Lesson 3: Practical Application of Microbiology

Section II: Disinfection and Sterilization

3-5

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3-5. CHEMICAL AGENTS

a. General. Numerous chemical agents are available as germicides, disinfectants, and antiseptics. Only a few of the principal ones will be discussed here. Antiseptics and disinfectants may act on microorganisms in anyone of or a combination of several ways:

(1) Oxidation.

 

(2) Hydrolysis.

 

(3) Combination with cell proteins to form salts.

 

(4) Coagulation of proteins

 

(5) Disruption of the cytoplasmic membrane.

 

(6) Inhibition or inactivation of enzymes.

b. Phenol. Phenol (carbolic acid) and its derivatives are among the most useful of the organic germicides. The germicidal efficiency of phenol is used as a standard of comparison for other antiseptics. A five percent aqueous solution of phenol kills vegetative bacteria readily and the spores more slowly. It is used primarily in disinfecting nonliving objects, because of its harmful effect on tissue. It is particularly useful in disinfecting sputum, blood, feces, and other organic discharges.

c. Alcohol. Alcohol is one of the most effective and extensively used antiseptics. It is interesting to note, however, that it is most effective at strength of 70 percent. At concentrations above 90 percent or below 50 percent, it is distinctively less effective.

d. Halogens. Iodine and chlorine are two of the best known and widely known disinfectants. Tincture of iodine (two percent in alcohol solution) is used to disinfect cuts and wounds. Strong or old solutions will burn the skin. Chlorine, in the form of sodium or calcium hypochlorite, is used in water purification, in sewage treatment, and in disinfecting food and dairy equipment.

e. Formaldehyde. Formaldehyde is a gas used in fumigation. A one percent concentration will kill all pathogenic bacteria in a room or chamber. A 37-40 percent solution in water, known as formalin, is used as a disinfectant, deodorant, and tissue preservative.

f. Detergents. Soap and numerous other detergents act upon microorganisms by reducing the surface tension of the water in which they are used, thereby reducing the ability of the microbial cell membrane to

control the passage of fluids in and out of the cell by osmosis. Soap tends to neutralize the effect of disinfectants; therefore, it should be thoroughly rinsed off before an antiseptic or disinfectant is applied. Hexachlorophene, a phenol derivative, is not affected by soap; therefore, it is incorporated into soaps and detergents used in scrubbing hands of surgical personnel and the skin of surgical patients. Detergents are not true disinfectants, but are used as adjuncts to other agents in order to remove organic matter that might otherwise interfere with the action of the antiseptic or disinfectant. Detergents are also the principal agents employed in sanitizing food service equipment, eating utensils, and patient care items not requiring sterility (bed pans, water pitchers, and so forth).

g. Antibiotics. Numerous chemotherapeutic agents, including penicillin, streptomycin, tetracycline, the sulfonamides, and many others, are used to inhibit or kill microorganisms which have already gained entrance into the body of a human or animal host. This usage, however, falls within the realm of therapy, rather than disinfection and sterilization.

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