• The Control of Microbial
Growth
2008
The Control of Microbial Growth
• Sepsis refers to microbial contamination.
• Asepsis is the absence of significant contamination.
• Aseptic surgery techniques prevent microbial
contamination of wounds.
2008
Terminology
• Sterilization: Removal of all microbial life
• Commercial Sterilization: Killing C. botulinum
endospores
• Disinfection: Removal of pathogens
• 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
2008
• Bacterial populations die at a constant logarithmic rate.
2008
Figure 7.1a
Effectiveness of antimicrobial treatment depends on:
• Number of
microbes
• Environment
(organic matter,
temperature,
biofilms)
• Time of
exposure
• Microbial
characteristics
2008
Figure 7.1b
Actions of Microbial Control Agents
• Alternation of membrane permeability
• Damage to proteins
• Damage to nucleic acids
2008
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
2008
Heat
• Moist heat
denatures
proteins
• Autoclave:
Steam
under
pressure
2008
Figure 7.2
Physical Methods of Microbial Control
• Pasteurization reduces spoilage organisms and
pathogens
• Equivalent treatments
• 63°C for 30 min
• High-temperature short-time 72°C for 15 sec
• Ultra-high-temperature: 140°C for <1 sec
• Thermoduric organisms survive
2008
Physical Methods of Microbial Control
• Dry Heat Sterilization kills by oxidation
• Flaming
• Incineration
• Hot-air sterilization
Equivalent treatments
2008
Hot-air
Autoclave
170˚C, 2 hr
121˚C, 15 min
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
2008
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)
2008
2008
Figure 7.5
Chemical Methods of Microbial Control
• Principles of effective disinfection
• Concentration of disinfectant
• Organic matter
• pH
• Time
2008
Chemical Methods of Microbial Control
• Evaluating a disinfectant
• Use-dilution test
2008
1.
Metal rings dipped in test bacteria are dried
2.
Dried cultures placed in disinfectant for 10
min at 20°C
3.
Rings transferred to culture media to
determine whether bacteria survived
treatment
Chemical Methods of Microbial Control
• Evaluating a disinfectant
• Disk-diffusion method
2008
Figure 7.6
Types of Disinfectants
• Phenol
• Phenolics. Lysol
• Bisphenols.
Hexachlorophene,
Triclosan
• Disrupt plasma
membranes
2008
Figure 7.7
Types of Disinfectants
• Biguanides. Chlorhexidine
• Disrupt plasma membranes
2008
Types of Disinfectants
• Halogens. Iodine, Chlorine
• Oxidizing agents
• Bleach is hypochlorous acid (HOCl)
2008
Types of Disinfectants
• Alcohols. Ethanol,
isopropanol
• Denature proteins,
dissolve lipids
2008
Table 7.6
Types of Disinfectants
• Heavy Metals. Ag, Hg, Cu
• Oligodynamic action
• Denature proteins
2008
Types of Disinfectants
• Surface-Active Agents or Surfactants
Soap
Degerming
Acid-anionic detergents
Quarternary ammonium compounds
Cationic detergents
Sanitizing
Bactericidal, Denature proteins,
disrupt plasma membrane
2008
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
2008
Types of Disinfectants
• Aldehydes
• Inactivate proteins by cross-linking with functional
groups (–NH2, –OH, –COOH, —SH)
• Glutaraldehyde, formaldehyde
2008
Types of Disinfectants
• Gaseous Sterilants
• Denature proteins
• Ethylene oxide
2008
Types of Disinfectants
• Peroxygens
• Oxidizing agents
• O3, H2O2, peracetic acid
2008
Microbial Characteristics and Microbial Control
2008
Figure 7.11
Microbial Characteristics and Microbial Control
Chemical agent
Effectiveness against
Endospores
Mycobacteria
Phenolics
Poor
Good
Quats
None
None
Chlorines
Fair
Fair
Alcohols
Poor
Good
Glutaraldehyde
Fair
Good
2008