Control of Microbial Growth

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Control of Microbial Growth

• Chapter 5
• Part I

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• Control mechanisms are either physical or
  chemical
• May be a combination of both
• Physical methods
• Heat
• Irradiation
• Filtration
• Mechanical removal
• Chemical methods
• Antimicrobial agents
• Chemical depends on circumstances and degree of
  control required

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• Sterilization
• Removal of all microorganisms
• Sterile item is absolutely free of microbes,
  endospores and viruses
• Can be achieved through filtration, heat,
  chemicals and irradiation
• Disinfection
• Eliminates most pathogens
• Some viable microbes may exist
• Disinfectants - used on inanimate objects and
  surfaces
• Antiseptics - used on living tissues
• Pasteurization
• Brief heat treatment used to reduce organisms
  that cause food spoilage
• Surfaces can also be pasteurized

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• Decontamination
• Treatment to reduce pathogens to a level
  considered safe
• Degerming
• Mechanism to decrease the number of microbes in a
  specific area
• Particularly the skin
• Sanitized
• Substantially reduced microbial population
• Not a specific level of control
• Preservation
• Process used to delay spoilage of perishable
  items
• Often includes the addition of growth-inhibiting
  ingredients

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• The approach used depends on situation and
• degree of control required
• Daily life
• Hospital
• Microbiology laboratories
• Food and food production facilities
• Water treatment

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• Daily life
• Hand washing single most important step to
  achieving control
• Soap and detergents aid in mechanical removal of
  microorganisms
• Removes numerous organisms from outer the layer
  of skin
• Normal flora is usually unaffected because it
  resides in deeper layers

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• Hospitals
• Danger of nosocomial infections
• Ill patients are more susceptible to infection
• Pathogens more likely found in hospital setting
• Numerous organisms develop antimicrobial
  resistance due to the use/abuse of antibiotics

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• Microbiology laboratories
• Strict methods of control that require
  elimination
• of microbial contamination to experimental
• samples and environment
• Aseptic technique and sterile media used for
  growth
• Contaminated material treated for disposal

Food and food production facilities Food
quality is retained via Physical and chemical
methods Heat treatment most common Irradiation
approved to treat certain foods Chemicals used
to prevent spoilage
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• Water treatment facilities
• Chlorine generally used to disinfect water
• Can react with naturally occurring chemicals
• Form disinfection by-products (DBP)

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• Selection of effective procedure is complicated
• Ideal methods do not exist
• Each has drawbacks and procedural parameters
• Choice of procedure depends on numerous factors
• Type of microbe
• Extent of contamination
• Number of organisms
• Environment
• Risk of infection
• Composition of infected item

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• Type of microorganism
• Most critical consideration
• Resistant microbes include
• Bacterial endospores
• Resistant to heat, drying and numerous
  chemicals
• Mycobacterium species (tuberculosis)
• Waxy cell wall structure is cause of chemical
  resistance
• Pseudomonas species
• Can grow in presence of many chemical
  disinfectants
• Naked viruses (poliovirus)
• Lack envelope and are more resistant to chemical
  killing

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• Number of organisms initially
• present
• Killing effectiveness is gauged
• by decimal reduction time
• D value
• Time required to kill 90
• of population under specific
• Conditions
• Effect of prior washing

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• Environmental conditions
• pH, temperature and presence of organic materials
• can increase or decrease effectiveness
• Most chemicals are more effective at higher
  temperatures and lower pH

• Potential risk of infection
• Medical items categorized as
• Critical items - come in contact with body
  tissues
• - Needles and scalpels
• Semicritical instruments - contact mucous
  membranes but do not penetrate body tissues
• -Endoscope
• Non-critical instruments - contact unbroken skin
  only

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Heat

• Heat treatment most useful for microbial control
• Relatively fast, reliable, safe and inexpensive
• Heat can be used to sterilize or disinfect
• Methods include
• Moist heat
• Dry heat

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• Moist heat
• Causes irreversible coagulation of proteins
• Includes
• Boiling
• Pasteurization
• Pressurized steam

