Chapter 7 Outline
Control of Microorganisms: Physical and Chemical Methods
Introduction
7.1 General Principles of Microbial Control
• Sterilization and Sanitation Are Key to Good Public Health
• Sterilization involves destruction of all living microbes, spores, and
viruses
• Sterile objects become contaminated when exposed to air and
surroundings
• Sanitization reduces the numbers of pathogens or discourages their growth
7.2 Physical Methods of Control
• Heat Is One of the Most Common Physical Control Methods
• At temperatures above the growth range:
• proteins and nucleic acids are destroyed
• water is removed
• Each microbial species has a thermal death time for a given temperature
• Each species has a thermal death point:
• the minimum temperature at which it dies in a given time period
• Incineration uses direct flame to kill microbes
• Dry and Moist Heat Are Applied Differently
• Dry heat requires long periods of exposure to high temperatures
• The heat changes microbial proteins and removes water, slowly
burning the microbes
• Moist heat (like boiling water) is faster and effective at a lower
temperature than dry heat
• It kills microbes by denaturing their proteins
• Boiling water may not kill all spores or inactivate all viruses
• Pressurized steam is used in an autoclave to sterilize a variety of objects
• The prevacuum autoclave decreases
• cycle time
• exposure of sensitive materials to steam
• Fractional sterilization is used for sterilization if materials are not suited to the
autoclave
• Pasteurization reduces bacterial populations in food and drink
• This reduces the chances of spoilage and disease
• Bacterial spores are not affected by pasteurization
• Filtration Traps Microorganisms
• As fluid passes through a filter, organisms above a certain size threshold
are trapped in the pores
• Air can be filtered using a high-efficiency particulate air (HEPA) filter
• Ultraviolet Light Can Be Used to Control Microbial Growth
• Exposure to UV light causes thymine molecules to link together
• Errors in protein synthesis and impairment of chromosome
replication occur
• Other Types of Radiation Also Can Sterilize Materials
•
X rays and gamma rays (ionizing radiations) force electrons out of
microbial molecules
• This affects cell metabolism and physiology
• Radiation is used to control microbes in food
• Preservation Methods Retard Spoilage by Microorganisms in Foods
• Drying removes the water necessary for microbes to live
• Salting causes water to diffuse out of organisms, causing dehydration and
death
• Low temperatures lower microbial metabolic and growth rates, retarding
spoilage
7.3 General Principles of Chemical Control
• Medicinal Chemicals Came into Widespread Use in the 1800s
• Chemical agents rarely achieve sterilization
• But they do disinfect (destroy pathogens)
• Antiseptics are used to destroy pathogens on living tissue
• Sepsis refers to contamination by microorganisms
• Sanitizing means reducing microbial population to a safe level
• Degerming means removing organisms from an object’s surface
• Antiseptics and Disinfectants have Distinctive Properties
• They should be
• Able to kill or slow growth of microbes
• Nontoxic to humans and animals
• Soluble in water
• Storable
• Effective quickly and at low concentration
• Also important in choosing
an agent are
• Temperature
• pH
• Duration of disinfection
• Antiseptics and Disinfectants Can Be Evaluated for Effectiveness
• The phenol coefficient (PC) indicates disinfecting ability compared to that
of phenol
• An in-use test to compare samples of substrate before and after
disinfection
7.4 Chemical Methods of Control
• Halogens Oxidize Proteins
• Chlorine keeps bacterial populations low in municipal water supplies
and swimming pools
• Iodine is used to disinfect
wounds, water, and
restaurant equipment
• Iodophores release
iodine over a long
period of time
• Phenol and Phenolic Compounds Denature Proteins
•
•
•
•
•
•
Phenol
• is expensive
• is caustic
• has a pungent odor
• Derivatives have
• greater germicidal activity
• lower toxicity than phenol
• Bisphenols are combinations of two phenol molecules
• They are commonly used in disinfection and antisepsis
Heavy Metals Interfere with Microbial Metabolism
• Mercury, copper, and silver are reactive with proteins
• They disrupt cellular metabolism, killing microbes
Alcohols Denature Proteins and Disrupt Membranes
• Ethanol is effective against vegetative cells but not spores
• Membrane disruption is caused by lipid dissolution
Soaps and Detergents Act as Surface-Active Agents
• Soaps remove microbes by emulsifying and solubilizing particles on the
skin
• Detergents are surfactants
• They also cause cytoplasm leakage from microbial membranes
• Quaternary ammonium compounds react with cell membranes and
destroy some bacteria and viruses
Hydrogen Peroxide Damages Cellular Components
• Catalase in wounded tissue transforms it to oxygen and water
• The oxygen is effective against anaerobic bacteria
Some Chemical Agents Can Be Used for Sterilization
• Aldehydes cause cross-linking that inactivates proteins and nucleic acids
• Ethylene oxide is effective but carcinogenic and explosive
• Chlorine dioxide is a non-toxic and non-carcinogenic agent
• It was used to decontaminate anthrax-exposed mail in 2001