control of microbial growth
TRANSCRIPT
Control of Micro organisms‐
Sahaya Asirvatham
Prevent contamination
Prevent transmission of pathogen
To prevent decomposition & spoilage of product
To prevent contamination in aseptic areas, processes like production of pharmaceuticals by fermentation.
To maintain aseptic condition in operation theaters, filling area of non sterile pharmaceuticals.
Reasons for Controling Microorganisms
Sterilization Sterilization is defined as the process by which an article, surface or the medium is freed of all living microorganism either in vegetative or spore state.
Essential stage in processing of any product used for parenteral administration, broken skin, mucosal surface or internal organ.
Fundamentals of Microbial Control
Important terms:
Thermal death point (TDP) It is the lowest temperature at which all of the microorganism present in the liquid suspension will be killed in just 10 minutes.
Thermal death time (TDT) It is the minimum length of time whereby all microorganism present in the liquid culture medium will be killed at given temperature.
Fundamentals of Microbial Control
Disinfection: A treatment that reduces the total number of microbes on an object or surface, but does not necessarily remove or kill all of the microbes
Sanitation: Reduction of the microbial population to levels considered safe by public health standards
Antiseptic: A mild disinfectant agent suitable for use on skin surfaces
–cide: to kill Bactericide: chemical that destroys bacteria
Fungicide: a chemical that can kill fungal spores, hyphae, and yeastsVirucide: a chemical that inactivates virusesSporicide: can destroy bacterial endosporesGermicide and microbicide: chemical agents that kill microorganisms
Stasis and static: to stand still
Bacteristatic: prevent the growth of bacteriaFungistatic: inhibit fungal growth
Bactericidal Vs Bacteristatic
Death (in terms of microorganisms)
Irreversible loss of ability to reproduce.
Determined by inoculating culture to the medium
Pattern of Death in a Microbial Population
Bacterial populations die at a constant logarithmic rate
Microorganisms were previously considered to be dead when they did not reproduce in conditions that normally supported their reproduction
However organisms can be in a viable but nonculturable (VBNC) condition
Once they recover they may regain the ability to reproduce and cause infection Pattern of Microbial death-an exponential
plot of survivors against mins of exposure to heating at 121 0C.
Conditions affecting Antimicrobial Activity
1. Type of agent: Physical or chemical
2. Nature of heat
3. Concentration, Dose, Time & temperature of the agent
4. Bioburdon
5. Mechnism of Action of the agent
6.Environment:Physical & chemical properties of the medium or substance carrying the organismEffectiveness of heat in acid is greater than in alkaliConsistency of the material (viscous: less penetration)Concentration of carbohydrates increases thermal resistancePresence of extraneous organic matter (inactivation of agent or protective effect)Action of chemical agent Increases with increase in temperature.
7. Kind of Microorganism:
Microbial spp. Differe in susceptibility to physical & chemical agentsVegetative cell of spore forming bacteria are more susceptible than sporesSpores are most resistant
8. Physiological State of Cells:
Young actively metabolizing cells are more susceptible In dormant cells the agent which causes damage through the interference with metabolism, non growing cell would not be affected
How Antimicrobial Agents Work: Their Modes of Action??
1. Damage to the cell wall or inhibition of cellwall synthesis
2. Alteration of permeability of cytoplasmic membrane
3. Alteration of physical /chemical state of proteins & nucleic acid
4. Inhibition of proteins & nucleic acid synthesis
Survivor Curve
When exposed to a killing process, populations of microorganismsgenerally lose their viability in an exponential fashion, independent of the initial number of organisms.
Of the typical curves obtained, all have a linear portion which may be continuous (plot A), or may be modified by an initial shoulder (B) or by a reduced rate of kill at low survivor levels (C).
Short activation phase, representing an initial increase in viable count, may be seen during the heat treatment of certain bacterial spores.
Survivor curves have been employed principally in the examination of heat sterilization methods, but can equally well be applied to anybiocidal process.
Fig. 20.1 Typical survivor curves for bacterial sporesexposed to moist heat or gammaradiation.
Expressions of resistance
Dvalue:
The resistance of an organism to a sterilizing agent can be described by means of the Dvalue.
For heat and radiation treatments, respectively, this is defined as the time taken at a fixed temperature or the radiation dose required to achieve a 90% reduction in viable cells (i.e. a 1 log cycle reduction in survivors).
The calculation of the Dvalue assumes a linear type A survivor curve, and must be corrected to allow for any deviation from linearity with type B or C curves.
Expressions of Resistance
In order to assess the influence of temperature changes on thermal resistance a relationship between temperature and log Dvalue can be developed leading to theexpression of a zvalue.
which represents the increase in temperature needed toreduce the Dvalue of an organism by 90% (i.e. 1 log cyclereduction).
For bacterial spores used as biological indicators for moist heat (B. Stearothermophilus) and dry heat (B. subtilis) sterilizationprocesses, mean zvalues are given as 10°C and 22°C, respectively.
The zvalue is not truly independent of temperature but may beconsidered essentially constant over the temperature ranges used in heat sterilization processes.
ZValue:
In food industry unit of lethality has been devised, called as Fvalue.
