Methods for Control of Microbial
Growth
Controlling Microorganisms
• Physical, chemical, and mechanical methods
can be used to destroy or reduce undesirable
microbes in a given area
• Primary targets are microorganisms capable
of causing infection or spoilage:
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vegetative bacterial cells and endospores
fungal hyphae and spores, yeasts
protozoan trophozoites and cysts
worms
viruses
Hierarchy of Resistance
• Most resistant = bacterial endospores
• Moderately resistant = cysts, fungal
zygospores, naked viruses
• Least resistant = vegetative bacterial
cells
Types of Control
• Sterilization – a process that destroys all viable
microbes, including viruses and endospores;
microbiocidal
• Disinfection – a process to destroy vegetative
pathogens, not endospores; for inanimate objects
• Antisepsis – disinfectants applied directly to
exposed body surfaces
• Sanitization – any cleansing technique that
mechanically removes microbes
• Degermation – reduces the number of microbes
A Number of Factors Influence the
Effectiveness of Control Agents
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Number of microbes
Nature of microbes in the population
Temperature and pH of environment
Concentration or dosage of agent
Mode of action of the agent
Presence of solvents, organic matter,
or inhibitors
Microbiocidals Cause Microbial
Death
Stasis Agents Slow or Retard
Growth, but Do Not Kill Microbes
Microbial Death
The Permanent Loss of
Reproductive Capability Under
Optimal Growth Conditions
Cellular Targets of Control
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Cell wall
Cell membrane
Nucleic Acids (DNA, RNA)
Proteins
Physical Control Practices
Heat-Based Microbial Control
Processes
• Incineration/ baking achieve sterility
• Autoclaving: sterilization with live
steam and pressure, very widely used
• Pasteurization: flash heat treatment
(63°C - 66°C for 30 minutes) that
reduces the bio-burden of food
materials (kills Salmonella and Listeria)
• Boiling disinfection – does not achieve
sterility
Radiation Sterilization
• Ionizing radiation (X-rays, gamma
particles) penetrates and damages DNA
and other vital cell components
• Non-ionizing radiation (UV light) causes
adjacent T-T pairs in DNA to fuse
• UV has limited penetration; use for
surfaces and films of liquids
Ultra-filtration
• Mechanically excludes organisms from
a liquid
• Membranes have a specific pore size;
any particle larger cannot pass through
• Achieves sterilization
Filtration
Targets of Chemical Agents
• Cell Membrane - detergents
• Key Proteins – denaturing and crosslinking agents
• Nucleic Acids – alkylating and crosslinking agents
Halogen Antimicrobials Denature Proteins
• Chlorine compounds (gaseous Cl,
bleach, chloramine); can be sporicidal
• Iodine (tincture, Betadine)
• Halogens can react with any organic
matter – a surface should be clean
before applying them!
Phenolics- Disrupt Cell
Membranes & Precipitate Proteins
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Phenol
Lysol
PhisoHex (not any more)
Benzalkonium chloride
Triclosan
Alcohols: Dissolve Membranes
and Coagulate Proteins
• Ethanol (70%)
• Isopropanol
• Act as surfactants dissolving
membrane lipids and coagulating
proteins of vegetative bacterial cells
and fungi
• Cannot destroy spores at room
temperature
Hydrogen Peroxide –
Attacks DNA and Proteins
3% solution is effective as
wound antiseptic, but is
potentially damaging to tissues
Heavy Metals
Mercury, Silver Salts Kill Vegetative
Cells Present in low Numbers by
Inactivating Proteins
Aldehydes – Cross-link DNA and
Proteins
• Formaldehyde – formalin
• Glutaraldehyde - Cidex
• A soak of dental or surgical
instruments in glutaraldehyde Does Not
guarantee sterility!
Gases and Aerosols
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Ethylene oxide, propylene oxide
Strong alkylating agents
High level
Sterilize and disinfect plastics and
prepackaged devices, foods