Class notes
MBY161 theme 5-9
- Institution
- University Of Pretoria (UP)
Lecture notes on themes 5-9 for MBY161 (microbiology)
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Theme 5: microbial growth controlSTERILIZATION: Destruction of all microbial life PATTERN OF MICROBIAL DEATH
COMMERCIAL STERILIZATION: killing Clostrium botulinum endospores
(heat)
DISINFECTION: focusing on removing pathogens
ANTISEPSIS: removing pathogens from living tissue
DEGERMING: removing microbes from a limited area
SANITIZATION: lowering microbial counts on eating utensils
BIOCIDE/GERMICIDE: killing microbes
BACTERIOSTASIS: inhibiting, not killing, microbes
SEPSIS: microbial contamination
ASEPSIS: the absence of significant contamination
ASEPTIC SURGERY TECHNIQUES: prevent microbial contamination of
wounds
EFFECTIVENESS OF TREATMENT
Depends on:
- Number of microbes (Initial number)
- Environment (organic matter, temperature, biofilms)
- Time of exposure
- Microbial characteristics (e.g. endospores)
ACTIONS OF MICROBIAL CONTROL AGENTS
- Alteration of membrane permeability
- Damage to proteins
- Damage to nucleic acids
METHODS OF MICROBIAL CONTROL
Physical Chemical
- Heat - Phenol & Phenolics
- Filtration - Halogens
- Low temp - Alcohols
- High pressure - Heavy metals
- Desiccation - Surfactants
- Osmotic pressure - Aldehydes
1. Heat
- Used in food preservation, sterilizing nutrient media, glassware, hospital instruments
- Kills microbes by denaturing enzymes (inactivates proteins)
Thermal death point (TDP):
lowest temp all cells in a culture are killed in 10 min
Moist heat Pasteurization Dry heat
1.1 Boiling 2.1 Equiv. 3.1 Direct
1.2 Steam treatment Flaming
flowing 2.2 HTST
,1.3 Autoclaving 2.3 UHT 3.2 Incineration
3.3 Hot air
Thermal death time (TDT):
time all cells in a culture (specific culture) are killed @ given temp
Decimal Reduction Time (DRT) – D value:
mins to kill 90% of a pop @ given temp
Moist heat sterilization:
Moist heat denatures proteins
Autoclave: steam under pressure
● 121 ºC for 15 min
● kill all organisms and endospores
● sterilize anything that can withstand high temps
Pasteurization
Reduces spoilage organisms and pathogens
Equivalent treatments:
63°C for 30 min
● High-temp short-time (HTST): 72°C for 15 sec
● Ultra-high-temp (UHT): 140°C for <1sec
Dry heat sterilization
Kills by oxidation
E.g. Flaming, Incineration, Hot-air sterilization
Equivalent treatments:
● Hot-Air: 170˚C, 2 hr
● Autoclave: 121˚C, 15 min
2. Filtration
Liquids/gases
Antibiotics, enzymes, vaccines
- HEPA (High efficiency particulate airfilter) removes microbes >0.3
µm
- Membrane filtration removes microbes >0.22 µm
3. Low temp
Inhibits microbial growth (bacteriostatic)
Refrigeration, Deep-freezing, Lyophilization
4. High pressure denatures proteins
5. Desiccation prevents metabolism -Lyophilization/freeze-drying
6. Osmotic pressure (hypertonic environment) causes plasmolysis
7. Radiation
- Ionizing radiation (X rays, gamma rays, electron
beams)
● Ionizes water to release OH
● Damages DNA
, CHEMICAL METHODS
- Use-Dilution test (evaluating disinfectants)
● Glass metal rings dipped in test bacteria are dried
● Dried cultures are placed in disinfectant (specific concentration) for 10 min at 20°C
● Rings are transferred to culture media (Incubated) to determine whether bacteria survived treatment
- Disc Diffusion test (evaluating disinfectants)
1. Phenol & Phenolics
Disrupt plasma membrane
2. Halogens (broad-spectrum activity)
Iodine
● Tinctures: in aqueous alcohol
● Iodophors: in organic molecules
● Alter protein synthesis and membranes
● Betadine
Chlorine: Used in drinking & recreational water, restaurant eating utensils, household
disinfectant (bleach), water purification tablets
3. Alcohols
● Ethanol, isopropanol
● Denature proteins, dissolve lipids
● Require water (e.g. 70% ethanol)
● Effective against bacteria & fungi (not endospores & enveloped viruses)
● Alcohol hand sanitizers
4. Heavy metals
- Ag, Hg, and Cu
● Silver nitrate may be used to prevent gonorrheal ophthalmia neonatorum
● Silver sulfadiazine used as a topical cream on burns
● Copper sulfate is an algicide (destroy green algae)
- Oligodynamic action – Denature proteins
5. Surface-active agents/surfactants
- Soap: Degerming
- Acid-anionic detergents: Sanitizing
- Quaternary ammonium compounds (cationic detergents): Bactericidal, denature proteins, disrupt plasma
membrane (also fungicidal & virocidal)
6. Aldehydes
Inactivate proteins by cross-linking w/functional groups (–NH2 , –OH, –COOH, –SH)
Use: medical equipment – Glutaraldehyde, formaldehyde, and ortho-phthalaldehyde
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