Sterilisation Methods Flashcards
Sterilisation by HEAT
- heat produces thermal energy that is lethal to m.o, damages cytoplasmic membrane, cellular enzymes, DNA, RNA
- Moist heat (Protein Coagulation)
- Dry heat (Oxidation)
- Factors (HOT): sufficient quantity of heat i.e. temp, Organisms i.e. need to know population and their heat resistance, Time needs to be sufficient heat and holding time to destroy them
Thermal death kinetics
At constant temp, same percentage of m.o will be destroyed per unit of time at the rate proportional to the number of surviving organisms
Log10Nt=Log10No-kTt
LogNT VS t
KT = thermal death rate constant at temp T
T in minutes
Decimal Reduction Time (D)
Heating time required from 10 fold reduction (1 log cycle) in number of surviving organisms
D=1/Kt
How do we know whether organisms are heat sensitive or heat resistant?
Z value
- defines heat sensitivity (small z) or resistance (large z) of m.o
- defined by increase in temp required to bring a 10 fold decrease in decimal reduction time
Fo value
- defines the lethality in terms of equivalent heating time in minutes at 121 deg using reference organism with a z value of 10 deg
- takes into account the equivalent lethal effect go lower temp compared with 121 deg
Fo = T x 10^( T-121 / z )
Sterility Assurance Level SAL
- Defines the degree of sterility in the treated batch of product
MOIST HEAT STERILISATION
- saturated steam at high temp holds load of latent heat which can transfer without drop in temperature when it condenses onto a cooler surface
- protein coagulation and denaturation of m.o
Autoclaving - moist heat sterilisation under high steam pressure in an autoclave oven
- steps involve remove air, heat to temp under high steam pressure, hold temp for required time, cool and depressurise
Used for sterilising: dressings, equipment, aq injections only
Monitoring steam sterilisation
Instrumental indicators: thermometers, pressure gauges, thermocouples
Chemical indicators: Browne’s tube red to green, Bowie-Dick tape (light-dark), Steam-Clox strip (purple-green)
Biological Indicator: Spore strips, B. stearothermophilus
Biological Indicators
- strain stable
- non- pathogenic
- Resistant to method used
- reproducible recovery
- not less than 10^5 viable spores per unit
Advantages of Moist Heat
Lower temp
Shorter exposure time
Aqueous thermostable preparations ie aq inj
Sealed in heat stable plastic containers
Disadvantages of Moist Heat
Heat labile prep corrosive containers must be penetrable by steam difficult to monitor damp load bursting of rigid containers
DRY HEAT STERILISATION
- Heat transfer by convection or radiation
- Oxidative destruction of bacterial cell wall
- higher pressure and longer holding time (no rubber, plastic packages)
- suitable for metal instruments, glass ware, powders, oils
- also used for depyrogenation of glassware
Hot air oven
Electric heater to heat the oven
Fan Circulation to distribute heat evenly, max variation in chamber <10 temp
Thermostat to maintain temp
Heat lining to prevent heat transfer from inside
Efficiency Indicator: B. Subtilis var niger
Advantages of Dry Heat
Non-aq systems Non-corrosive Penetration Dry process Oily thermostable products i.e. oily inn
Disadvantages of dry heat
Slow heating
Longer holding times
Higher temp
Not for aq preparations
moist heat vs Dry heat
Condensation, protein coagulation MOA, 121 for 15 mins under high steam pressure 15psi, B. Stearothermophilus, D121 = 2 mins
Convection and radiation, oxidative destruction, 160-170 for 1-2 hours, B subtilise var niger, D160 = 5-10 mins
Radiation
- electromagnetic radiation: energy in motion in the form of non-particulate electromagnetic rays
- Energy photons can induce excitation of atoms in living cells depending on level of energy and penetrating power
- chemical reactions in nucleic acid, cell mutation and death
- UV, GAMMA
UV Radiation
Electromagnetic radiation of lower energy and poor penetrating power
- non-ionising, can cause excitation of atoms to higher energy state
- cause chemical reactions in nucleic acid, alter DNA, mutate, cell death
Uses:
Air - lower m.o counts in clean rooms by UV lamps fitted ducted air supply systems
Water - UV water steriliser equipment in water supply
Surfaces - Direct irradiation of sterile cabinet surfaces (LFC)`
