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Chapter: Essentials of Anatomy and Physiology: An Introduction to Microbiology and Human Disease

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Methods of Control of Microbes

Microorganisms are everywhere in our environment, and although we need not always be aware of their presence, there are times when we must try to dimin- ish or even eliminate them.

METHODS OF CONTROL OF MICROBES

 

Microorganisms are everywhere in our environment, and although we need not always be aware of their presence, there are times when we must try to dimin- ish or even eliminate them. These situations include the use of chemicals for disinfection, especially in hospitals, and the protection of our food and water supplies.

 

 

ANTISEPTICS, DISINFECTANTS, AND STERILIZATION

 

We are all familiar with the practice of applying iodine, hydrogen peroxide, or alcohol to minor cuts in the skin, and we know the purpose of this: to prevent bac-terial infection. The use of such chemicals does indeed destroy many harmful bacteria, although it has no effect on bacterial spores. The chemicals used to pre-vent infection may be called antiseptics or disinfec-tants. An antiseptic (anti5 against; septic 5 infection) is a chemical that destroys bacteria or inhibits their growth on a living being. The chemicals named above are antiseptics on skin surfaces. A disinfectant is a chemical that is used on inanimate objects. Chemicals with antibacterial effects may be further classified as bactericidal or bacteriostatic. Bactericides kill bacteria by disrupting important metabolic processes. Bacteriostatic chemicals do not destroy bacteria, but rather inhibit their reproduction and slow their growth. Alcohol, for example, is a bactericide that is both an antiseptic and a disinfectant, depending upon the particular surface on which it is used.

 

Some chemicals are not suitable for use on human skin because they are irritating or damaging, but they may be used on environmental surfaces as disinfec-tants. Bleach, such as Clorox, and cresols, such as Lysol, may be used in bathrooms, on floors or coun-tertops, and even on dishes and eating utensils (if rinsed thoroughly). These bactericides will also des-troy certain viruses, such as those that cause influenza. A dilute (10%) bleach solution will inactivate HIV, the virus that causes AIDS.

 

In hospitals, environmental surfaces are disinfected, but materials such as surgical instruments, sutures, and dressings must be sterilized. Sterilization is a process that destroys all living organisms. Most med-ical and laboratory products are sterilized by autoclav-ing. An autoclave is a chamber in which steam is generated under pressure. This pressurized steam penetrates the contents of the chamber and kills all microorganisms present, including bacterial spores.

 

Materials such as disposable plastics that might be damaged by autoclaving are often sterilized by expo-sure to ionizing radiation. Foods such as meats may also be sterilized by this method. Such food products have a very long shelf life (equivalent to canned food),and this procedure is used for preparing some military field rations.

 

PUBLIC HEALTH MEASURES

 

Each of us is rightfully concerned with our own health and the health of our families. People who work in the public health professions, however, consider the health of all of us, that is, the health of a population. Two traditional aspects of public health are ensuring safe food and safe drinking water. Two more recent aspects are tracking and learning about emerging dis-eases, and preparing defenses against the possible use of biological weapons.

 

Food

 

The safety of our food depends on a number of factors. Most cities have certain standards and prac-tices that must be followed by supermarkets and restaurants, and inspections are conducted on a regu-lar basis.

 

Food companies prepare their products by using specific methods to prevent the growth of microor-ganisms. Naturally, it is in the best interests of these companies to do so, for they would soon be out of business if their products made people ill. Also of importance is the willingness of companies to recall products that are only suspected of being contami-nated. This is all to the benefit of consumers. For example, since 1925 in the United States, only five fatal cases of botulism have been traced to commer-cially canned food. If we consider that billions of cans of food have been consumed during this time, we can appreciate the high standards the food industry has maintained.

 

Milk and milk products provide ideal environments for the growth of bacteria because they contain both protein and sugar (lactose) as food sources. For this reason, milk must be pasteurized, that is, heated to 1458F (62.98C) for 30 minutes. Newer methods of pasteurization use higher temperatures for shorter periods of time, but the result is the same: The pathogens that may be present in milk are killed, although not all bacteria are totally destroyed. Milk products such as cheese and ice cream are also pas-teurized, or are made from pasteurized milk.

 

When a food-related outbreak of disease does take place, public health workers try to trace the outbreak to its source. This stops the immediate spread of dis-ease by preventing access to the contaminated food, and the ensuing publicity on television or in the news-papers may help remind everyone of the need for care-ful monitoring of food preparation.

 

Some foods meant to be eaten raw or briefly cooked, such as fruit or rare beef, do carry a small risk. Consumers should realize that food is not sterile, that meat, for example, is contaminated with the animals’ intestinal bacteria during slaughtering. Meat should be thoroughly cooked. Fruit and vegetables should be washed or peeled before being eaten raw.

