of sterilization i.e. Sterility Indicators or Sterility Monitors.

    • Sterility testing of the products.

  • Monitoring of sterilization can be done by employing either of,

    • Physical Indicator

    • Chemical Indicator,

    • Biological Indicator, for the sterilization process employed.

  1. Physical Indicators:

  1. Moist Heat:

  • A main “Master Process Record (MPR)” is prepared for a particular autoclave and for each specified product and load configuration.

  • This is then used as a reference for “Batch Process Record (BPR).

  • Nowadays sterilization cycles are controlled by microprocessors and pressure is measured through gauges or transducers.

  1. Dry Heat:

  • A Master temperature record is prepared for a particular equipment which can be used as reference for each batch temperature setting.

  1. Radio Sterilization:

  • Sterilization by radiation can be monitored by use of plastic dosimeters which can detect amount of dose absorbed during the process.

  1. Gaseous Methods:

  • For gaseous sterilization process following steps are monitored,

    • Elevated temperatures during the sterilization cycle are recorded by “Temperature Probes”.

    • Gas leakage tests are done to ensure “Gas-tight” seals.

    • Gas concentration, weight of gas under pressure, humidity are measured and recorded.

  1. Filtration Methods:

  • Bubble Point pressure test is conducted immediately after the process which ensures the integrity and determines the pore size of the filter in use. 

  • Bubble Point pressure test:

    • Filter is soaked in an appropriate fluid and then the pressure is applied.

    • The pressure difference at which the first bubble of air breaks away is equivalent to maximum pore size of the filter.

    • On further increase of air pressure causes eruption of air bubbles all over the surface of filter, the air pressure is equivalent to mean pore size.

2) Chemical Indicators:

  • Chemical monitoring of the sterilization process involves change in chemical or physical properties of the chemical monitors by sterilization process.

  • The most common used chemical monitors are as follows,

    • Brown’s Tubes

    • Witness Tubes

    • Heat Sensitive Tape.

    • Royce Sachet

    • Chemical Dosimeters.

  1. Brown’s Tubes:

  • Commonly used for heat processes.

  • These are small tubes containing reaction mixture and an indicator.

  • Exposure to high temperature causes a color change in the tube from red, yellow, brown to green showing a specific temperature.

  1. Witness Tubes:

  • These tubes contain a substance of known melting point at a specific temp. e.g. Sulphur (115°C), Benzoic Acid (121°C) etc.

  • Melting of the substances indicates a specific temp. Achieved in the heat process.

  • Sometimes a dye like methylene blue is added for clear indication of melting.

  1. Heat Sensitive Tape:

  • It is an adhesive tape used in autoclaving to indicate a specific temperature.

  • Autoclave tape works by changing color after exposure to temperatures commonly used in sterilization processes, typically 121°C in a steam autoclave.

  • Small strips of the tape are applied to the items before they are placed into the autoclave.

  • The tape is similar to masking tape but slightly more adhesive, to allow it to adhere under the hot, moist conditions of the autoclave. 

  • One such tape has diagonal markings containing an ink which changes colour (usually beige to black) upon heating.

  1. Royce Sachet:

  • It's a chemical indicator used for “Ethylene Oxide Sterilization”.

  • Its a polyethylene sachet containing magnesium chloride and bromophenol blue indicator.

  • Ethylene oxide penetrates polyethylene bags and reacts with the contents of the sachet.

  • At a given concentration - time exposure the color of the mixture changes from yellow to purple due to formation of “Ethylene chlorohydrin”.

  1. Chemical Dosimeter:

  • It is the best technique available to measure radiation dose absorbed during sterilization by radiation.

  • The radio sensitive materials are impregnated in a plastic container and changes color from yellow to red on exposure to the radiations.

3) Biological Indicators.

  • It consists of a suitable microorganism deposited on a carrier and placed in load.

  • After sterilization they are collected and checked for viable microorganisms indiacating effectiveness of the sterilization process.

  • The microorganism to be used should possess following abilities,

    • Highly resistant to the sterilizing agent to be used.

    • Genetically stable.

    • Non pathogenic.

  • The viability of the organisms, storage condition, incubation and culture conditions must be standardized before the use.

  • The organisms usually used are spores as they are tougher life forms.

  • Most common biological indicators and sterilization processes are as follows,


Sterilization Process

Species Used

Autoclaving at 121°C

Bacillus stearothermophilus

Clostridium sporogenes

Dry heat at 160°C

Bacillus subtilis var.niger

Ethylene Oxide

Bacillus subtilis var.niger

Ionizing radiations

Bacillus pumilus

Membrane filter (0.45 micrometer)

Serratia marcescens

Membrane filter (0.22 micrometer)

Pseudomonas diminuta

Commonly asked Questions:

  1. Explain different sterilization indicators.

  2. Write a note on Chemical Indicators / Biological indicators of sterilization process.

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