Disadvantages of UV radiation
Hazard to operators as they are also irradiated
- use reflectors to protect operator and direct and intensify UV exposure to defined areas
Poor penetration
Shielding and reflection
GAMMA IRRADIATION
- High energy, powerful and penetrating electromagnetic radiation
- shorter wavelength than X-rays
- Emitted by decaying unstable nucleus (cobalt 60 - unstable radioactive form of cobalt 59)
- ionisation of vital cell components and cell death –> ionisation of DNA, ionisation of cell water and produce free radicals to react with DNA
Biological Indicator: B. Pumilus
used in: drugs in dry state i.e. freeze dried antibiotics, vaccines, oil, creams
Factors: Amount of energy absorbed Oxygen Temperature Organic substances (protective)
Advantages of Radiation
- materials not wetted or heated
- operated at ambient temps (not extreme heat)
- packed and sealed container in packaging
- continuous and automated
Disadvantages
- Hazardous
- chemical changes in drug
- container deterioration
- small volume, low density
- expensive, needs a plant
Chemical Sterilisation
Ethylene Oxide
Formaldehyde
Glutaldehyde
MOA
- alkylating agents
- Addition of hydrocarbons to reactive groups of protein molecules
- acts on cell during DNA replication
- Biocidal, mutagenic, carcinogenic
Ethylene Oxide
Flammable Highly explosive used with inert diluent gases as non-flammable mixture highly diffusable through packaging freely soluble in water dissolve in rubber, plastic
Uses: dressings, containers, equipment, implants e.g. breast implants
Factors affecting ethylene oxide sterilisation
Concentration
Temperature
Moisture - dissolves gas to form conc solution for optimal effect at 33% RH
Monitoring of ethylene oxide
Thermocouples
Humidity sensors
Chemical indicators - strip changes colours on exposure
Biological indicators - attest vials, spore strips, indicator organisms B. subtilis var niger
Limitations of ethylene oxide
Toxicity - carcinogenic
Difficult to monitor
Removal of residues
Time consuming
Sterilisation by filtration
- liquid and gas substances are removed by filters
- no killing and no growth inhibited, only physical removal
Use: Cleanroom air supply, LFC heat labile liquids heat labile ointments QC tests using membrane filtration method purification of water
Desired properties of filters
Inert Non-pyrogenic Efficiently removes particles Good flow rate Resistant to clogging Sterilisable Reusable Do not retain drug Do not release filter components
Depth Filters
Compacted pads of fibres with pores and channels or variable diameters and directions
Eg HEPA filters
Mechanism:
Sieving, adsorption, retention
Under vacuum or positive pressure to assist flow
Arrest depends on size, charge, velocity of particles
High porosity
Good pore size range
Good dirt handling capacity
Depth Filters Limitations
- Need uniform pressure, if too high pressure, filter compress and decrease flow rate
- Retain liquid in filter = loss of solvent, can not suck out liquid in filter otherwise filter will crack
- loss of formulation ingredients
- slow speed of sterilisation
- may not be suitable for sterilisation or reuse
- filter fibres may shed into filtrate
Membrane Filters
Polymeric membranes (one layer) Hydrophilic, hydrophobic, solvent resistant, gridded High porosity Fast flow and speed Low fluid retention no shedding Minimum solute adsorption narrow pore size range Poor dirt handling capacity rapidly clogging
Uses: screening/ sieving, retention, all particles greater than pore size retained
Monitoring Filters
Bubble point pressure test
Bacterial retention test (microbial challenge test)
Bubble point pressure
- the pressure of a test gas (N2) to push liquid out of a wet membrane filter
- gas bubbles appear in liquid at that pressure
- gas pressure is a measure of pore diameter
- smaller pore size requires higher pressure (to overcome surface tension)
Pass filter integrity test if bubble pressure is >/= to recommended pressure i.e. actual pore size is = or < specified test
Microbial Challenge/ retention test
Test efficiency of sterilising filter (0.22um) using a challenge organism Br. diminuta 0.3um size in high conc
slow filtration then the entire filtration is cultured in broth medium, or filtered again and culture the filter
Filter passes if no growth after 72 hours at 30 deg