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• Boiling (100 C)
• Destroys most microorganisms and viruses
• Not effective means of sterilization
• Does not destroy endospores
• Pasteurization
• Does not sterilize but significantly reduces
  organisms
• Used to increase shelf life of food (milk)
• Most protocols employ High-Temperature-Short-Time
  (HTST) method
• Heated to 72C and held for 15 seconds

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• Pressurized steam
• Autoclave
• Achieves sterilization at
• 121C and 15psi in
• 15 minutes

Effective against endospores Prions destroyed at
132C for 4.5 hours
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• Dry heat
• Not as effective as moist heat
• 200C for 1.5 hours (dry) vs. 121C for 15
  minutes (moist)
• Incineration method of dry heat sterilization
• Oxidizes cell to ashes
• Used to destroy medical waste and animal
  carcasses
• Flaming laboratory inoculation loop incinerates
  organism

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• Filtration
• Liquid filtration
• Membrane filters allow
• liquids to flow through

• Filtration of air
• High efficiency
• particulate air (HEPA)
• filter removes nearly all
• microbes from air
• Filter has 0.3µm pores
• to trap organisms

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• Radiation
• Electromagnetic radiation
• Energy released as waves
• (microwaves, radio, gamma, X rays, ultraviolet
  light etc)
• Shorter wavelength, higher frequency more
  energy
• Radiation can be ionizing or non-ionizing

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• Ionizing radiation
• Gamma radiation
• X-rays
• Electron accelerators
• Causes direct damage to DNA and potentially to
• cell membrane
• Causes indirect damage by producing reactive
• molecules such as superoxide and hydroxyl
  radicals
• Used to sterilize heat sensitive materials
• Medical equipment, surgical supplies, medications
• Some endospores can be resistant

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• Ultraviolet radiation
• Non-ionizing (NI) radiation
• Damages DNA (causes thymine dimers)
• Used to destroy microbes in air, drinking water
  and
• on surfaces
• Limitation
• Poor penetrating power (thin films or coverings
  can limit its effectiveness)

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Chemicals

• Chemicals can be used
• to disinfect and sterilize
• Called germicidal
• chemicals
• React with vital cell
• structures and components
• Proteins
• DNA
• Cell membrane

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• Sterilants
• Destroy all microorganisms
• High-level disinfectants
• Destroy viruses and vegetative cells but not
  endospores
• Intermediate-level disinfectants
• Kill vegetative cells fungi, most viruses but not
  endospores
• Low-level disinfectants
• Kill fungi, vegetative bacteria and enveloped
  viruses
• No effect on mycobacteria, naked viruses or
  endospores

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

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• Classes of chemicals
• Alcohols
• Aldehydes
• Biguanides
• Ethylene oxide
• Halogens
• Metals
• Ozone
• Peroxides
• Phenolics
• Quaternary ammonium compounds

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• Alcohols
• Solutions of 60 - 80 isopropyl or ethyl alcohol
  kill
• Vegetative (actively growing) bacteria and fungi
• Not effective against endospores and some naked
  viruses
• Mode of action
• Coagulation of proteins ands 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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• Aldehydes
• Destroy organisms by inactivating proteins and
  DNA
• 2 glutaraldehyde solution most widely used
  liquid
• sterilant
• Formalin used to kill bacteria and inactivate
  viruses
• Also used for specimen preservation
• Formalin is a 37 aqueous solution of formaldehyde

Biguanides Most effective member is
chlorhexidine Extensively used in
antiseptics Relative low toxicity Destroys wide
range of organisms
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• Ethylene oxide
• Useful gaseous sterilant
• Destroys microbes including endospores and
  viruses
• Mode of action
• Reacts with proteins
• Useful in sterilizing heat or moisture sensitive
  items
• Limitations
• Mutagenic and potentially carcinogenic

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• Halogens
• Common disinfectants
• Mode of action
• Oxidize proteins and other cell components
• Include chlorine and iodine
• Chlorine
• Destroys all types of organisms and viruses
• Used as disinfectant
• Caustic to skin and mucous membranes
• Chlorine dioxide replacing chlorine in many
  applications
• Iodine
• Kills vegetative cells
• Not reliable with endospores
• Used in tincture or iodophore on skin