Bringing Sanitary Conditions to Hospitals

During the Crimean War in the 1850s, poor sanitary conditions led to the rapid spread of typhus. More soldiers were dying of disease than as a direct result of warfare. Florence Nightingale was called on to help in the military hospitals. She attempted to create sanitary conditions, believing in the healing power of pure air. Her reforms in hospital sanitation markedly improved the survival rate of patients and limited the further spread of disease. Although Nightingale had only contempt for the germ theory of disease, the sanitary conditions she fostered worked because she was stemming the spread of disease-causing mi­croorganisms.

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312 CHAPTER 11 CONTROL OF MICROBIAL GROWTH AND DEATH

isms, many items that are free of microorganisms are kept sterile by wrapping them in paper, plastic, or foil. Such wrappings are impermeable to microor­ganisms and prevent recontamination. For example, foods are packaged in cans, jars, or other containers that prevent contamination with microorganisms that could contaminate the foods, causing them to spoil and rendering them a threat to human health. Similarly, syringes, needles, scalpels, and other med­ical instruments are sealed in sterile packages, as are Petri dishes, pipettes, and many other items used in the microbiology laboratory. Surgical gloves and gowns help prevent microorganisms from passing from one person to another, especially during surgi­cal procedures or where a hospital worker comes in contact with a patient that has a contagious disease.

High Temperatures

Temperature is one of the most important environ­mental factors affecting the rates of microbial growth and death. Temperature influences the rates of chem­ical reactions by altering the three-dimensional shapes of proteins, thereby affecting the rates of en­zymatic activities. Heat can kill microorganisms by denaturing their enzymes. Consequently, high tem­peratures can be used to kill microorganisms in order to control their proliferation. At temperatures ex­ceeding the maximal growth temperature, the death rate exceeds the growth rate. The higher the temper­ature above the maximal growth temperature, the higher the death rate for that microorganism.

Temperature influences the rates of chemical reac­tions by affecting the three-dimensional configura­tion of proteins and thus enzymatic activity; high temperatures kill microorganisms by denaturing their enzymes.

High temperatures can be used to reduce the numbers of microorganisms or to eliminate all viable microorganisms. When using heat to kill microorganisms, the degree of the microorganism's heat resis­tance must be considered so that the correct exposure times and temperatures are used. Heat resistance varies among different microorganisms; these differences can be expressed through the concept of thermal death point. Thermal death point (TDP) is the lowest temperature required to kill all of the microorganisms in a liquid suspension in 10 minutes. Heat is the most widely applicable and effective agent for killing microorganisms and also the most economical and easily controlled.

The heat killing of microorganisms can be described by the decimal reduction time (D value) (FIG. 11-1). D is defined as the time required for a tenfold reduction in the number of viable cells at a given

FIG. 11-1 The D value is the time in minutes needed to duce the number of viable microorganisms by a factor ten (one log unit).

temperature, that is, the time required for a log г duction in the number of microorganisms. This is time required to bring about a 90% reduction in numbers of viable microorganisms. As the temper, ture is increased above the maximal growth tempe ature for a microorganism, the decimal reductio time is shortened. The decimal reduction time varu for different microorganisms. In the food industr the decimal reduction time is important in establising appropriate processing times for sterilizing food products.

The D value describes the rate of death at a given temperature.

Pasteurization

Pasteurization is a process that uses relatively brie exposures to moderately high temperatures to reduce the numbers of viable microorganisms and tieliminate human pathogens (FIG. 11-2). Such procedures prolong shelf life and ensure safety of the foocas long as human pathogens are eliminated. A pateurized food, however, retains viable microorganisms, which means that additional preservation methods are needed to extend the shelf life of the product. These other preservation methods, such a refrigeration, are used to reduce the growth rates a the surviving microorganisms. Pasteurization ol milk, for example, is required by law in the U.S. ti eliminate pathogenic bacteria, namely Brucella sp Coxiella burnetii, and Mycobacterium tuberculosa. These bacteria are associated with the transmission of disease via contaminated milk. They are relativeli sensitive to elevated temperatures, and pasteurizaj

