CONTROL OF MICROBIAL GROWTH Flashcards
Sterilisation
Sterilization: The killing or removal of ALL living organisms and viruses from on/in a material.
from surfaces,
from gasses
from liquids (e.g. growth media).
Criterion of death is the irreversible loss of the ability to multiply under ANY circumstances
Disinfection
Disinfection: The elimination of most pathogenic microorganisms from inanimate objects or surfaces (inanimate = not alive).
Disinfectants are usually chemicals but disinfection can also be done by using steam or boiling water etc.
There are different levels of disinfectant (high, intermediate and low).
STERILISATION vs DISINFECTION
Sterilizationdescribes a process that destroys or eliminates ALL forms of microbial life (including bacteria, viruses, fungi, and spores) and is carried out by physical or chemical methods.
Steam under pressure,
Dry heat,
EtO gas,
Hydrogen peroxide gas
plasma,
Liquid chemicals
Disinfectiondescribes a process that eliminates most pathogenic microorganisms, except bacterial spores, on inanimate objects. Objects are usually disinfected by
Liquid chemicals,
Wet pasteurization.
Decontamination
What is contamination?
The action or state of making or being made impure by polluting or poisoning.
“the risk of contamination by dangerous bacteria”
Decontamination: Removal of dangerous substances to make an object or inanimate surface safe to handle.
Sanitisation
Sanitization: The use of agents that reduce, but may not eliminate, microbial numbers to a safe level.
Sanitizing is typically recommended for food contact surfaces as the chemicals are in concentrations that are considered safe.
Sanitisation is essential to avoid food poisoning!
Antiseptics
Antiseptics: Chemical agents that kill or inhibit the growth of microorganisms but are sufficiently non-toxic to be applied to living tissues.
They are (in theory) similar to disinfectants but used at lower concentrations.
Do not confuse antiseptic with aseptic.
Aseptic means the absence of microbial contamination.
Aseptic Technique
Aseptic means the absence of microbial contamination as in
‘aseptic techniques’.
- Handwashing
- Autoclaving
SEPSIS
Sepsis is a life-threatening reaction to a bacterial infection.
It happens when your immune system overreacts to an infection and starts to damage your body’s own tissues and organs.
It is sometimes called septicaemia or blood poisoning
BACTERIA KILLERS - MODE OF ACTION
Agents that destroy or kill bacteria are referred to as bacteriocides.
-cide/cidal suffix:
Fungicide kills fungi.
Virucide deactivates or
destroys viruses.
Algicide kills algae.
Pesticide kills pests (insects).
Agents that inhibit bacterial growth are said to be bacteriostatic.
(Prevent growth but cells may not be killed.)
MICROBIAL DEATH
Bacterial death occurs at a constant, usually, exponential rate.
Killing curves usually plotted as log10 number of survivors vs time.
Factors affecting death rate
Microbial characteristics, growth phase, culture conditions, presence of resting cells.
Environmental factors.
Concentration or intensity of treatment.
Time of exposure.
Number of organisms does not affect rate of death but will affect time of treatment.
Methods to Kill Pathogens
Killing methods
Physical methods
e.g. heat, irradiation, filtration.
Chemical methods
e.g. gases or liquids
MECHANISMS OF KILLING:
Alteration of membrane permeability.
Damage to proteins.
Damage to nucleic acids.
Methods to Kill Pathogens
Thermal sterilisation = Heat
Non-thermal sterilisation
Irradiation
Filtration
Chemical (Gas – liquid)
Metals
1- HEAT STERILISATION
One of the most widespread method used for controlling microbial growth is the application of heat.
For all microorganisms there is a maximum temperature for growth, beyond which viability decreases.
At very high temperatures virtually all macromolecules lose their structure and their ability to function.
In other words: macromolecules are denaturated. This process is called denaturation
DECIMAL REDUCTION TIME – D VALUE
D-value (decimal reduction time) isthe time (in minutes) required to reduce viable bacteria by 90% at a defined temperature.
It is used to calculate the time that is required to achieve sterilisation. For example in the canning industry.
This is the most precise way to characterise heat sterilisation.
DRY HEAT vs MOIST HEAT
Heat can be in the form of ‘dry heat’ or ‘moist heat’ and the nature of the heat is important.
Moist heat has better penetrating power than dry heat and produces a faster reduction in the number of living organisms at a given temperature.
