Chapter 7: The Control of Microbial Growth

1
Chapter 7The Control of Microbial Growth
2
The Control of Microbial Growth

• Asepsis is the absence of significant microbial
  contamination
• Aseptic surgery techniques prevent microbial
  contamination of wounds
• Aseptic lab techniques prevent contamination of
  lab samples/specimens

3
Terminology

• Sterilization Removal of all forms of microbial
  life
• Disinfection Destruction of pathogenic microbes
• Usually refers to vegetative pathogenic microbes
• Antisepsis Disinfection of living tissue
• Sanitization Lower microbial counts on eating
  utensils to safe public health levels to minimize
  disease transmission
• Biocide/Germicide Kills microbes
• Bacteriostasis Inhibiting microbial growth (not
  killing)

4
Effectiveness of antimicrobial treatment depends
on

• Number of microbes
• Time of exposure
• Constant rate of death in response to
  antimicrobial treatments
• Microbial environment
• Organic matter (biofilms) can inhibit chemical
  antimicrobials
• Chemical treatments rely on temp-sensitive
  reactions
• Microbial characteristics

Figure 27.11
Figure 7.1b
5
Microbial Characteristics and Microbial Control
Due to waxy, lipid-rich outer layer of cell wall
Due to outer membrane porin selectivity
Figure 7.11
6
Actions of Microbial Control Agents

• Alteration of membrane permeability
• Cytosol leakage
• Interferes with microbial growth
• Damage to proteins
• Ionic, hydrogen and covalent bonds may be
  disrupted
• Damage to nucleic acids
• Disrupts protein synthesis and cells ability to
  replicate

7
Methods of Microbial Growth Control

• Physical
• Chemical

8
Physical Methods of Microbial Control

• Heat
• Heat resistance varies among different microbes
• Pasteurization (limited heat treatment)
• Moist heat
• Dry heat

9
Physical Methods of Microbial ControlHeat
Pasteurization

• Pasteurization reduces spoilage organisms and
  eliminates pathogenic organisms
• Thermoduric microbes survive, but dont cause
  disease or significant spoilage
• Equivalent treatments (i.e. for milk
  pasteurization) yield the same effects
• 63C for 30 min
• 72C for 15 sec (High-temp short time
  pasteurization)
• 140C for lt1 sec (Ultra-high temp pasteurization)

10
Physical Methods of Microbial ControlMoist Heat

• Moist heat denatures proteins
• Autoclave Steam under pressure
• Temperature of steam rises above 1000C
• Can kill all microbes

Sterilization
Figure 7.2
11
Physical Methods of Microbial ControlDry Heat

• Dry Heat Sterilization kills by oxidation
• Flaming (like in lab)
• Hot-air sterilization (oven)

Hot-air Autoclave
Equivalent treatments 170C, 2 hr 121C, 15 min
12
Physical Methods of Microbial ControlSterilizing
Radiation

• Radiation damages DNA ? lethal mutations
• Ionizing radiation (X rays, gamma rays, electron
  beams) shorter wavelengths
• Ionization of water? reactive hydroxyl radicals
• Medical supply sterilization
• Nonionizing radiation (UV)

Ionizing
Figure 7.5
13
Other Physical Methods of Microbial Control

• Filtration removes microbes
• Useful for heat-sensitive solutions
• Low temperature inhibits pathogenic microbial
  growth (bacteriostatic)
• Refrigeration (0-70C)
• Deep freezing
• High pressure denatures proteins
• Desiccation prevents metabolism/growth
• Osmotic pressure causes plasmolysis

14
Chemical Methods of Microbial Control

• Joseph Lister performed the first aseptic surgery
  (1865)
• Used phenol (carbolic acid), which was shown to
  reduce odor from sewage
• Father of modern surgery

http//web.ukonline.co.uk/b.gardner/Lister.html
15
Chemical Methods of Microbial ControlTypes of
Disinfectants

• Phenolics (Triclosan)
• Disrupt lipids in the PM
• Halogens (Iodine, Chlorine)
• Oxidizing agents disrupt cell membranes
• Alcohols (Ethanol, Isopropanol)
• Dissolve lipids in the PM denature proteins
• Heavy metals (Ag, Hg, Cu, Zn)
• Denature proteins
• Surface-active Agents (Surfactants/Quats)
• Denature proteins disrupt the PM

16
Chemical Methods of Microbial Control

• No single disinfectant is effective in all
  circumstances
• Evaluating disinfectants Disk-diffusion method
• Disinfectant-soaked filter paper is placed onto
  inoculated agar, then incubated to allow growth
• Look for zone of growth inhibition

17
(No Transcript)