CONTROL OF MICROORGANISMS BY PHYSICAL ENVIRONMENTAL FACTORS 313

FIG. 11-2 Pasteurization reduces the numbers of viable microorganisms and eliminates human pathogens. The same reduction of viable microorganisms is achieved by heating to 63° С for 30 minutes as is accomplished by ex­posure to 71.5° С for only 0.33 minutes. The long time hold (LTH) pasteurization process uses 63° С and the high tem­perature-short time (HTST) pasteurization process uses 71.5° С

tion is, therefore, normally achieved by exposure of milk to 62.8° С for 30 minutes (low temperature-hold or LTH process) or 71.7° С for 15 seconds (high tem­perature-short time or HTST process). Milk pro­duced in the United States is normally preserved by pasteurization and requires refrigeration to extend its shelf life.

Pasteurization uses brief exposures to moderately high temperatures to reduce numbers of viable mi­croorganisms and eliminate human pathogens but does not eliminate all viable microorganisms.

Sterilization

Sterilization is the complete elimination of living or­ganisms. In many microbiological procedures, high temperatures are used to kill all viable microorgan­isms, that is, to sterilize materials. As long as there are no endospore-forming bacteria, boiling at 100° С for 10 minutes is adequate to eliminate microorganisms from water. When the potential for the contamination of water supplies with enteric pathogens exists, boiling ensures the bacteriological safety of the water. In many cases, higher temperatures are needed to ensure that all microorganisms, including endospore producers, are killed.

Sterilization eliminates all living organisms.

Excessive heat destroys the quality of milk, for ex­ample, boiling alters the smell and taste of milk, but brief exposure to high temperatures kills all the mi-

croorganisms in milk without destroying its quality. In the ultra high temperature or UHT process, exposure to 141° С for 2 seconds is used to sterilize milk. Sterilized milk is currently marketed in several European countries and has been introduced in the United States. It has an indefinite shelf life, provided the container remains sealed.

The heat used in sterilization can be moist or dry heat. Dry heat kills by oxidation. This is, for example, what happens when paper slowly chars in a heated oven when the temperature is below the ignition point of paper. Moist heat kills microorganisms more quickly because the water hastens the breaking of hydrogen bonds that hold proteins in their three-dimensional structure.

Dry heat sterilization requires higher tempera­tures for much longer exposure periods to kill all the microorganisms in a sample. Exposure in an oven for 2 hours at 170° С (328° F) is generally used for the dry heat sterilization of glassware and other items. The longer period of time and higher temperature (relative to moist heat) are required because heat in water is more readily transferred to a cool body than heat in air. Heat sterilization is very important in medical microbiology where sterile instruments are required for surgical procedures and where contaminated materials must be sterilized before they can be reused or safely discarded.

We routinely sterilize transfer loops by flaming them red hot before aseptically transferring a culture from one site to another. In this case we use a very high temperature for a short time. A similar principle is used in incineration, an effective way to sterilize and dispose of contaminated paper cups, bags, and dressings. Medical wastes are safely disposed by using incineration to kill any viable microorganisms.

Dry heat sterilization uses higher temperatures and often takes longer than moist heat sterilization.

Moist heat is far more penetrating than dry heat and, hence, more effective for killing microorganisms. Steam under pressure is frequently used in sterilization procedures. Steam at 121° С for 15 minutes is at least as effective as a dry oven treatment at 170° С for 2 hours and kills all microorganisms, including endospores (FIG. 11-3). Culture media in bacteriological laboratories are normally prepared by heat sterilization in an autoclave, an instrument that permits exposure to steam under pressure. The autoclave is basically a chamber that can withstand pressures of greater than two atmospheres. The materials to be sterilized are placed in a chamber, and the chamber is sealed. Steam then is transferred from a jacket into the chamber, forcing out all the air. The steam is held in the chamber for the necessary time and then vented from the chamber. In the normal heat steril-