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Control of Microbial Growth
Chapter 5
5.1 Approaches to Control
Physical methods
Heat
Irradiation
Filtration
Mechanical (e.g., washing)
Chemical methods
Disinfectants
Antibiotics
Principles of Control
Sterilization
Disinfectants
Germicides
Antiseptics (topical)
Pasteurization
Decontamination
Sanitation
Preservation
Situational Considerations
Daily life
Hospitals
Microbiology laboratories
Food production facilities
Water treatment facilities
Others
5.2 Selection of an Antimicrobial
Procedure
Type of microorganism
Bacterial endospores and
Mycobacterium species are
heat and chemical resistant
Vegetative cells are
susceptible to most
disinfectants
Number of microbes
initially present
“99.9% effective” - still a lot of
live bacteria!
Environmental conditions
(dirt, grease, etc.)
Potential risk of infection
Critical instruments - sharps
Semicritical instruments - mucous
membranes
Noncritical instruments - intact skin
Composition of the item
Metals are damaged by some disinfectants
Plastics are damaged by irradiation
5.3 Using Heat to Destroy
Microorganisms and Viruses
Moist heat
Boiling kills most bacteria and viruses
Pasteurization is effective for many food-borne
microbes
Heat to 72° C for 15 sec for most liquids
Other objects can withstand higher temps and
durations
Autoclave
High pressure increases boiling temp of water
Typically reach 120° C at 15 psi
Moist heat (cont.)
Commercial canning process
Most serious threat is Clostridium
botulinum endospores
Canning facilities use retorts, which
are large autoclaves
Prevents spores from germinating
inside the can, thus prevents
botulism toxin production
Dry heat
Heating items to 200° C or more
5.4 Using Other Physical Methods to
Remove or Destroy Microbes
Filtration of fluids
Nylon membranes of defined
pour sizes
0.45 µm
0.20 µm
Filters are sterilized by
irradiation
Filter out bacteria
Will not filter viruses or
prions
Filtration of air
High-efficiency particulate air filters
(HEPA)
Used in a variety of settings
Room ventilation
Laminar flow hoods
Powered Air-Purifying Respirator
(PAPR)
Radiation
Ionizing
Ultraviolet
Microwaves
High pressure (up to 130,000 psi)
5.5 Using Chemicals to Destroy
Microorganisms and Viruses
Potency of germicidal chemical formulations
Sterilants - everything
High-level disinfectants - viruses and all
vegetative bacteria
Intermediate-level disinfectants - all
vegetative bacteria and most viruses
Low-level disinfectants - all vegetative
bacteria and enveloped (membrane-bound)
viruses
Selecting the appropriate germicidal
chemical
Toxicity
Activity in the presence of organic matter
Residue
Cost and availability
Storage and stability
Environmental risk
Classes of germicidal chemicals
Alcohols - denature proteins, solubilize
membranes
Aldehydes - chemically modify proteins
and nucleic acids
Glutaraldehyde
Formaldehyde
Orthophthaladehyde
Classes of germicidal chemicals (cont.)
Biguanides adhere to skin and mucous
membranes
Chlorhexidines
Surgical scrubbing
Mouthwash
Skin abrasions
Ethylene oxide
Highly oxidative
Used for surgical garbs
Classes of germicidal chemicals (cont.)
Halogens
React with organic molecules to form
toxic substances
Chlorine
Iodine
Ozone
Oxidizing agent
Drinking water and wastewater
Classes of germicidal chemicals (cont.)
Peroxygens
Oxidizing agents
Hydrogen peroxide (H2O2)
Peracetic acid (CH3C[=O]OOH)
Phenolic compounds
Dissolve membranes
Denature proteins
Quaternary ammonia compounds
Cationic (+ charge) detergents
Bind to negatively-charged membrane
lipids
5.6 Preservation of Perishable Products
Chemical preservatives
Benzoic and sorbic acids - breads,
cheeses, juices
Nitrates - inhibit spore germination
Low temperature - reduces biochemical
activities of microbes
Reduce availability of water - salting/drying