For comparison:
sterilisation with dry heat at 160-170°C takes 1-2 hrs
sterilisation with moist heat at 121°C takes 15 minutes.
MOIST HEAT
Can be done with water at liquid state or vapour state (steam).
A temperature of > 63°C is sufficient to kill the vegetative cells of most pathogens BUT not spores or thermoduric or thermophilic organisms.
Boiling for 10 min will kill most vegetative cells, fungi and viruses.
Functions by denaturation of proteins by breakage of hydrogen bonds.
Probably kills by denaturing membrane proteins.
Disadvantage: Increased rate of chemical reactions (e.g. in nutrients).
Thermophilic: Bacteria those thrive at 50-110 C growing conditions and this needs for their optimum growth.
Thermoduric: those bacteria which withstand higher temperatures that actually not suitable for their optimal growth
MOIST HEAT + PRESSURE = AUTOCLAVING
Autoclave is a pressure vessel.
Pressure increases the boiling point of water – higher temperatures can be reached!
121°C 15-20 min 15 psi or 103.5
kPa (most common).
132°C 2-3 min 30 psi or 207 kPa.
Kills all organisms including endospores
A special autoclave cycle called ‘vacuum cycle’
used for dry objects such as dressings. The vacuum inhibits condensation of water so that the objects do not get wet
How to monitor efficiency of autoclave?
Physical indicators: pressure and temperature recording devices,
Chemical indicators: indicators that change colour after being exposed to specific temperatures, such as temperature sensitive tape.
Biological indicators: Bacillus stearothermophilus spores are used, due to their resistance to heat.
ENDOSPORES AND HEAT STERILISATION
Under typical autoclave conditions (121°C, 15-20 min., 15 psi):
Endospores require about 4-5 min. for a decimal reduction.
Vegetative cells require only 0.1-0.5 min. at 65°C.
Water content and the concentration of SASPs (small acid soluble proteins) of the endospore determine heat resistance of endospores.
Low water content + high SASP concentration = high heat resistance.
pH is also an important factor. Acid foods such as tomatoes fruits and pickles are much easier to sterilise than neutral pH foods such as corn and beans.
TYNDALLISATION
Tyndallisation is a method for killing endospores without need for very high temperatures. (Not considered totally reliable and not often used today.)
Step 1: Kill vegetative state cells at 80oC (10 mins)
Step 2: Cool and incubate at 30-37oC – trigger germination.
Step 3: Heat at 80oC (10 mins) – kills all germinated cells (now at vegetative state!)
Step 4: Repeat the cycle 2-3 times.
Used for liquids that support endospore germination.
Used for heat sensitive materials (e.g. vitamins, plant seeds).
PASTEURISATION
Pasteurisation is NOT sterilisation.
Pasteurization isthe process of heating, and then rapidly cooling, liquids or food in order to kill microbes that may expedite their spoilage or cause disease.
Objective is to kill pathogens and increase shelf life
Low temperature pasteurisation: 63°C for 30 min (Milk, ham, fermented sausages e.g. salami).
High temperature Short time (HTST) pasteurisation: 72°C for 15 sec. Refrigerated shelf life of approximately two weeks.
Ultra High temperature (UHT) 135°C for 1-2 sec. provides same level of protection, but along with packaging, increases shelf life to 3 months under refrigeration.
DRY HEAT
Dry heat kills microorganism by oxidizing molecules.
Several types of dry heat sterilisation are:
1- Hot air oven
2- Incineration
3- Open flame
Ovens: 180°C for 1 hr or 160°C for 2 hr or 121°C for 16 hr
Used for glassware, surgical instruments, oils and heat resistant powders.
IRRADIATION
Ionising radiation e.g. gamma radiation or high energy beta particles (electron beams) are used for sterilisation of medical devices and treat wastewater.
Often cobalt 60 high-energy photons are used for gamma radiation.
Used on disposable glass and plastic-ware, pharmaceuticals, sutures, syringes and needles etc.
Lead shielding is needed to protect process workers.
Water can be sterilised by ionising radiation. Ionisation of water forms highly active free radicals such as hydroxyl radicals, ·OH and hydride radicals ·H. . These radicals react with proteins, nucleic acids and lipids – and impair their structure/function – and lead to death of pathogens.
