Disinfection: Factors affecting action of disinfectants.
Introduction:
Disinfection is a process of removal or destruction of microorganisms and reducing their number to a non harmful level.
Disinfection usually kills the vegetative form of microorganisms and doesn't affect the endospores.
The chemical which is used for disinfection of the nonliving objects (Inanimate objects) is called a “Disinfectant”.
The chemical which is used for disinfection of the living objects is called an “Antiseptic”.
Mostly the disinfectants are “Bactericidal” while some may be “Bacteriostatic”.
Factors affecting action of Disinfectants:
The rate and extent of antibacterial action of the disinfectant depends on many factors like,
Concentration of the disinfectant.
Chemical Structure of the disinfectant.
Formulation of the disinfectant.
Interfering substances in the environment.
pH of the surrounding.
Potentiation and antagonism of the disinfectants.
Surface Tension.
Temperature.
Time of Contact.
Type and no. of microbes present.
Concentration of the disinfectant:
The rate of killing the microbes varies directly with concentration of the disinfectant.
The rate of killing is related exponentially with the concentration of the disinfectant, not linearly.
There is an optimum concentration of the disinfectant at which it shows maximum efficacy, below and beyond this concentration the efficacy decreases.
The Dilution Coefficient is an important characteristic of a disinfectant which determines how much dilution is to be made for maximum efficiency.
The dilution coefficient is calculated using following formula,
Where,
n= Dilution Coefficient of the Disinfectant.
t1= Death time @ Concentration C1.
t2= Death time @ Concentration C2.
Chemical Structure of the disinfectant:
Chemical activity is largely dependent on the chemical structure of the disinfectant.
Introduction of an alkyl chain at para position of Phenol increases the activity, however, when the chain increases more than 6 carbon atoms it decreases the solubility and disinfectant action.
Halogenation increases antimicrobial action of the phenol while nitration reduces it.
Formulation of the disinfectant:
A good formulation increases the effectiveness of the disinfectant.
Iodine is virtually insoluble in water and hence is made to dissolve by using alcohol and potassium iodide solution.
Addition of a surfactant in iodine solution decreases its odour, staining problem and increases stability of the preparation.
Chlorhexidine and Quaternary ammonium compounds show increased efficiency in 70% alcohol solution than in the aqueous solution.
Interfering substances in the environment:
The organic materials like pus, blood etc present at site of action of disinfectant greatly reduces the activity of disinfectant.
Presence of fats and oils at the site of action of phenol greatly reduces its activity.
pH of the surrounding:
Most bacteria show optimum growth at 6-8 pH.
Acidic disinfectant shows maximum activity at an acidic pH as they remain ionized.
Basic dyes like Acridine and Quaternary ammonium compounds show maximum activity at basic pH as they remain ionized at that pH.
Amphoteric surfactants e.g. Tego compounds show good activity at variety of pH.
Potentiation and antagonism of the disinfectants:
Some disinfectants potentiate the activity of other disinfectants while some antagonize actions of each other.
Surface Tension:
Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible.
The lowered surface tension in an aqueous solution of the disinfectant increases its adsorption on the microbial cell and increases wetting properties and solubility of the solution.
A combination of Phenol with soap shows increased disinfectant action as soap by its property lowers surface tension.
Temperature:
The action of disinfectant normally increases with temperature at a certain point above which it decreases.
The effect of temperature on disinfectant action is expressed by using “Temperature coefficient” which is denoted by “𝞡” , where as per 10℃ is denoted by Q10.
The Q10 is calculated using following formula,
Time of Contact:
Sufficient time of contact must be allowed to the disinfectant to show its action.
The lesser time of contact results in decreased activity of the disinfectant.
Type and no. of microbes present:
Disinfectants are mainly active against vegetative forms of microbes and not their spore form.
Bacterial spores are very difficult to destroy however, aldehydes like formaldehyde are known sporicidals.
Acid fast bacilli due to presence of fats in their cell membrane are virtually immune to aqueous solutions of disinfectants but can be killed using phenols, aldehydes and halogen derivatives.
Labels: Microbiology
