Eprac 3B: Control Of Microorganisms Flashcards
Method of microbial control: physical agents
Heat: dry (sterilisation), moist (disinfection, sterilisation)
Radiation: ionising (radiation) and nonionising (disinfectant)
Methods of microbial control: chemical agents
Gases: disinfection and sterilisation
Liquids: animate (chemotherapy, antisepsis) and inanimate (disinfection, sterilisation)
Methods of microbial control: mechanical removal methods
Filtration:
- > air: disinfection
- > liquids: sterilisation
Factors which may affect efficacy of disinfection
- Pop size
- Pop volume
- Type/structure of organisms - Gram +ve, Gram -ve, structures that may interfere with disinfection- spores, capsules, biochemical properties
- Exposure time
- Conc of agent
- pH
- Presence of organic matter
Autoclave
Moist heat: kill by degrading nucleic acids and by denaturing proteins
To ensure death of endospores: chamber must be packed such that steam can circulate freely, all the air can be evacuated from chamber and enough time is allowed for volume of materials being treated to reach required temp
Steps in autoclaving
- Autoclave is correctly loaded (no air pockets, bundles separated to allow penetration of steam)
- Efficacy of indicators are included in each load with biological indicators packed within loads to ensure steam penetration
- Door is closed and locked
- Air is evacuated from chamber
- Steam is pumped in until pressure is reached
- Timing begins and pressure is maintained until end of programmed time period
- Steam is evacuated from chamber
- Warm filtered air is released into chamber to prevent excessive drying
- Once temp is low enough (below 80degrees), the door can be released
Monitoring autoclaves
Biological indicators (indicators ampoules or spore strips) should be packed within test loads -> ensure adequate steam penetration of loads
Chemical indicators include autoclave tape and temp dependent colour changes on bags
Dry heat vs Moist heat
- Different mode of actions for killing mechanisms
- Different exposure time
- Different uses of treatment
Different mode of action for killing mechanisms (dry heat vs moist heat)
Dry heat kills by oxidation
Moist heat: kills organisms more rapidly because water hastens breaking of HB that hold proteins in 3D structure
Both denature proteins
Different exposure time (dry heat vs moist heat)
Dry heat: requires higher temp. for longer exposure periods to kill all micro-organisms in a sample
Moist heat: more penetrating than dry heat -> more effective and faster for killing microorganisms
Different uses of treatment (dry heat vs moist heat)
Dry heat: preferred for sterilising pipettes and other heat stable objects (autoclaves leaves them moist -> corrosion)
Moist heat: preferred for preparation of culture media in microbiology lab (dry heat at high temp -> destroy elements of media -> useless)
Disinfection by Pasteurisation (physical)
Substances treated at temps below boiling
Involves brief heating at 60-70degrees for approx 15 sec
Does not sterilise
Kills pathogens present and slows spoilage by reducing no. of non-pathogenic spoilage microorganism
Pasteurisation methods
- Low temp longer time (LTLT): 65 degrees for 30 mins (holding or batch method)
- High temp short time (HTST): heat treatment of 72degrees for 15 sec, followed by rapid cooling to below 10 degrees (continuous system or flash pasteurisation)
- Ultra high temp (UHT): 149 degrees for 1 sec or 93.4 degrees for 3 sec
Sterilisation by ionising radiation (physical)
Cause discharge of e which yield active radicals -> breaks in HB, destruction of ring structures and polymerisation of some molecules -> death of organism
Properties of ionising radiation (physical)
- Gamma rays, x rays or accelerated e
- Kills all types of microorganisms including endospores
- High penetration into solids and liquids
- Used for: reusable heat-sensitive equipment, medical plastics, prosthetics and some pharmaceuticals
- High start up and capital costs
