Chapter 11

Control of Microbial Growth and Death

Chapter Outline

Preview to Chapter 11

Control of Microorganisms by Physical Environmental Factors 311

Physical Exclusion or Removal of Microorganisms Newsbreak: Bringing Sanitary Conditions to Hospitals High Temperatures

Pasteurization

Sterilization Methodology: Sterility Testing

Canning Historical Perspective: Development of Canning

Low Temperatures Newsbreak: Refrigeration Fails to Protect Against

Yersinia enterocolitica Removal of Water—Desiccation Newsbreak: Salem Witch Hunts Radiation

Highlight: Safety of Irradiated Food Control of Microbial Growth by Antimicrobial Agents 320 Historical Perspective: Discovery of Antiseptics Food Preservatives

Salt and Sugar

Acids

Nitrates and Nitrites Disinfectants and Antiseptics

Halogens—Chlorine, Bromine, and Iodine Methodology: Evaluation of the Effectiveness of Disinfectants

Phenolics

Detergents

Alcohols

Aldehydes

Acids

Ethylene Oxide

Hydrogen Peroxide

Ozone

Dyes

Heavy Metals Antibiotics

310

In this chapter we will:

• Learn that microbial populations can be controlled b limiting growth or increasing death rates.

• Examine the factors that control rates of microbial growth and death.

• See how physical environmental conditions can be modified to control microbial populations.

• Study the chemical approaches for killing or prevent microbial growth.

• Review various types of chemicals used to control microorganisms, including pathogens.

• Learn the following key terms and names:

algicides high temperature-shorl

antibiotics time or HTST proce

incineration infrared radiation low temperature-hold

(LTH process) ozonation pasteurization preservative quaternary ammonium

compounds (quats) sanitizer shelf life sporicidal sterilization thermal death point (П ultra high temperature

process (UHT proce virucides

antimicrobial agents

antiseptics

autoclave

bactericides

bacteriostatic

chloramination

decimal reduction time (D

value) desiccation disinfectant dry heat sterilization ethylene oxide

sterilization fungicides fungistatic germicides high efficiency particulate

air filters (HEPA

filters)

Control of microorgani environmental factors

Rates of microbial growth and death are greatly in­fluenced by several environmental factors. Some en­vironmental conditions favor rapid microbial repro­duction; others preclude microbial growth or even result in microbial death. Each microorganism has a certain tolerance range for specific environmental pa­rameters. Outside the range of environmental condi­tions under which a given microorganism can repro­duce, it may either survive in a relatively dormant state or may lose viability. Loss of viability means that it will lose the ability to reproduce and conse­quently die.

By adjusting environmental conditions, one can increase the death rate of microorganisms. This is an important consideration when trying to kill microor­ganisms. The ability to kill microorganisms is very important in many instances, such as when trying to reduce the numbers of microorganisms in foods so that they do not spoil, and when it is necessary to to­tally eliminate microorganisms from pharmaceuti­cals and medical instruments to make them sterile (free of living organisms) and safe for use with pa­tients. Microbial populations also may be physically removed or excluded so as to limit the numbers of microorganisms that can multiply.

Microbial populations can be controlled by modify­ing environmental conditions.

It is also possible to alter environmental condi­tions so that microorganisms do not die but also do not reproduce. This method is used for the preserva­tion of microorganisms, such as in culture collections and food preservation, and for preventing spoilage. Many times the conditions needed to heat sterilize a product alter the texture and color of the desired product. It is for this reason that we use freezing to preserve many foods whose taste and textural quali­fies are destroyed if sterilized at high temperatures.

There are many factors that determine the effec­tiveness of a particular agent in controlling microor­ganisms. These factors include the type of microor­ganism, the amount and type of material to be treated, the duration of the treatment, the concentra­tion or intensity of the agent, and environmental fac­tors such as pH, temperature, and water availability.

Physical Exclusion or Removal of Microorganisms

I An effective method for controlling microorganisms is by physically excluding them. Filtration can be used to remove microorganisms from liquids and I gases. Generally, filtration is accomplished by pas-

Ms by physical

sage of the substance through a filter with 0.2 to 0.45 |xm diameter pores. Many pharmaceuticals, such as solutions that are administered intravenously to pa­tients, are sterilized by passage through such filters. Bacteria and other living organisms are eliminated from the solution by trapping them on the filter, but viruses and some very small bacteria may pass through the filter.

Microorganisms can also be removed from air by passage through high efficiency particulate air fil­ters (HEPA filters), which remove particulate mater­ial larger than about 0.3 |xm. Clean rooms, such as operating theaters and rooms where drugs are pack­aged, often employ HEPA filters. Many microbiology laboratories also have laminar flow hoods in which air that is filtered through a HEPA filter is blown across the work area to prevent contamination dur­ing culturing of microorganisms. While not nearly as effective as a HEPA filter, wearing a face mask helps decrease the exchange of microorganisms between people. Surgical staff wear face masks to prevent ex­haling microorganisms into the open surgical wound. Staff and visitors wear masks when they are with patients who have infections that may be trans­mitted through the air. These precautions generally are adequate. However, greater precaution may be warranted for contact with patients with tuberculo­sis, and it is now required in many situations that a respirator with a HEPA filter be worn rather than a simple surgical mask.

Regardless of whether filtration or other methods, discussed below, are used to eliminate microorgan-