Controlling Bacterial Growth

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Controlling Bacterial Growth
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Review

• Bacterial growth is increase in cell numbers
• Growth depends on temperature, pH, osmotic
  pressure, oxygen, and nutrients
• Log phase cultures are most sensitive to growth
  inhibition

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Bacteria Undergo Exponential Growth
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Growth measurements direct

• Plate counts
• detect viable organisms
• Direct microscopic counts
• detect total numbers

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Growth measurements Indirect

• Turbidity
• light absorption compared to a standard
• Metabolic activity
• amount of product produced in a given time
• Dry weight
• remove water and weigh

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Dry Weight Fungi
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Terminology

• Sterilization destruction of all forms of
  microbial life
• Commercial sterilization sufficient heat to kill
  Clostridium botulinum endospores (some
  non-pathogenic thermophilic bacteria may survive)
• Disinfection destruction of vegetative pathogens
  on inert substances

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Microbe of the Day

• Clostridium botulinum
• Common in environment

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Clostridium botulinum

• Gram positive rod
• Forms spores
• Strict anaerobe

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Clostridium botulinum

• Exotoxin causes botulism
• paralysis - blocks nerve signals to muscles

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C. botulinum Exotoxin Botox

• Used cosmetically to reduce wrinkles
• Used medically to relieve muscle spasm, excessive
  production of sweat

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Terminology

• Antisepsis chemical destruction of vegetative
  pathogens on living tissue
• Degerming mechanical removal of microbes from
  limited area
• Sanitization lowering microbial counts on eating
  and drinking utensils to safe levels

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Terminology

• Biocide or germicide kills microorganisms
• Fungicide kills fungi
• Virocide inactivates viruses
• Bacteriostatic agent stops growth of bacteria

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Terminology

• Sepsis bacterial contamination
• Asepsis absence of significant contamination
• Aseptic technique minimizes contamination

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Microbial Death

• Microbes die at a constant rate
• Factors affecting how long it takes to kill
  bacteria
• number of microbes
• environment
• slowed by organic materials, biofilms
• hastened by heat

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Microbial Death

• Factors affecting how long it takes to kill
  bacteria
• time of exposure
• characteristics of microbes most resistant are
• spores
• thick lipid coats
• protozoan cysts

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Actions of Microbial Control Agents

• Alteration of membrane permeability
• Damage to proteins and nucleic acids
• Mutation

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Physical Control Methods

• Temperature
• Desiccation
• Osmotic pressure
• Radiation

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Heat

• Must know temperature and time needed to kill
  critical bacteria
• Moist heat - boiling, autoclave
• Dry heat - oven

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Moist Heat

• Coagulates proteins by breaking hydrogen bonds
• Boiling 10 minutes kills most pathogens
  (hepatitis virus needs 30 minutes and spores need
  20 hours!)
• Autoclave 15 psi for 15 minutes (121 C)

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Autoclave (Fig 7.2)
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Pasteurization

• Used when taste of product would be damaged by
  longer heating lowers numbers of pathogens (63C
  for 30 minutes)
• High-temperature short-term (HTST) pasteurization
  (72C for 15 seconds)
• UHT sterilization (140C for 3 seconds)

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Dry Heat

• Flaming
• Oven (170C 2 hours)

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Low Temperature

• Refrigeration is bacteriostatic
• Most pathogens do not grow
• Exception?
• Freezing slow freezing creates ice crystals

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Membrane Filtration

• Pore size controls which microbes are removed
• HEPA filters used in safety hoods and operating
  theaters

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Limiting Water

• Desiccation bacteriostatic
• lyophilization used to preserve cultures
• Osmotic pressure high concentrations of salt or
  sugar
• Molds and yeasts most resistant

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Radiation
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Radiation

• Ionizing radiation gamma rays, X rays, high
  energy electron beams most energetic
• Ionization of H2O to form OH- radicals --gt
  mutations and death
• Low level ionizing radiation used on spices,
  certain meats and vegetables
• High energy electron beams used for medical
  supplies

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UV Radiation

• Thymine dimers in DNA
• Germicidal lamps, vaccine disinfection
• Not penetrating
• Can damage eyes

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Microwaves

• Very little effect on microbes
• Microwave ovens kill vegetative pathogens by
  heating
• Solid foods heat unevenly

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Chemical Methods

• Disinfectants and antiseptics
• Surface-active agents (surfactants)
• Chemical food preservatives
• Aldehydes
• Gas sterilization
• Oxidizing agents
• Antibiotics

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Disinfectants

• Kill/inhibit growth of microbes on surfaces
• Phenols and phenolics damage lipid membranes
• Active in presence of organic matter
• Stable
• Persist for long periods after application

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Disinfectants/Antiseptics

• Bisphenols
• Hexachlorophene (pHisoHex) and triclosan
• Antibacterial soaps and toothpaste
• Broad spectrum of activity

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Antiseptics

• Biguanides Chlorhexidine
• Low toxicity
• Used on skin and mucous membranes

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Disinfectants

• Halogens iodine and chlorine
• Iodine used in solution Betadine and Isodine
• Chlorine is a gas that forms bleach
  (hypochlorite) in water
• Chloramines are chlorine and ammonia

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Antiseptics

• Alcohol protein denaturation and membrane
  damage
• evaporate quickly
• ethanol and isopropanol
• not effective if taken internally

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Heavy Metals

• Denature proteins
• silver nitrate (topical cream)
• mercuric chloride (paint)
• copper sulfate (algaecide)
• zinc (mouthwash, paints)

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Surfactants

• Decrease surface tension
• Soaps and detergents
• Quaternary ammonium compounds microbicidal

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Chemical Food Preservatives

• Used in foods to inhibit microbial growth
• Sulfur dioxide
• Sodium benzoate
• Sorbic acid
• Calcium propionate
• Sodium nitrate and sodium nitrite

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Aldehydes

• Formaldehyde (formalin) and glutaraldehyde
• Disinfect instruments
• Used to preserve tissues for pathology
• Cross-link protein molecules

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Gas Sterilization

• Ethylene oxide
• Denatures proteins
• Kills all spores and microbes with lengthy
  exposure
• High penetration

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Oxidizing Agents

• Ozone
• Hydrogen peroxide
• Benzoyl peroxide
• Peracetic acid
• Interfere with metabolism (especially of
  anaerobes)

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Antiseptic Effectiveness
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Microbial Sensitivity to Chemical Biocides
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What You Should Know

• Terminology
• Types of physical and chemical control agents
  (not specific names)
• Example of when each would be used (internal,
  external, inanimate objects)