The Control of Microbial Growth

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Chapter 7

• The Control of Microbial Growth

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Terminology

• Sterilization Removal of all microbial life
• Commercial Sterilization Killing C. botulinum
  endospores
• Disinfection Removal of pathogens
• Sepsis refers to microbial contamination
• Antisepsis Removal of pathogens from living
  tissue
• Degerming Removal of microbes from a limited
  area
• Sanitization Lower microbial counts on eating
  utensils
• Biocide/Germicide Kills microbes
• Bacteriostasis Inhibiting, not killing, microbes

3

• Bacterial populations die at a constant
  logarithmic rate

Figure 7.1a
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Effectiveness of antimicrobial treatment depends
on

• Number of microbes
• Environment (organic matter, temperature,
  biofilms)
• Time of exposure
• Microbial characteristics

Figure 7.1b
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Actions of Microbial Control Agents

• Alternation of membrane permeability
• Damage to proteins
• Damage to nucleic acids

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Physical Methods of Microbial Control

• Heat
• Thermal death point (TDP) Lowest temperature at
  which all cells in a culture are killed in 10
  min.
• Thermal death time (TDT) Time to kill all cells
  in a culture
• Decimal reduction time (DRT) Minutes to kill 90
  of a population at a given temperature

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Heat

• Moist heat denatures proteins
• Autoclave Steam under pressure

Figure 7.2
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Physical Methods of Microbial Control

• Pasteurization reduces spoilage organisms and
  pathogens
• Equivalent treatments
• 63C for 30 min
• High-temperature short-time 72C for 15 sec
• Ultra-high-temperature 140C for lt1 sec
• Thermoduric organisms survive

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Physical Methods of Microbial Control

• Dry Heat Sterilization kills by oxidation
• Flaming
• Incineration
• Hot-air sterilization

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Physical Methods of Microbial Control

• Filtration removes microbes
• Low temperature inhibits microbial growth
• Refrigeration
• Deep freezing
• Lyophilization
• High pressure denatures proteins
• Desiccation prevents metabolism
• Osmotic pressure causes plasmolysis

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Physical Methods of Microbial Control

• Radiation damages DNA
• Ionizing radiation (X rays, gamma rays, electron
  beams)
• Nonionizing radiation (UV)
• (Microwaves kill by heat not especially
  antimicrobial)

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Figure 7.5
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Chemical Methods of Microbial Control

• Principles of effective disinfection
• Concentration of disinfectant
• Organic matter
• pH
• Time

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Chemical Methods of Microbial Control

• Evaluating a disinfectant
• Use-dilution test
• 1. Metal rings dipped in test bacteria are
  dried
• 2. Dried cultures placed in disinfectant for 10
  min at 20C
• 3. Rings transferred to culture media to
  determine whether bacteria survived
  treatment

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Chemical Methods of Microbial Control

• Evaluating a disinfectant
• Disk-diffusion method

Figure 7.6
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Types of Disinfectants

• Phenol
• Phenolics. Lysol
• Bisphenols. Hexachlorophene, Triclosan

Figure 7.7
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Types of Disinfectants

• Biguanides. Chlorhexidine
• Disrupt plasma membranes

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Types of Disinfectants

• Halogens. Iodine, Chlorine
• Oxidizing agents
• Bleach is hypochlorous acid (HOCl)

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Types of Disinfectants

• Alcohols. Ethanol, isopropanol
• Denature proteins, dissolve lipids

Table 7.6
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Types of Disinfectants

• Heavy Metals. Ag, Hg, Cu
• Oligodynamic action
• Denature proteins

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Types of Disinfectants

• Surface-Active Agents or Surfactants

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Types of Disinfectants

• Chemical Food Preservatives
• Organic Acids
• Inhibit metabolism
• Sorbic acid, benzoic acid, calcium propionate
• Control molds and bacteria in foods and cosmetics
• Nitrite prevents endospore germination
• Antibiotics. Nisin and natamycin prevent spoilage
  of cheese

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Types of Disinfectants

• Aldehydes
• Inactivate proteins by cross-linking with
  functional groups (NH2, OH, COOH, SH)
• Glutaraldehyde, formaldehyde

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Types of Disinfectants

• Gaseous Sterilants
• Denature proteins
• Ethylene oxide

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Types of Disinfectants

• Peroxygens
• Oxidizing agents
• O3, H2O2, peracetic acid

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Microbial Characteristics and Microbial Control
Figure 7.11
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Microbial Characteristics and Microbial Control