The rate and extent of antimicrobial action of chemical disinfectants are influenced by several factors.
Factors Affecting Disinfectant Action
- Concentration of disinfectant.
- Time of contact.
- pH of the environment.
- Surface tension.
- Formulation of the disinfectant.
- Chemical structure of disinfectant.
- Type and number of microorganisms present.
- Interfering substances in the environment.
- Potentiation, synergism, and antagonism of disinfectants.
1. Concentration of disinfectant:
The rate of killing microorganisms varies directly with the concentration of the disinfectant. However, the effectiveness is generally related to the concentration exponentially, not linearly. There is an optimum concentration of phenol at about 1%. Beyond this concentration, the disinfecting effectiveness becomes less. The dilution coefficient can be calculated from the following equation:
Where n is the concentration exponent or dilution coefficient for the disinfectant, t1 is the death time with disinfectant concentration C1 and t2 is the death time with disinfectant concentration C2-.
The concentration exponent or dilution coefficient is an important characteristic of each disinfectant and is useful in determining the effect of dilution on the disinfectant.
The rate of disinfection normally increases with the temperature. The effect of temperature on bactericidal activity may be expressed quantitatively using a temperature coefficient. The temperature coefficient per degree rise in temperature is denoted by ‘θ’ whereas per 10 oC rise in temperature is expressed by θ10 or Q10 values.
where T2 and T1 are two temperatures differing by 10 oC; t2 and t1 are the corresponding lethal times. The value for Q10 for phenol is 4, which means that over the 10 oC range used to determine the Q10 (20 to 30 oC), the activity will be increased by a factor of 4.
3. Time of contact:
Sufficient time of contact must be allowed for the disinfectant to exert its action. It is shown that the principles of first-order kinetics may be applied to the disinfection process and the rate or velocity constant, K, is a measure of the efficiency of the disinfectant.
where t = time for the viable count to fall from No to Nt
No = initial number of microorganisms
Nt = final number of microorganisms
The survivor/time curve may not be constant and its shape is influenced by several factors, mainly the concentration of the disinfectant (Fig.1).
4. pH of the environment:
A change in pH during the disinfection process can affect the rate of growth of the inoculum, the potency of the disinfectant, and the ability of the disinfectant to combine with a site on the cell surface. A pH of 6 – 8 is optimal for the growth of many bacteria and the rate of growth declines on either side of this range. Phenolic and acidic antimicrobial agents usually have the greatest activity in acidic conditions. Acridine dyes and quaternary ammonium compounds are usually more active in alkaline than in acidic solutions. Amphoteric surface-active antibacterials (e.g. Tego compounds) have optimum activities at widely differing pH values, depending on the number of nitrogen groups.
5. Surface tension:
The contact between aqueous solutions of disinfectants is facilitated if they have surfactant properties. This helps in the adsorption of surface-active disinfectants on the surfaces of cells as well as in the wetting and spreading properties of the solutions. A combination of soap with crude phenol (carbolic acid) has excellent disinfecting properties. Soaps can be used to lower the surface tension and the extinction time until the soap concentration is equivalent to the critical concentration for micelle formation.
6. Formulation of the disinfectant:
The formulation may be important for the effective use of disinfectants. The effectiveness of chlorhexidine and quaternary ammonium compounds may be greater in 70% alcohol than in aqueous solutions. Iodine is virtually insoluble in water and is dissolved in alcohol, potassium iodide solution, or solutions of surface active agents. The presence of a suitable surfactant can moderate the staining and corrosive properties of iodine and increase the stability of the preparation. For convenience and economy, it is essential to formulate a disinfectant solution to be as concentrated as possible which is suitable for dilution with water immediately before use.
7. Chemical structure of disinfectant:
The chemical structures of compounds affect the disinfectant activity. Substitution of an alkyl chain up to 6 carbons in length in para position to phenolic – OH group increases activity but greater than 6- carbons in length decrease water solubility and disinfectant activity. Generally, halogenation increases the antibacterial activity of phenol but nitration increases antibacterial activity and systematic toxicity also.
8. Type and number of microorganisms present:
The efficiency of disinfection greatly depends on the nature and number of contaminating microorganisms and especially on the presence or absence of bacterial spores. It can be seen that most vegetative bacteria except acid-fast bacilli are rapidly killed by most chemical disinfectants. Bacterial spores are difficult to destroy but some disinfectants (e.g. aldehydes) are sporicidal. Aldehydes and halogens, together with β-propiolactone are the most active virucides. Mycobacterium tuberculosis and other acid-fast bacilli are fairly resistant to many aqueous bactericides but are susceptible to iodine, formaldehyde, alcohol and phenolic compounds.
9. Interfering substances in the environment:
Material such as blood, body fluids, pus, milk, food residues, or colloidal proteins may reduce the effectiveness of disinfectants if present in small amounts. This may be due to adsorption or chemical interaction or the shielding of microorganisms from adequate contact with the germicide. The presence of oils and fats markedly reduces the disinfecting ability of phenolics.
10. Potentiation, synergism, and antagonism of disinfectants:
Potentiation of a disinfectant leads to enhanced antimicrobial activity e.g. polysorbate 80, low concentration of non-ionic surfactants, etc. Synergistic effects are often shown by two antimicrobial agents which is giving an increased activity. Different phydroxybenzoate esters show synergistic activity and mixtures are often used to obtain adequate preservation. Antagonism leads to decreased antimicrobial activity and use is made of antagonists in the elimination of antimicrobial properties of materials (tested for sterility) e.g. sodium thiosulphate, lubrol W+ lecithin, etc.
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