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7 Control of Microorganisms by Physical and Chemical Agents

 

CHAPTER OVERVIEW

This chapter focuses on the control and the destruction of microorganisms by physical and chemical agents. This is a topic of great importance, because microorganisms may have deleterious effects, such as food spoilage and disease. It is therefore essential to be able to kill or remove microorganisms from certain environments in order to minimize their harmful effects.

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o compare and contrast the processes of disinfection, sanitization, antisepsis, and sterilization

o compare the difficulties encountered when trying to kill endospores with those encountered when trying to kill vegetative cells

o discuss the exponential pattern of microbial death

o discuss the influence of environmental factors on the effectiveness of various agents used to control microbial populations

o discuss the uses and limitations of various physical and chemical agents used to control microbial populations

o describe the procedures used to evaluate the effectiveness of various antimicrobial agents

 

CHAPTER OUTLINE

I. Definition of Frequently Used Terms

A. Sterilization-destruction or removal of all viable organisms from an object or from a particular environment

B. Disinfection-killing, inhibition, or removal of pathogenic microorganisms (usually on inanimate objects)

C. Antisepsis-prevention of infection of living tissue by microorganisms

D. Sanitization-reduction of the microbial population to a safe level as determined by public health standards

E. -cide-a suffix indicating that the agent will kill the kind of organism in question (e.g., viricide, fungicide)

F. -static-a suffix indicating that the agent will prevent the growth of the type of organism in question (e.g., bacteriostatic, fungistatic)

II. The Pattern of Microbial Death-microorganisms are not killed instantly when exposed to a lethal agent; rather, the population decreases by a constant fraction at constant intervals (exponential killing); a microorganism is considered dead when it is unable to grow in conditions that would normally support its growth

III. Conditions Influencing the Effectiveness of Antimicrobial Agent Activity

A. Population size-larger populations take longer to kill than smaller populations

B. Population composition-microorganisms differ markedly in their sensitivity to various agents

C. Concentration or intensity of the antimicrobial agent-higher concentrations or intensities are generally more efficient, but the relationship is not linear

D. Duration of exposure-the longer the exposure, the greater the number of organisms killed

E. Temperature-a higher temperature will usually (but not always) increase the effectiveness of killing

F. Local environment-environmental factors, such as pH, viscosity, and concentration of organic matter can profoundly influence the effectiveness of a particular antimicrobial agent

IV. The Use of Physical Methods in Control

A. Heat

1. Moist heat

a. Boiling water is effective against vegetative cells and eucaryotic spores

b. Autoclaving (steam under pressure) is effective against vegetative cells and most bacterial endospores

c. Pasteurization, a process involving brief exposure to temperatures below the boiling point of water, reduces the total microbial population and thereby increases the shelf life of the treated material; it is often used for heat-sensitive materials that cannot withstand prolonged exposure to high temperatures

1) Low-temperature long-term (LTLT) pasteurization-63°C for 30 min

2) High-temperature short-term (HTST) flash pasteurization-72°C for 15 sec

3) Ultrahigh temperature (UHT) pasteurization-140 to 150°C for 1 to 3 sec

d. Tyndallization (fractional steam sterilization), a process that kills spore-forming microorganisms, involves exposing the material to elevated temperatures (killing the vegetative cells), then incubation at 37°C (to allow spores to germinate to form new vegetative cells), and then exposure to elevated temperatures again (to kill the newly germinated vegetative cells); it is used to sterilize heat-sensitive materials or to kill particularly resistant endospores

2. Dry heat can be used to sterilize moisture-sensitive materials such as powders, oils, and similar items; it is less efficient than moist heat because it usually requires higher temperatures (160 to 170°C) and longer exposure times (2 to 3 hrs)

3. The thermal death time (TDT) is the shortest time necessary to kill all microorganisms in a suspension at a specific temperature and under defined conditions

4. The decimal reduction time (D, or D value) is the time required to kill 90% of the microorganisms or spores in a sample at a specific temperature

5. The Z value is the increase in temperature required to reduce D to 1/10 of its previous value

6. The F value is the time in minutes at a specific temperature (usually 250°F or 121.1°C) necessary to kill a population of cells or spores

B. Filtration sterilizes heat-sensitive liquids and gases by removing microorganisms rather than destroying them

1. Depth filters are thick fibrous or granular filters that remove microorganisms by physical screening, entrapment, and/or adsorption

2. Membrane filters are thin filters with defined pore sizes that remove microorganisms, primarily by physical screening

3. High-efficiency particulate air (HEPA) filters are used in laminar flow biological safety cabinets to sterilize the air circulating in the enclosure

C. Radiation

1. Ultraviolet (UV) radiation is effective, but its use is limited to surface sterilization because UV radiation does not penetrate glass, dirt films, water, and other substances

2. Ionizing radiation (X rays, gamma rays, etc.) is effective and penetrates the material; the Food and Drug Administration and the World Health Organization have approved food irradiation and declared it safe; however, it is not widely used because of cost and concerns about the effects of the radiation on food

V. The Use of Chemical Agents in Control

A. Phenolics-laboratory and hospital disinfectants; act by denaturing proteins

B. Alcohols-widely used disinfectants and antiseptics; will not kill endospores; act by denaturing proteins and possibly by dissolving membrane lipids

C. Halogens-widely used antiseptics and disinfectants; iodine acts by oxidizing cell constituents and iodinating cell proteins; chlorine acts primarily by oxidizing cell constituents

D. Heavy metals-effective but usually toxic; act by combining with proteins and inactivating them

E. Quaternary ammonium compounds-cationic detergents used as disinfectants for food utensils and small instruments, and because of low toxicity, as antiseptics for skin; act by disrupting biological membranes and possibly by denaturing proteins

F. Aldehydes-reactive molecules that can be used as chemical sterilants; may irritate the skin; act by combining with proteins and inactivating them

G. Sterilizing gases (e.g., ethylene oxide, betapropiolactone)-can be used to sterilize heat-sensitive materials such as plastic petri dishes and disposable syringes; act by combining with proteins and inactivating them

H. Recently, vapor-phase hydrogen peroxide has been used to decontaminate biological safety cabinets

VI. Evaluation of Antimicrobial Agent Effectiveness-agents' effectiveness and safety must be tested under a variety of conditions, including conditions approximating those of normal use

 

 

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