 

Finally, the safety of our food may depend on some-thing we often take for granted: our refrigerators. For example, a Thanksgiving turkey that was carved for dinner at 3 P.M. and left on the kitchen counter until midnight probably should not be used for turkey sand-wiches the next day. Although we have to rely on oth-ers to ensure that commercially prepared food will be safe, once food reaches our homes, all we really need (besides the refrigerator) is our common sense.

 

Water

 

When we turn on a faucet to get a glass of water, we usually do not wonder whether the water is safe to drink. It usually is. Having a reliable supply of clean drinking water depends on two things: diverting human sewage away from water supplies and chlori-nating water intended for human consumption.

 

Large cities have sewer systems for the collection of wastewater and its subsequent treatment in sewage plants. Once treated, however, the sludge (solid, par-ticulate matter) from these plants must be disposed of. This is becoming more of a problem simply because there is so much sewage sludge (because there are so many of us). Although the sludge is largely free of pathogens, it ought not to be put in landfills, and because ocean dumping is being prohibited in many coastal areas, this is a problem that will be with us for a long time.

 

Drinking water for cities and towns is usually chlo-rinated. The added chlorine kills virtually all bacteria that may be present. The importance of chlorination is shown by a 1978 outbreak of enteritis (diarrhea) in a Vermont town of 10,000 people. The chlorina-tion process malfunctioned for 2 days, and 2000 of the town’s inhabitants became ill. (The bacterium was Campylobacter, a common intestinal inhabitant of animals.)

You may now be wondering if all those bottled spring waters are safe to drink. The answer, in general, is yes, because the bottling companies do not wish to make people ill and put themselves out of business. Some bottled waters, however, do have higher mold spore counts than does chlorinated tap water. Usually these molds are not harmful when ingested; they are destroyed by the hydrochloric acid in gastric juice.

 

In much of North America, nearly everyone has easy access to safe drinking water. We might remind ourselves once in a while that our water will not give us typhoid, dysentery, or cholera. These diseases are still very common in other parts of the world, where the nearest river or stream is the laundry, the sewer, and the source of drinking water. The World Health Organization (WHO) estimates that more than 2.5 billion people do not have basic sanitation, and more than 1 billion do not have dependable access to clean water to drink.

 

Emerging Diseases and Biological Weapons

 

The term emerging diseases may be a bit misleading, because some of the pathogens are not really new. What has happened is that we have become aware of them. A good example of this is Lyme disease. Lyme disease in the United States was named for the town in Connecticut where a cluster of cases was discovered in the 1970s. When the causative bacteria were later found and the disease fully characterized, health offi-cials realized that the disease had been described in Scandinavia in the early years of the 20th century. Lyme disease was not a new disease after all, merely a newly encountered disease for Americans.

 

Other emerging diseases are those caused by the Ebola and Marburg viruses, by Hantavirus, West Nile virus, and the SARS (severe acute respiratory syn-drome) virus, by certain strains of E. coli, and by antibi-otic-resistant strains of enterococci. The tasks of public health officials are to keep track of all cases of these diseases and to educate medical personnel about them.

 

Anthrax is no longer in the news, as it was in the autumn of 2001 when spores were sent through the U.S. mail. As of 2006, no other instances of the use of biological weapons have been reported, though use of the smallpox virus as a weapon remains a concern. Public health officials had hoped that improvements to public health policies and procedures that were spurred by the use of anthrax as a weapon would not be wasted but would be applicable to the emerging diseases as well. Unfortunately, that has not proven true, because as of 2006 most nations were not at all prepared for a possible pandemic of avian flu.

Avian influenza A (H5N1) is truly an emerging dis-ease. The virus is a natural parasite of wild birds and has infected domestic flocks in Asia, Europe, and Africa. Although not a frequent event, people have caught the virus from infected chickens or ducks, and the human mortality rate has been 50% (though with a relatively small number of cases, fewer than 500, this may not be accurate). The great concern is that the virus will mutate and become contagious for peo-ple, that is, easily transmissible from person to person. By way of plane travel, the virus could be spread throughout the world in a matter of weeks. The avian flu virus H5N1 may never mutate into a “people” virus, but we have to be prepared for just that. As of 2006 we have no vaccine for this virus (some are in the testing stages) and only one antiviral medication that is even somewhat effective. Because this would be a “new” disease for people, it is possible that no group of people will have evolved any kind of resistance to it.

Bird flu may be for us what measles was to the Hawaiian islanders in the 18th and 19th centuries. The measles virus was brought to the islands by Europeans (for whom it was usually not fatal, reflecting thousands of years of coexistence) and nearly wiped out the native population, for whom it was a new virus.

 

However, this is by no means certain. It may be that milder cases of human avian flu have been overlooked thus far, that H5N1 causes a wide spectrum of disease in people, from mild to fatal. It may even be possible that some human cases are asymptomatic (antibody studies would be needed to determine that). We have much to learn.

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