Rate of Death of Organisms & Factors Influencing it

  The death of microorganisms, due to the effect of antimicrobials, always occur in a logarithmic manner. An antimicrobtal agent causes death of micro-organisms by killing a specific proportion of organisms present in the object under a given set of conditions.

  Therefore, the decrease in the number of organisms become logarithmic or exponential. For example, a given agent is able to kill 90% of the population under a given set of conditions per unit time the number of survivors will be as under.
10(n), 10 (n-1), 10 (n-2), 10(n-3) and so on
Where, n indicates initial logarithmic value of organisms present in the object / material.

The death curve of organisms exposed to an antimicrobial agent. Curve A shows arithmetic number of organisms surviving per unit volume against time. Curve B shows logarithmic value of organisms surviving per unit volume against time.

This probability of death of micro-organisms is also proportional to the concentration of the antimicrobial chemical agent or intensity of the physical agent. If the concentration of the chemical or intensity of the physical agent is more, rate of death will be more, and vice versa.
   Further, rate of death is also influenced by various other factors such as
- age of organisms,
- physiology of cells,
- types of organisms as well as environmental conditions of treatment.
  The overall death of organisms will also depend on the time of treatment as well as initial microbial load.

Importance of study of kinetics of microbial death
  The study indicates the probability of surviving micro- organisms at the end of treatment. The probability Increases with higher iInitial microbial load. Hence time required to achieve effective sterilization depends on Initial microbial load. Thus the study allows to calculate the time required to achleve effective sterilization.

Factors influencing death of micro-organisms

  Various factors are known to infuence the death of micro-organisms on treatment with antimierobial agents. They include :
1. Type of microorganisms
2. Biological and physiological characteristics of the organisms
3. Initial microbial load
4. Time of treatment
5. pH
6. Viscosity of the material
7. Chemical composition of the material
8. Temperature
9. Concentration or intensity of antimicrobial agent

1. Type of micro-organisms -

  The species of micro-organisms present in the material is one of the important factors that decides rate of death. Sensitivity of an organism to an agent varies with the species. Some species are most sensitive to an agent. Usually spores of bacteria are most resistant to any antimicrobial agent than the vegetative cells.
   Amongst bacteria, usually cocet and gram-positive organisms are found more resistant to an agent than gram-negative organisms.

2. Biological and physiological Characteristics of the organisms - 

   The biological and physiological properties of organisms are also one of the factors that decide effectivity of antimicrobial agents and hence rate of death. In general, old cells as well as resting or dormant organisms are resistant, whereas actively growing young cells are easily killed. This is because of the fact that most agents act by interfering with cell's metabolism, which is most active in actively growing young cells.

3. Initial microbial load -

   If the initial microblal load in the material is high, a more number of organisms is required to be killed. Since, any antimicrobial agent kills organisms in a specific proportion per unit time of treatment, more treatment is required to kill all organisms from the material if initial microbial load is high and vice versa.

4. Time of treatment -

  The organisms present in a material are always killed function of time, when treated with antimicrobial agent. Therefore, if duration of treatment with antimicrobial agent is increased, more organisms will be killed and vice versa.

5. pH -

  pH is inverse log of H ion concentration. It affects the potency of any antimicroblal agent. Usually, the effectivity of most antimicrobial agents increase with increase in H ion concentration of material, i.e. at lower pH. However, It also depends on the property of the agent.
i).  e.g. Heat, radiations etc. are more effective at low pH values.
ii). Effect of certain agents such as penicillin decrease at low pH values as they get inactivated.

6. Viscosity of the material -

   Another physical factor affecting efficiency of an antimicrobial agent is viscosity. High viscosity has a protective effect and hence reduces the death rate of organisms.

7. Chemical composition of material -

  Presence of large amount of organic matter in the material has the protective effect against the antimicrobial agent. It may adsorb the antimicrobial agent and retard its eflectivity.

8. Temperature -

   Temperature of treatment is another important factor which influences death rate of bacteria by antimicrobtal agent. Usually. the effectivity increases with the increase in temperature of treatment.

9. Concentration or intensity of the agent -

   The death rate of bacteria is always Increasing with the increase in the concentration of antimicrobial chemical or intensity of the physical agent.

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