Chemical Methods of Control

Chemical agents are generally not intended to achieve sterilization. Most reduce the microbial populations to safe levels or remove pathogens from objects. An ideal disinfectant or antiseptic (chemical agent) kills microorganisms in the shortest possible time without damaging the material treated.


Among the important criteria for selecting an antiseptic or disinfectant are the concentration of disinfectant to be used, whether the agent is bactericidal or bacteriostatic, the nature of the material to be treated, whether organic matter will be present, the temperature and pH at which the chemical agent will be used, and the time available in which the chemical agent will be left in contact with the surface tested.

Evaluation methods. To evaluate an antiseptic or disinfectant, the phenol coefficient test is used. In this test, various dilutions of the chemical agent are prepared and tested against equivalent dilutions of phenol with such bacteria as Staphylococcus aureus and Salmonella typhi. A phenol coefficient (PC) greater than one indicates that the chemical agent is more effective than phenol and less than one that it is less effective.

An alternative test is the in-use test. Various dilutions of the chemical agent are made and tested against a standardized preparation of test bacteria on the type of material later to be disinfected in normal use.

Phenol. One of the first chemicals to be used for disinfection was phenol. First used by Joseph Lister in the 1860s, it is the standard for most other antiseptics and disinfectants. Phenol derivatives called phenolics contain altered molecules of phenol useful as antiseptics and disinfectants. The phenolics damage cell membranes and inactivate enzymes of microorganisms, while denaturing their proteins. They includecresols, such as Lysol, as well as several bisphenols, such as hexachlorophene, which is particularly effective against staphylococci (Figure 1 ). 


Figure 1

A selection of chemical disinfectants and antiseptics.

A chemical agent resembling the phenols is chlorhexidine (Hibiclens), which is used for skin disinfection as an alternative to hexachlorophene. It persists on the skin and is effective against vegetating bacteria, but not spores.

Halogens. Among the halogen antiseptics and disinfectants are chlorine and iodine.Iodine is used as a tincture of iodine, an alcohol solution. Combinations of iodine and organic molecules are called iodophors. They include Betadine and Isodyne, both of which contain a surface active agent called povidone. Iodine combines with microbial proteins and inhibits their function.

Chlorine also combines with microbial proteins. It is used as sodium hypochlorite (bleach). As calcium hypochlorite, chlorine is available to disinfect equipment in dairies, slaughterhouses, and restaurants. The chloramines contain chlorine together with ammonia. They are used to sanitize glassware and eating utensils and are effective in the presence of organic matter. Chlorine is also used as a gas to maintain a low microbial count in drinking water.

Alcohols. Alcohols are useful chemical agents when employed against bacteria and fungi, but they have no effect on bacterial spores. The type of alcohol most widely used is 70 percent ethyl alcohol (ethanol). Isopropyl alcohol (rubbing alcohol) is also useful as an antiseptic and disinfectant. Because alcohols evaporate quickly, they leave no residue and are useful in degerming the skin before injections (Figure 1 ).

Heavy metals. A number of heavy metals have antimicrobial ability. For example,silver is used as silver nitrate in the eyes of newborns to guard against infection byNeisseria gonorrheae. It is also used to cauterize wounds. Copper is used as copper sulfate to retard the growth of algae in swimming pools, fish tanks, and reservoirs. Zincis useful as zinc chloride in mouthwashes and as zinc oxide as an antifungal agent in paints. The heavy metals are believed to act by combining with sulfhydryl groups on cellular proteins.

Soaps and detergents. Soaps and detergents decrease the surface tension between microorganisms and surfaces, and thereby help cleanse the surface. Soaps emulsify the oily film on the body surface, carrying the oils, debris, and microorganisms away in a degerming action. The cationic detergents are quaternary ammonium compounds.They solubilize the cell membranes of microorganisms. Among the popular compounds are Zephiran (benzalkonium chloride) and Cepacol (cetylpyridinium chloride) (Figure 1 ).

Aldehydes. Two aldehydes, formaldehyde and glutaraldehyde, inactivate microbial proteins by crosslinking the functional groups in the proteins. Formaldehyde gas is commonly used as formalin, a 37 percent solution of formaldehyde gas. It is widely employed for embalming purposes. Glutaraldehyde is used as a liquid to sterilize hospital equipment. However, several hours are required to destroy bacterial spores (Figure 1 ).

Ethylene oxide. Sterilization can be achieved with a chemical known as ethylene oxide (ETO). This chemical denatures proteins and destroys all microorganisms, including bacterial spores. It is used at warm temperatures in an ethylene oxide chamber. Several hours are needed for exposure and flushing out the gas, which can be toxic to humans. ETO is widely used for plastic instruments such as Petri dishes, syringes, and artificial heart valves (Figure 1 ). Propylene oxide, a similar compound, is also valuable as a sterilant.

Oxidizing agents. Oxidizing agents such as hydrogen peroxide kill microorganisms by releasing large amounts of oxygen, which contributes to the alteration of microbial enzymes. Hydrogen peroxide is useful on inanimate objects and in foods, but on the skin surface, it is quickly broken down by the enzyme catalase, liberating oxygen. This oxygen causes a wound to bubble and thereby removes microorganisms present. However, the chemical activity on the skin is limited compared to that on inanimate surfaces. Contact lenses can be disinfected with hydrogen peroxide.

Two other oxidizing agents are benzoyl peroxide and ozone. Benzoyl peroxide is applied to the skin to treat acne due to anaerobic bacteria. The oxygen released by the compound inhibits anaerobic growth. Ozone can be used to disinfect water, where it oxidizes the cellular components of contaminating microbes.

Food preservatives. Foods can be preserved by using a number of organic acids to maintain a low microbial population. Sorbic acid is used in a number of acidic foods, including cheese, to prevent microbial growth. Benzoic acid also inhibits fungi and is used in acidic foods and soft drinks. Calcium propionic acid prevents the growth of mold in breads and bakery products.