Topic 6: Control of Microbial Growth Flashcards
Why do we need microbial control?
- Prevent cross contamination of cultures in a lab
- Prevent contamination of equipment used in medical and food prep procedures.
- To increase the shelf life of food and beverages.
- To limit the detrimental effect the microbes or their metabolic products would have on us and our food.
Microbial Growth Control Terms
Sterilisation: Killing or removal of all microoganisms
Commercial Sterilisation: Killing C. botulinum ensospores (no pathogens left, growth stopped).
Disinfection: Removing pathogens (inanimate objects).
Antisepsis: Removing pathogens from living tissue
Degerming: Removing microbes in a limited area
Sanitisation: Lowering microbial counts, e.g. on eating utensils.
Biocide/germicide: Kills microbes
Bateriostasis: inhibiting, not killing, microbes
Sepsis: refers to microbial contamination.
Asepsis: is the absence of significant contamination
Aseptic surgery: techniques prevent microbial contamination of wounds.
Effectiveness of Treatment
Depends on
- Number of microbes
- Environment (organic matter, temperature, biofilms)
- Time of exposure
- Microbial characteristics
Actions of Microbial Control Agents
- Alteration of membrane permeability
- Damage to proteins
- Damage to nucleic acids
Physical control of mechanisms
- Temerature +/- Moisture
- Filtration
- Desiccation
- Osmotic Pressure
- High Pressure
- Radiation
Heat
- Denatures and coagulates proteins
- Microbes differ in their resistance to the affects of temperature.
- Thermal death point (TDP): Lowest temperature at which all cells in a culture are killed in 10 min.
- Thermal death time (TDT): Time during which all cells in a culture are killed.
Decimal Reduction Time (DRT)
DRT = Time in (MInutes) to kill 90% of a population at a given temperature.
Autoclaving
- Moist heat (steam) under pressure
- temperature acting on items is raised by increasing the pressure around the item
- 15 psi, 121 degrees Celsius @ 15min - kills all (except prions).
- Sterilisation depends on volume
- 10ml- 15min
- 6L- 70min
Steam Sterilisation
-Steam must contact the item’s surface.
Dry Heat Sterilisation
- Kills by oxidation
- Flaming
- Incineration
- Hot air sterilisation
Pasteurisation
- Kills pathogens and reduces spoilage organisms
- Equivalent treatments
- 63 degrees Celsius for 30 minutes
- High temperature short time: 72 degrees Celsius @15sec
- Ultra High Temperature: 140 degrees Celsius @<1sec
*Thermoduric (heat resistant) organisms can survive and cause spoilage
Low Temperatures
- Refrigeration:
1. Slows metabolic activity
2. Generally bacteriostactic, rather than bacteriocidal. - Freezing
1. Slow Freezing more effective than flash freezing as forming ice crystals disrupts the cells.
2. Thawing more damaging than freezing. - Freeze drying:
1. Long term preservation
2. Decrease water availability.
Filtration
- HEPA: removes microbes >0.3um
- Membrane filtration removes microbes > 0.22um
- Does not inactivate any metabolic products e.g. toxins.
- Will not remove viruses.
Disk diffusion method
- Will determine the most strongest antibiotic to combat a microbe.
e. g. Streptococcus pyogenes grown on HBA plate, with antibiotic discs.
Phenol and Phenolics
- Disrupt plasma membranes
e. g. Phenol and O-Phenylphenol
Bisphenols
- Hexachlorophene, triclosan
- Disrupt plasma membranes.
Biguanides
- Biguanide can refer to a a molecule, or to a class of drugs based upon this molecule.
- Acts on membranes by blocking lipid synthesis and disrupting plasma membranes.
- Often used in combination with other agents
- Antiseptic uses
- Disinfectant uses
- Preservative uses
- Example - Chlorhexidine
Surface-Active Agents or Surfactants
-Soap: Degerming (cleaning/washing)
-Acid-anionic detergents: Sanitizing
-Quarternary ammonium compunds (cationic detergents):
Batericidal, denature proteins, disrupt plasma membrane.
Quats
- Quaternary ammonium compounds
- Effective against bacteria, fungi, viruses (enveloped and non) and amoebae.
- Do not kill endospores or mycobacteria
- Widely used in both home and industry
- Examples- Benzalkonium chloride (Lysol), Cetylpyridinium chloride (Cepacol).
Microbial Resistance
-Most Resistant (Decreasing Order) Prions Endospores of Bacteria Cysts of Protozoa Vegetative protozoa Gram Negative Bacteria Fungi, including most fungal spores Viruses without envelopes Gram Positive Bacteria Viruses with lipid envelopes Least resistant
Aseptic Technique
-A set of practices and procedures performed under carefully controlled conditions with the aim of minimising contamination by unwanted pathogens or cells.
e.g. Culturing Microoraganisms Cell structure Sterile liquid transfers Surgery Wound care.
Ehrlich and Fleming’s contribution to antibiotics
- Paul Ehrlich, discovered Salvarsan
- Ehrlich came up with the concept that different organisms have different types of cells and therefore should have unique drug targets within those cells. “Magic Bullet” idea - kill an invading pathogen, but leave us alive (Salvarsan, 1910).
- Fleming Discovered the most famous of all magic bullets, penicillin in 1928. Florey and Chain were able to develop this for use in the second world war. Modern Antibiotic era was born.
