control of microbial growth some terms to start with
Post on 21-Dec-2015
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Control of microbial growth
Some terms to start with.
terms
• Biocide: kills biologicals
• Germicide: Kills bacteria
• Bacteriostasis: stops growth
• Asepsis: absence of growth
Some terms.
• Sterilization: destruction of all life
• Disinfecting: destruction of vegetative pathogens
• Antisepsis: disinfection of living tissue
• Sepsis: refers to microbial contamination.
• Aseptic surgery techniques prevent microbial contamination of wounds.
Factors that influence effectiveness
• Number of microbes
• Environmental influences
• Time exposure
• Microbial characteristics
• Bacterial populations die at a constant logarithmic rate.
Figure 7.1a
Methods of Control
• Physical
• Chemical
• Biological
• Why is prevention best?
Physical Controls:
• Not for use on living organisms
• Somehow, alter membrane permeability and / or structure of proteins and nucleic acids
The autoclave: Moist heat and pressure
• 15psi, 121’c, 15 minutes• 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
Other controls
• Heat, dry heat takes longer
• Low temperature
• Desiccation
• Osmotic Pressure
• Radiation– Ionizing– No ionizing (UV)
•Radiation damages DNA
•Ionizing radiation (X rays, gamma rays, electron beams)
•Nonionizing radiation (UV)
•(Microwaves kill by heat; not especially antimicrobial)
• Filtration Removes Microbes
Chemical Methods of Control
• May be used on living organisms
• Factors that may effect disinfecting– Concentration of the disinfectant– What is to be disinfected
• pH• Organic materials
– Contact with microbe– time
Testing
• Use dilution test– Three microbes are used
Salmonella choleraesuis
Staphylococcus aureus
Pseudomonas aeruginosa– Metal carriers dipped and dried– Standard conditions 10 min at 20’C
Disk Diffusion Method
Types of disinfectants
• Phenol and Phenolics
• Halogens (iodine, chlorine) bind to aa of enzymes– Betadine iodophor (iodine bound to organic
molecules)
Types of Disinfectants
Figure 7.7
• Phenol
• Phenolics. Lysol
• Bisphenols. Hexachlorophene, Triclosan– Disrupt plasma
membranes
• Alcohol’s usually by protein denaturizing
• Heavy metals– Oligodynamic action– Denature proteins
• Surface active Agents (detergents)Soap DegermingAcid-anionic detergents SanitizingQuarternary ammonium compounds
Cationic detergentsBactericidal, Denature proteins, disrupt plasma membrane
• Antibiotics– Worry about resistance– Not many used as preservative (Nisin, Natamycin)
• Gaseous Chemostaerilizers– Ethylene oxide denatures proteins
• Resistance to these methods depends on the microbe.
• Peroxygens– Oxidizing agents
– O3, H2O2, peracetic acid
Types of Disinfectants
• Aldehydes– Inactivate proteins by cross-linking with
functional groups (–NH2, –OH, –COOH, —SH)
– Glutaraldehyde, formaldehyde
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
Types of Disinfectants
Resistance to chemical Biocides