CHAPTER7
MICROBIAL CONTROL
Microbial contamination
• Sepsis refers to microbial contamination.
Also a term for blood infection/septicemia
• Asepsis is the absence of significant contamination.
• Aseptic surgery techniques prevent microbial contamination of wounds.
Terminology
• Bacterio static : inhibits bacterial growth
• Bacteri cidal : something capable of killing bacteria
• Antiseptic: an agent that is used to inhibit/kill bacterial growth on skin and mucus membranes
• Disinfectant: an agent that is used to inhibit/kill bacterial growth on inanimate objects
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Terminology
• Sterilization: Removal of all microbial life
• Commercial Sterilization: Killing C. botulinum endospores
• Degerming: Removal of microbes from a limited area
• Sanitization: Lower microbial counts to safe public health standards
• Biocide/Germicide: Kills microbes
History behind microbial control
• Joseph Lister was the first to introduce the use of carbolic acid to reduce bacterial infections in hospitals (1860s)
• Ignatz Semmelweis regarded as the “Father of Infection Control”, physicians used chlorinated lime to cleanse hands (1850s)
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What factors influence the success of microbial control?
• Is the organism an endospore former?
• What type of organism are you targeting?
• What type of environment are you treating?
Bacteria die at a constant logarithmic rate
Figure 7.1a
Effectiveness of treatment
• Number of microbes
• Environment
(organic matter, temperature, biofilms)
• Time of exposure
• Microbial characteristic
Figure 7.1b
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Are all bacteria equally sensitive?
What parts of a bacterial cell are sensitive to physical treatments and chemicals?
• Plasma membrane
• DNA and proteins
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
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Physical Methods
• Heat
– Dry: kills by oxidation (incineration, flaming, hot air)
– Moist: promotes coagulation of proteins
• Boiling (100 o C)
• Autoclave(121 o C, 15lbs/sq inch)
• Pasteurization reduces spoilage organisms and pathogens
• Equivalent treatments
– Classic: 63°C for 30 min
– High-temperature short-time/HTST: 72°C for 15 sec
– Ultra-high-temperature/UHT: 140°C for <1 sec
– Thermoduric organisms survive
Autoclave
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Autoclaves work due to steam under pressure
What if the substance is heat labile?
• Filtration is the best choice
• Pore sizes can be either .45um or .22um
Physical Methods
• Low temperature: freezing does not kill bacteria, most cultures are stored at -80 o C
– Refrigeration (0
⁰
C to 7
⁰
C)reduces metabolic rate
– lyophilization
• High Pressure: apply to liquids such as fruit juice; denatures proteins
• High Osmotic Pressure: use high concentration of salt or sugar; plasmolysis
• Dessication: remove the water and bacteria can remain viable but do not grow
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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)
Chemical Methods
• Most are only able to reduce the numbers of organisms, not achieve sterility
• Types of chemicals
– Phenol and phenolics
– Halogens
– Alcohols
– Heavy metals
– Soaps
Figure 7.5
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Chemical Methods of Microbial
Control
• Evaluating a disinfectant
– Use-dilution test
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
Evaluation of chemicals: disc diffusion method
• Phenol
• Phenolics
– coal tar (cresols)
– O-phenylphenol
(Lysol)
• Bisphenols.
Hexachlorophene
(Phisohex),
Triclosan
– Disrupt plasma membranes
Figure 7.7
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Types of Disinfectants
• Biguanides. Chlorhexidine
– Disrupt plasma membranes
Chemical control
• Halogens
– Oxidizing agents
– Iodine
• Tincture
• Iodophor
• Skin disinfectant and wound treatment; some patients are allergic to iodine
-Chlorine
Drinking water, swimming pools, etc.
Bleach is hypochlorous acid (HOCl)
Chloride dioxide-gas used for area disinfectant
Chemical Control
• Alcohols
– Mechanism of action is protein denaturation, can also dissolve lipids
– Most common are ethanol and isopropanol
– Does not kill endospores or viruses
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Chemical control
• Heavy metals exert oligodynamic action
• Denature proteins
• Silver, copper and mercury
– Silver nitrate: NBs
– Silvadine: burns
– Mercury: paints
– Zinc: mouthwashes
Chemical control
• Surfactants (soaps)
– Little value as an antiseptic
• Acid-anionic surface-active sanitizers are important in cleaning of dairy equipment and utensils
• Quats: if foams, contains quats
– Mouthwashes
– Some bacteria (Pseudomonas) can grow
DISINFECTANTS
• Surface-action agents or Surfactants
Soap Degerming
Acid-anionic detergents
Sanitizing
Quarternary ammonium compounds
Cationic detergents
Bactericidal,
Denature proteins, disrupt plasma membrane
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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
• Aldehydes
– Inactivate proteins by cross-linking with functional groups (–NH
2
, –OH, –COOH, —
SH)
– Glutaraldehyde: sterilizing, used on hospital equipment
– Formaldehyde:preserve biological specimens
– Both used in embalming
Types of Disinfectants
• Gaseous Sterilants
– Denature proteins
– Ethylene oxide: Sterilizes; used to sterilize spacecraft
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Types of Disinfectants
• Peroxygens
– Oxidizing agents
– O
3
, H
2
O
2
,
– peracetic acid (sterilize food processing and medical equipment)
Are all bacteria equally sensitive?
Which antiseptic works the best?
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Microbial Characteristics and
Microbial Control
Chemical agent
Phenolics
Quats
Chlorines
Alcohols
Glutaraldehyde
Effectiveness against
Endospores
Poor
None
Fair
Poor
Fair
Mycobacteria
Good
None
Fair
Good
Good
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