Transcript Ch 7 Control of Microbial Growth

Control of microbial growth
Some terms to start with.
terms
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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
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Number of microbes
Environmental influences
Time exposure
Microbial characteristics
• Bacterial populations die at a constant
logarithmic rate.
Figure 7.1a
Methods of Control
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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
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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
• Phenol
• Phenolics. Lysol
• Bisphenols.
Hexachlorophene,
Triclosan
– Disrupt plasma
membranes
Figure 7.7
• Alcohol’s usually by protein denaturizing
• Heavy metals
– Oligodynamic action
– Denature proteins
• Surface active Agents (detergents)
Soap Degerming
Acid-anionic detergents Sanitizing
Quarternary ammonium compounds
Cationic detergents
Bactericidal, 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.
Types of Disinfectants
• 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
Resistance to
chemical
Biocides