Transcript Control of Microbial Growth

Chapter 7
The Control of Microbial Growth
TERMINOLOGY
• Sepsis refers to microbial contamination.
• Asepsis is the absence of significant
contamination.
• Sterilization: Removal of all microbial life
• Commercial Sterilization:
• Disinfection: Killing of vegetative microbes on
nonliving objects
•Antisepsis: Killing of vegetative microbes in/on
living tissue
•Degerming: Removal of microbes from a limited
area
•Biocide/Germicide: Kills microbes
• Bacteriostasis: Inhibits, does not kill microbes
Microbe Death by Antimicrobial Agents
• Factors influencing death rate are:
• Number of microbes
• Environmental influences (ie. Organic substances,
biofilms, temperature, pH)
• Biofilms:
• Time of exposure
• Microbial characteristics
• [antimicrobial agent]
Actions of Microbial Control Agents
• Alteration of membrane permeability
• Lipids and proteins are altered
• Intracellular substances leak out of cell
• Damage to proteins
• Denaturation of enzymes, etc.
• Loose their structure and function
• Damage to nucleic acids
• Damaged DNA and RNA prevents production of
vital proteins
• Heat denatures
proteins
• Boiling water
• (May not kill
spores)
Moist Heat
• Autoclave:
generates very high
pressure and
temperature via
steam
• Steam must
contact surface to
kill microbes
Figure 7.2
Pasteurization
• Pasteurization reduces spoilage organisms and
pathogens via heating
• Thermoduric (i.e. thermophiles) organisms survive
• Pasteurization treatments
• High-temperature short-time (HTST): 72°C for 15s
• Ultra-high-temperature (UHT): 140°C for 3s
Dry Heat
• Dry Heat Sterilization:
• Direct flaming
• Incineration
• Hot-air sterilization
Equivalent treatments
Hot-air
Autoclave
170˚C, 2 hr
121˚C, 15 min
Other Physical Methods of Microbial Control
• Filtration removes microbes
• Low temperature inhibits microbial growth:
• Refrigeration
• Deep freezing
• Desiccation prevents metabolism
• Hypertonicity causes plasmolysis
Other Physical Methods of Microbial Control
• Radiation damages DNA and other organic molecules
• Ionizing radiation (X rays, gamma rays, electron
beams)
• Nonionizing radiation (UV)
• Microwaves kill by heat; not especially antimicrobial
Chemical Methods of Microbial Control
• Evaluating a disinfectant
• Use-dilution test
• 1. Stainless steel cylinders are inoculated with
Staphylococcus aureus, Salmonella choleraesuis,
Pseudomonas aeruginosa
• 2. Cylinders are dried in an incubator
• 3. Cylinders are placed in test disinfectant
• 4. Cylinders are incubated in broth media
• Disk-diffusion method
Types of Disinfectants/Antiseptics
• Phenol
• Phenolics: Modified phenol
• reduced irritation, enhanced
antibacterial action
• ie. Lysol
• Bisphenols:
• Hexachlorophene
• Triclosan (2nd link)
• Found in many
household items
Figure 7.7
Types of Disinfectants/Antiseptics
• Chlorhexidine
• surgical hand scrub
• preoperative patient scrub
• Halogens:
• Iodine: skin disinfectant; water treatment
• Chlorine (ie. Bleach): water, sewage treatment;
swimming pools; restaurants, etc.
Types of Disinfectants/Antiseptics
•
Alcohols: Ethanol,
isopropanol
•
May not be
appropriate for
wound cleaning
•
•
May “seal in”
microbes
Alcohol-based
handrubs
Table 7.6
Types of Disinfectants/Antiseptics
• Heavy Metals: ie. Ag, Cu
• Silver compounds: impregnated dressings; burn
creams; catheters
• Copper sulfate: kills green algae in reservoirs, pools,
ponds
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 in meats
• Antibiotics. Nisin and natamycin prevent spoilage of
cheese
Types of Disinfectants
• Aldehydes:
• Glutaraldehyde:
• sterilizes*
• hospital instrument disinfection
• used for embalming
Types of Disinfectants
• Gaseous Sterilants
• Ethylene oxide:
• Peroxygens
• Ozone = O3: ie. water treament
• Hydrogen peroxide = H2O2: ie. Food packaging;
NOT good for open wounds
Types of Disinfectants/Antiseptics
• Surface-Active Agents or Surfactants
Soap
Degerming
Cationic detergents:
Quaternary ammonium compounds
= “Quats”
ie. Pseudomonads can grow in
Quat solutions; can Norovirus?
Relative Resistances to Antimicrobials
Figure 7.11