Control of Microbial Growth

1
Control of Microbial Growth

• Chapter 5

2

• Why do we need to control bacteria growth? To
  what degree?
• How do we control bacteria growth?

3

• Daily life
• Hospital
• Microbiology lab
• Food facilities
• Water facilities

4
Approaches to Control

• Control mechanisms either physical or chemical
  (or both)
• Physical methods
• Heat
• Irradiation
• Filtration
• Mechanical removal
• Chemical methods
• Use a variety of antimicrobial chemicals
• Chemical depends on circumstances and degree of
  control required

5
Approaches to Control

• Degree of control
• Sterilization
• Disinfection
• Pasteurization

6
Selection of Antimicrobial Procedure

• Choice of procedure depends on numerous factors

Situational considerations Type of microbe Extent
of contamination Number of organisms Environment R
isk of infection Composition of infected item
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Selection of Antimicrobial Procedure

• Type of microorganism
• resistant microbes
• Endospores
• Protozoan cysts and oocysts
• Mycobacterium species
• Pseudomonas species
• Naked viruses

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Selection of Antimicrobial Procedure

• Number of organisms initially present
• Time it takes to kill depends on population size
• decimal reduction time
• A.k.a D value
• Washing effect

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Quiz

• If the D value is 3 min, how long does it take to
  reduce 10,000 to 1?

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Selection of Antimicrobial Procedure

• Environmental conditions
• Environmental conditions strongly influence
  effectiveness
• pH, temperature and presence of organic materials
  affect effectiveness

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Selection of Antimicrobial Procedure

• Potential risk of infection
• Medical items categorized according to potential
  risk of disease transmission
• Critical items
• Semicritical instruments
• Non-critical instruments

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Selection of Antimicrobial Procedure

• Composition of the item
• Some sterilization and disinfection methods
  inappropriate for certain items
• Heat inappropriate for plastics and other heat
  sensitive items

13
Physical control--Heat

• Heat treatment most useful for microbial control
• Heat can be used to sterilize or disinfect
• Methods include
• Moist heat
• Dry heat

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Heat as Control

• Moist heat
• mechanism
• Moist heat includes
• Boiling
• Pasteurization
• Pressurized steam

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Heat as Control

• Boiling (100 C)
• Destroys most microorganisms and viruses
• Not effective means of sterilization
• Pasteurization
• Pasteur developed to avoid spoilage of wine
• significantly reduces organisms
• increase shelf life of food
• Heated to 72C and held for 15 seconds
• Other protocol UHT
• Heated to 140C - 150C, held for several seconds
  then rapidly cooled

16
Heat as Control

• Pressurized steam
• Autoclave used to sterilize using pressurized
  steam
• Achieves sterilization at 121C and 15psi in 15
  minutes
• Prions destroyed at 132C and 15psi for 4.5 hours

17
Heat as Control

• Dry heat
• Not as effective as moist heat
• 200C for 1.5 hours vs. 121C for 15 minutes
• Incineration method of dry heat sterilization
• Flaming laboratory inoculation loop incinerates
  organism

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Control of bacteria growth---physical method

• Low temperature storage
• Microbial growth is temperature dependent
• Freezing means of food preservation
• Essentially stops microbial growth
• Irreversibly damages cell

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Other Physical Methods of Control

• Heat sensitive materials require other methods of
  microbial control
• Filtration
• Irradiation
• High-pressure treatment

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Other Physical Methods of Control

• Filtration
• Membrane filtration used to remove microbes from
  fluids and air
• Liquid filtration
• Heath sensitive
• Filtration of air
• High efficiency particulate air (HEPA) filter
  remove nearly all microbes from air

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Other Physical Methods of Control

• Radiation
• Electromagnetic radiation
• Energy released from waves
• Range of wavelength is electromagnetic spectrum
• Radiation can be ionizing or non-ionizing

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Other Physical Methods of Control

• Ionizing radiation
• Radiation able to strip electrons from atoms
• Three sources
• Gamma radiation
• X-rays
• Electron accelerators
• mechanism
• Used to sterilize heat resistant materials
• Medical equipment, surgical supplies, medications
• Some endospores can be resistant

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Other Physical Methods of Control

• Ultraviolet radiation
• Non-ionizing radiation
• Damages DNA
• Used to destroy microbes in air, drinking water
  and surfaces
• Limitation
• Poor penetrating power

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Chemicals as Control

• Chemicals can be used to disinfect and sterilize
• Called germicidal chemicals
• Reacts with vital cell sites
• Proteins
• DNA
• Cell membrane

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Chemicals as Control

• Potency of chemicals
• Formulations generally contain more than one
  antimicrobial agent
• Regulated by
• FDA
• Antiseptics
• EPA
• Disinfectants
• Germicidal agents grouped according to potency

• Sterilants
• Destroy all microorganisms
• High-level disinfectants
• Destroys viruses and vegetative cells,
• Not endospores
• Intermediate-level disinfectants
• Kills vegetative cells fungi, most viruses,
• Not endospores
• Low-level disinfectants
• Removes fungi, vegetative bacteria and enveloped
  viruses
• Not mycobacteria, naked viruses or endospores

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Chemicals as Control

• Selecting appropriate chemical
• Points to consider
• Toxicity
• Benefits must be weighed against risk of use
• Activity in presence of organic material
• Compatibility with material being treated
• Residue
• Cost and availability
• Storage and stability
• Environmental risk

27
Chemicals as Control

• Classes of chemicals
• Germicides represent a number or chemical
  families
• Alcohols
• Aldehydes
• Biguanides
• Ethylene oxide
• Halogens
• Metals
• Ozone
• Peroxides
• Phenolics
• Quaternary ammonium compounds

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Chemicals as Control

• Alcohols
• Solutions of 60 - 80 isopropyl or ethyl alcohol
  kill vegetative bacteria and fungi
• Not effective against endospores and some naked
  viruses
• Mechanism
• Coagulation of proteins and essential enzymes
• Damage to lipid membranes
• Commonly used as antiseptic and disinfectant
• Limitations
• Evaporates quickly limiting contact time
• May damage material such as rubber and some
  plastics

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Chemicals as Control

• Aldehydes
• Mechanism
• 2 glutaraldehyde solution most widely used
  liquid sterilant
• Formalin used to kill bacteria and inactivate
  viruses
• A solution made from formaldehyde

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Chemicals as Control

• Hydrogen peroxide
• Powerful oxidizing agents
• biodegradable
• Less toxic
• Effectiveness depends on surface being treated
• More effective on inanimate object
• Useful as disinfectant
• Leaves no residue
• Doesnt damage most materials
• Hot solutions used in food industry
• Vapor-phase can be used as sterilant material

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Chemicals as Control

• Chemical preservatives
• Weak organic acids often used as food
  preservatives
• Benzoic, ascorbic and propionic acids
• Mode of action
• Alter cell membrane function
• Interfere with energy transformation
• Nitrates and nitrites used in processed meats
• Inhibits germination of endospores and growth of
  vegetative cells
• Have been shown to be potent carcinogen

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Control of bacteria growth--chemical

• Reducing water availability
• Salting
• drying

33
Review

• Important concepts
• Sterilization, pasteurization, D value etc.
• Methods to control bacteria growth
• Physical
• chemical

34
Review

• Considering factors when choose control methods.
• Moister heat.
• Radiation (ionizing, nonionizing).
• The difference between different levels of
  disinfectants.

35
Review

• Chemicals
• Ethanol
• Aldehyde
• Ethylene oxide
• Hydrogen Peroxides