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Renin Angiotensin Mechanism.

Sterilization Criteria and terms used for expression of microbial resistance.

 

Introduction:

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

  • Dr. Joseph Lister the British Surgeon is considered as a pioneer in antiseptic surgery.

  • Various criterias are applied to improve the efficiency of the sterilization process, some of them are as follows,

    • The Bioburden.

    • Sensitivity of microorganisms.

    • Death Rates or Survivor Curve.


  1. The bioburden:

  • The initial number of microorganisms present in the product or material before sterilization is called “Bioburden” or “Bioload”.

  • The bioburden of the material gives the idea of selecting the temperature, time, concentration of sterilant i.e. deciding the sterilization cycle.

  • e.g.  The usual temperature of autoclave is designed for killing heat resistant microbes while the products containing less heat resistant microbes don't require that temperature conditions.

  1. Death Rate or Survivor Curve.

  • Death in a microbial population is determined by assessing the reduction in the number of viable organisms resulting from contact with given sterilising agent.

  • This can be represented graphically with a “Survivor Curve” by plotting a graph of log of number of survivors against exposure time or dose. 

  • It is observed that the death rate follows “First Order Kinetics” i.e. depends on concentration of sterilising agent or the exposure time. 

  • It is used to find out what is the minimum time needed to achieve an acceptable level of sterilization for a specific purpose. 

  • Death rate determination is used to compare the resistance of the organism to different temperatures of the dame method and also to compare resistance of different microbes to the same sterilizing process.

  • Death rate also provides quantitative measurement of different environment factors like pH, osmolarity, presence of other chemicals on the sterilization process.

  1. Sensitivity of Microorganisms:

  • Microbes show varying degrees of resistance to the various processes of sterilization.

  • Viable cell form of microbes is more sensitive while spore form is very resistant.

  • These forms show varying degrees of resistance to the different methods of sterilization, hence the correct method should be selected for sterilization by considering the microbes sensitivity.

Terms used for expression of microbial resistance:

  • Various terminologies are widely used to detect microbial resistance to the sterilization process, some of the important are as follows,

    • D Value / Decimal Reduction Time

    • Z Value or Thermal destruction Value

    • Inactivation factor.

  1. D Value / Decimal Reduction Time:

  • Definition: Time in minutes at any given temperature to destroy 90% of the viable microorganisms is called D value.

  • The D value is shown by temperature in subscription e.g. D115°C . 

  • D value is considered as one of the important functions to indicate the efficiency of sterilization.

  1. Z Value or Thermal destruction Value:

  • A Z-value is defined as the number of degrees (Celsius or Fahrenheit) required to change a D-value by ten folds. 

  • In the practical sense, it is a measure of how susceptible a spore population is to changes in temperature. 

  • For example, if the Z-value of a population is 10 degrees, then increasing the sterilization temperature 10 degrees will result in a log reduction of the D-value.

  • Bacterial spores have a Z value in the range of 10 to 15°C while the viable cells have Z value of 4 to 6°C.

  1. F Value:

  • Used in the food industry, also called “Unit of Lethality”.

  • The F value for a process is the number of minutes required to kill a known population of microorganisms in a given food under specified conditions. 

  • This value is also used to calculate the probable number of bacterias remaining after the process in food.

  1. Inactivation Factor:

  • It is the degree to which the viable population of organisms is reduced by applying a sterilization process.

  • It is obtained by dividing the initial viable count by final viable count.

  • Inactivation factor (IF) = 10 t/D 

    • Where, t = exposure time in minutes , 

    • D = Decimal reduction time for the same temperature and conditions.

  1. Q10 Value or Temperature Coefficient: 

  • It is defined as the increase in rate of reaction or killing rate of a sterilization process brought after increasing the temp. by 10°C.

Commonly asked questions.

  1. Define: D Value, Z value, Q10, F value, Bioburden.

  2. Explain the terms Inactivation factor, Z value, Death Rate. D value.
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