Experimental Parameters Influencing the Antimicrobial Agent Activity

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Chapter: Pharmaceutical Microbiology : Microbial Control by Physical and Chemical Methods

antimicrobial agent is invariably and predominantly affected by the following six cardinal factors, namely :



It has been amply demonstrated, proved, and well documented that the actual prevalent destruction of various pathogenic/nonpathogenic microorganisms and their subsequent inhibition of the resulting ‘microbial growth’ are not simple matters at all, as the underlying efficacy of an antimicrobial agent is invariably and predominantly affected by the following six cardinal factors, namely :


1. Population Size


It may be observed that usually an equal fraction of a microbial population gets killed during each stipulated period (interval); and, therefore, a larger population certainly needs a relatively longer duration to die than a smaller one. Importantly, the same principle holds good for the chemical antimi-crobial agents.


2. Population Composition


Importantly, the overall effectiveness of an antimicrobial agent exclusively changes with the prevailing nature of the microorganisms under investigation due to the fact that they differ distinctly in their susceptibility.


Salient Features : These are as follows :


(a) Microbial endospores are found to be much more resistant to a large segment of the antimi-crobial agents in comparison to the vegetative forms.


(b) Younger cells are invariably more prone to rapid destruction than the corresponding mature organisms.


(c) Certain specific species may withstand adverse experimental parameters better than others.


Example : Mycobacterium tuberculosis (causative organism for tuberculosis is found to be much more resistant to antimicrobial agents vis-a-vis other microorganisms.


3. Concentration of Antimicrobial Agent


One may observe quite often that the more concentrated a ‘chemical agent’ or ‘intense a physi-cal agent’—the more quickly the microorganisms get destroyed. Nevertheless, the ‘agent effective-ness’ is not normally associated with either concentration or intensity directly.


Salient Features—are as given under :


(1) Spread over a short-range a rather small increase in the concentration of antimicrobial agent ultimately leads to a definite exponential rise in its effectiveness ; however, beyond a certain critical point one may not observe any more increase in the rate of killing.


(2) Occasionally, an antimicrobial agent is found to be more effective even at much lower con-centrations.


Example : Ethanol 70% (v/v) is more effective in comparison to 95% (v/v), by virtue of the fact that its (EtOH) activity gets markedly enhanced by the presence of water.


4. Duration of Exposure


The longer a particular population of microbes is duly exposed to a microcidal agent, the more number of microorganisms would be killed. In order to accomplish perfect sterilization, an exposure duration just sufficient to reduce the ensuing survival probability to either 10–6 or less must be em-ployed effectively.


5. Temperature


It has been noticed that an increase in the temperature at which a particular chemical agent invariably exerts its action often increases its activity. Quite often a lower concentration of either a sterilizing agent or disinfectant may be suitably employed at a higher temperature effectively.


6. Local Environment


It is, however, pertinent to state here that the population to be controlled is not isolated by sur-rounded by several environmental factors which may cause :


·        offer due protection, and


·        afford destruction.


Examples :


(a) As heat kills more rapidly at an acidic pH, hence the acidic beverages and food products viz., tomatoes and fruits are much convenient and easy to get pasteurized in comparison to such foods having higher pHs e.g., milk.


(b) Organic matter present in a surface-biofilm would eventually afford due protection of the biofilm’s microorganisms ; besides, the biofilm together with its associated microorganisms often shall be difficult to remove efficaciously.


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