Controlling Microbial Growth Chapter 11

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Controlling Microbial GrowthChapter 11
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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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Generation Time

• Time required for cell to divide/for population
  to double
• Average for bacteria is 1-3 hours
• E. coli generation time 20 min
• 20 generations (7 hours), 1 cell becomes 1
  million cells!

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Exponential Growth
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Exponential Growth
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Case Baby Caroline
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Case Baby Caroline

• Flaccid paralysis
• Possible sources of infection
• Formula
• Honey
• Dirt
• Diagnosis infant botulism

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Microbe of the Week

• Clostridium botulinum
• Gram rod, spore-former, lives in soil - common
  in environment
• Spores ingested, germinate and produce botulinum
  toxin
• Strict anaerobe

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Clostridium botulinum

• Exotoxin causes botulism
• flaccid paralysis

Sources - improperly canned produce - honey
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Clostridium botulinumtoxin

• Clinical use Botox
• Muscle spasms
• Excessive perspiration
• Wrinkles

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Control of Microbial GrowthTerminology

• 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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Terminology

• Antisepsis 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
• E.g. microbiocides for HIV used in spermacides
• 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 (e.g.,
  feces, sewage)
• hastened by prior cleaning, 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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Effect of Population Load on Death Curve (Fig 7.1)
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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 Preservation

• Must know temperature and time needed to kill
  critical bacteria
• The thermal death time

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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 at 121 C (psi
  pounds per square inch)

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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 (170 C 2 hours)

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Low Temperature

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

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Membrane Filtration (Fig 7.4)

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

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Limiting Water

• Desiccation bacteriostatic
• Review 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
• Cause mutations and death

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Radiation

• 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
• A good summary of physical methods in Table 11.5

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Chemical Methods

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

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Disinfectants

• Phenols and phenolics damage lipid membranes
• Active in presence of organic matter
• Stable
• Persist for long periods after application

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PhenolsOrignially used by Joseph Lister, as
carbolic acid vapors during surgery
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Disinfectants

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

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Disinfectants

• 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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Disinfectants

• Alcohol protein denaturation and membrane
  damage
• evaporate quickly
• ethanol and isopropanol

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Evaluation of Disinfectants Fig 7.6
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Heavy Metals

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

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Heavy Metals
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Surfactants

• Decrease surface tension, damage lipid membranes
• Soaps and detergents
• Quaternary ammonium compounds microbicidal

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Quaternary Ammonium Compounds (Quats)
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Chemical Food Preservatives

• 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 (Fig
7.11)
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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