Disinfection

W. L. MALLMANN.

A FARMER can do much for the health of his animals by keeping his barns and sheds absolutely clean.

The initial step in sanitation is to know (and to have workers who know) what proper cleaning is, why it is important, and how to do it.

Disinfection destroying the disease germs is possible only after thorough cleaning. Disinfectants spread over unclean surfaces kill only the microorganisms on the surface; the embedded organisms are untouched; the disinfectant is wasted.

Effective cleaning begins with the removal of all gross waste, such as manure and bedding. Then the surfaces are scrubbed with brushes and a good detergent solution until they are visibly clean. The surfaces then can be flushed with clean water and a disinfectant applied.

Any good alkaline detergent in warm water is satisfactory, but lye (caustic soda) alone may be used. Because lye is very caustic, rubber boots should be worn and the solution should be handled carefully lest it come in contact with the skin. The surfaces should be treated with the alkaline solution and allowed to react for a few minutes before brushing. The surfaces should be thoroughly flushed with clean water to remove all traces of the detergent.

Lye tends to destroy most microorganisms; it kills as it cleans. It corrodes metals and so cannot be used as a general cleaner.

The value of good cleaning before disinfection was shown in a test made in a meatpacking plant. On a badly contaminated concrete wall, 28 million bacteria were counted per 2-inch square. A section of the wall was sprayed with a good germicidal solution. The bacterial population was reduced to 11 million, which demonstrated the disinfectant had failed. Another section was washed with an alkaline detergent solution. Before rinsing, the bacterial count was 380,000- After rinsing with clean water, the count was 53,000. The wash operation removed the visible soil and 99.8 percent of the bacteria. The same disinfecting solution applied to the clean-washed and rinsed surface reduced the population from 53,000 per 2-inch square to 100. The wall that had been washed-rinsed and sanitized was properly treated, but the wall that had been sanitized only was still unclean and bad many bacteria.

In another experiment to demonstrate the ineffectiveness of a sanitizer on an unclean surface, two milking machines were carefully cleaned. Each day one was rinsed with an ineffective sanitizer and the other with an acceptable sanitizer. Both machines were rinsed with cold water after each milking and stored for the next milking without washing. For the first 4 days there were very few bacteria in the milk from either machine. From the fifth day on, the number of bacteria increased in the milk from both machines, but more rapidly in the machine treated with the ineffective sanitizer. The accumulating milk soil in both machines began to interfere with the sanitizing action, however proof that disinfection was impossible when the surfaces of a milking machine had not been cleaned of all milk soil.

THE DISINFECTION PROCESS is a chemical reaction between the disinfectant and the micro-organisms. The disinfectant has to touch the organism.

If the organism is embedded in organic soil, no contact is made and no disinfection results. All soil that contains embedded organisms therefore should be removed from surfaces by cleaning. Thus only the exposed organisms are left on the clean surfaces and the disinfectant can reach them.

Some disinfectants come in contact with the organisms readily. Others need the addition of other chemicals to obtain rapid contact. These agents are called wetting agents, because they cause the water to wet the soil and the organisms more rapidly than water alone. The wetting agents such as soap tend to improve contact with the organisms, and they also serve as detergents to aid in the removal of any organic soil from the surface of the organism. Thus the disinfecting agent is given penetrating power.

Because soaps may react with the disinfectant and reduce its activity, the cleaning operation should precede the disinfection process. Only the products that are called detergent-sanitizers should be used as combination cleaners and sanitizers.

Some disinfectants, such as the quaternary ammonium compounds, (sometimes called "quats") are wetting agents. They exhibit some detergency, which aids in better contact with organisms. Compatible wetting agents have been added to these compounds to increase the detergency-sanitizer effect. They are used to clean and disinfect dairy utensils.

Disinfectants work best when they and the surfaces to be disinfected are warm. Phenol in a concentration of 1:90 kills the typhoid bacillus in 5 minutes at 68 F., but a weaker concentration of 1:60 kills it at 100 . The difference at lower temperatures is greater. A 1:70 solution of phenol kills staphylococcus in loo minutes at a temperature of 50 but at 68 the killing time is 20 minutes.

Lye is one of the few disinfectants that is only slightly affected by temperature. A 2-percent lye solution can be used at 32 without little reduction in germicidal efficiency. Disinfection should not be done when the temperature is below 32 .

THE ACIDITY OR ALKALINITY of the disinfectant or the thing to be disinfected should be considered in the selection of the disinfectant. For example, a hypochlorite solution is active in slightly acid solution and inactive in alkaline solution. But quaternary ammonium compounds generally are more active in alkaline solution than in acid solutions.

In a series of tests, a quaternary ammonium compound was tested at pH values of 5 (acid), 7 (neutral) and 9 (alkaline) in a 1:1,000 dilution. A culture of Micrococcus pyogenes var. aureus was used and the percentage of kill was determined after exposures of 1 minute. The kills were 55.6 percent, 96.2 percent, and 100 percent when the pH was 5, 7, and 9, respectively.

Only in the alkaline solution was complete kill obtained in 1 minute. If the solution had been allowed to act for a longer period, kill would have occurred in the acid (pH 5) solution. If the concentration had been increased materially, kill would have occurred in the acid solution in 1 minute. One can use large amounts of most disinfectants and get satisfactory results at varying pH values if the disinfectant is not excessively acid or alkaline.

MOST ORGANIC-TYPE DISINFECTANTS like the cresols--react slowly even when present in large amounts. The fastest acting disinfectants known are the halogens, such as the hypochlorites.

In the absence of ,organic matter, 1 part per million (p. p. m.) of free chlorine at 6 pH killed 100,000 cells of Escherichia coli in 15 seconds. The cresols and their derivatives are much slower. Where contact periods of three or more minutes occur, the cresols do an equally good job in proper concentration.

THE CONCENTRATION of the disinfectant also is important. In general, the greater the concentration is, the shorter is the time of kill. An example is the action of mercuric chloride (HgCl₂) on Salmonella paratyphi. The times taken for disinfection (in minutes) were 1.5, 7, 10, 13, 65, and 230 when the solution was, respectively, 1, 0.50, 0.10, 0.05, 0.01, and 0.005 parts of HgC1₂ per 1,000 parts of water. Thus, diluting the mercuric chloride solution 50 percent lowers its germicidal activity about 50 percent. Diluting a phenol solution 50 percent, however, lowers the germicidal activity approximately 64 times.

Disinfectants should always be used in proper concentrations. The user should follow recommended dilutions closely unless he knows how dilution will affect the compound he is using.

THE PRESENCE OF ORGANIC MATTER in the disinfecting solution or on the surfaces to be disinfected may reduce germicidal activity to a point where no disinfection results. The organic matter may cover the organism so the disinfectant cannot reach the organism; it may neutralize the disinfecting action by combining with the disinfectant; or it may serve as an adsorbing surface to reduce the amount of active disinfectant present.