The concentration of a disinfectant plays an important role to kills microorganisms.
In general, a higher concentration of disinfectant increases its ability to destroy microbes effectively, because at higher concentration, more disinfective agents are available to attack the cells. Some disinfectants work best only within a specific concentration range, and if the concentration becomes too high, their activity may decrease, i.e. alcohol concentrations over 91% coagulate cell wall proteins instantly and created a protective layer which protects disinfectant molecule to enter inside the microbial cell, and reduced its effectiveness. In most cases, using too low a concentration will slow down the killing process because the disinfectant becomes too weak to damage all microorganisms effectively.The effectiveness of a disinfectant depends greatly on its contact time, which is the amount of time the disinfectant remains in direct contact with microorganisms.
If the contact time is too short, the disinfectant may not penetrate or damage the microbes enough to kill them, resulting in incomplete disinfection. Most disinfectants need sufficient time to act on the cell wall, cell membrane, or internal components of the microorganism. When the required contact time is provided, the disinfectant works more effectively and ensures complete killing of microbes.Many disinfectants need to stick to the surface of microbes before they can kill them. If there are too many microbes together, they can trap most of the disinfectant, and other microbes do not get exposed to disinfecting agent molecules.
So, in this condition, when the number of microorganisms is very high, the disinfectant takes more time to kill all of them completely.Most chemical disinfectants work best at temperatures above 68°F.
However, too high of temperatures may break down the chemical structure of some disinfectants, or increase evaporation and reduce the effectiveness. Low temperatures can also reduce activity of some products.A change of pH during the disinfection process can affect the potency of the disinfectant and the ability of the disinfectant to combine with a site on the cell surface.
Phenolic and acidic antimicrobial agents usually have greatest activity in acidic conditions. Acridine dyes and quaternary ammonium compounds are usually more active in alkaline than in acidic solutions.The contact between aqueous solutions of disinfectants is enhanced if they have surfactant properties.
A combination of soap with crude phenol (carbolic acid) has excellent disinfecting properties. Soaps can be used to lower the surface tension which enhance the contact of disinfecting agent to the microbial cell.Formulation may be important for the effective use of disinfectants.
Effectiveness of chlorhexidine and quaternary ammonium compounds may be greater in 70% alcohol than in aqueous solutions.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 with the exceptions of 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 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.The presence of organic material such as blood, body fluids, pus, milk, food residues or colloidal proteins may reduce the effectiveness of disinfectants even if they present in small amounts.
This may be due to adsorption or chemical interaction or the shielding of microorganisms from adequate contact with the disinfectant molecules. The presence of oils and fats markedly reduces the disinfecting ability of phenolics. Presence of dead bacterial cell in the medium, adsorbs some amount of disinfectant molecules and reduces its effective concentration and ultimately reduces its effectiveness.