This is defined as the equivalent in minutes of 1210C of all heat considered with respectto its capacity to destroy spores or vegetative cells of perticular organism
F = D (log N0 – log N)
Where D = Dvalue at 1210C of the organism N0 = Initial population Number/unit volume N = Final Population number/unit volume
It is used to calculate probable number of survivors remaining in a load
FValue:
The inactivation factor is the degree to which the viable population of organisms isreduced by the treatment applied and is obtained by dividing the initial viable countby the final count
IF = 10t/D
Where t = holding time at sterilizing temp. D = Dvalue for the marker organism at that temp.
Inactivation Factor
Sterilization Methods
I. Physical Methods
1. Heat Sterilization methods
Moist Heat
Dry Heat
Low Temperatures
l Moist Heat Sterilization
Mechanism of killing is a combinantion of protein/nucleic acid denaturation and membrane disruption
Effectiveness Heavily dependent on type of cells present as well as environmental conditions (type of medium or substrate)
Bacterial spores are much more difficult to kill than vegetative cells
Methods for Moist heat Sterilization
a) Temperature at 100° C – Boiling
b) Temperature below 100° C – Pasteurization
c) Steam Under Pressure – Autoclave
d) Steam at Atmospheric Pressure Tyndallization
Temperature at 100° C – Boiling
Most vegetative microorganism are killed in 23 mins, but spores make take 23 hrs.
Endospores, protozoan cysts, and some viruses can survive boiling
Pasteurization
Used to reduce microbial numbers in milk and other beverages while retaining flavor and food quality of the beverage
Retards spoilage but does not sterilize
Traditional treatment of milk, 63°C for 30 min
Flash pasteurization (hightemperature short term pasteurization); quick heating to about 72°C for 15 sec, then rapid cooling
Tyndallisation
The media containing sugar and gelatin are exposed to 1000 C for 20 minutes on three successive days is known as tyndallization or intermittent sterilization.
In first exposure it kills vegetative bacteria and spores, since they are in favorable medium, will germinate and killed on subsequent occasion.
Steam under pressure : Autoclave
Principle:
The lethal effect of moist heat is due to the denaturation and coagulation of protein.
When steam makes contact with the cooler articles in the autoclave it condenses into water on their surface and in their interstices.
This condensation has three effects of critical importance for sterilization process
1. It wets the microorganisms on the articles and so provides this essential condition for their killing by moist heat.
2. It liberates the very large latent heat of steam and so rapidly heats up the articles to the sterilizing temperature.
3. It causes a great contraction in the volume of the stream and so promotes the ingress of fresh steam.
Autocalave
Method :
Materials to be sterilized are placed in perforated diaphragm
The testtubes are plugged with non absorbent cotton and covered with paper to avoid drenching of plugs during the release of steam when condensation occurs
After adjusting the water level, lid is kept in position and is closed tightly by means of screw clamps.
Autoclave is switched on, steam valve is kept open until all the air is expelled out and steam comes out.
The steam valve is closed and the pressure begins to rise
As the pressure begins to rise to 15lbs/sq.in (psi), time is noted. One of the electrical coils is switched off (when 2 coils are used) or steam valve is opened partially so as to
maintain the pressure constant at 15psi.
After 15 min the autoclave is switched off and is allowed to cool down
After all the steam has escaped from the open steam valves, autoclave is opened and all the sterilized materials are removed
Pharmaceutical applications:
Sterilization of culture media, apron and rubber tubing etc
Sterilization of injections solution and suspensions.
Sterilization of surgical dressings and fabrics such as cotton wool, gauze swabs, ribbon gauze, masks, and caps etc.
Sterilization of packaging materials such as containers like metal drum, card board boxes and wrappings such as nylon film, paper etc.
Dry Heat Sterilization
Principle: The killing effect of dry heat is because of denaturation of protein, oxidative damage and toxic effect of elevated levels of electrolyte.
Method: there are three methods for dry heat sterilization.
Flaming
Incineration
Hot air oven
Flaming: inoculating loop or wire, tip of forceps and searing spatulas are held in Bunsen flame till they become red hot.
Incineration: it is excellent method for safely destroying materials such as contaminated cloths, pathological materials etc.
Hot Air Oven
It consists of a double walled chamber insulated with asbestos sheet or glass wool to prevent radiation of heat.
The chamber is heated electrically and maintained thermostatically. The temperature can be noted on the thermometer which can be inserted from top.
The hot air oven can be operated at 150˚C for 150 min, 160˚C for 2 Hrs, 170˚C for 60 min and 180˚C for 30 min for sterilization of the articles.
Hot air oven is generally used for sterilization of glassware, metal devices and other articles which are spoiled by autoclaving.
Precautions:
All the glassware must be clean and dry to prevent internal contamination after sterilization.
It is essential to wrap the apparatus before placing them into the oven.
The oven must be loaded when it is cool.
The temperature must not raise above 180˚C in any case otherwise the cotton plugs and papers may be charred.
Before removing the apparatus, oven should be cooled to room temperature.
Pharmaceutical Applications:
It is used to sterilize glassware’s forceps, scissors, scalpels.
It is used to sterilize all glass syringes.
Pharmaceutical product such as liquid paraffin, dusting powder, fats and grease can be sterilize. Sterilization of anhydrous material.
Sterilization of powders.
Decrease microbial metabolism, growth, and reproduction
Chemical reactions occur slower at low temperatures
Liquid water not available
Psychrophilic microbes can multiply in refrigerated foods
Refrigeration halts growth of most pathogens
Slow freezing more effective than quick freezing
Organisms vary in susceptibility to freezing
Refrigeration and Freezing
Dessication is drying (98% of the water is removed) inhibits growth due to removal of water
Lyophilization (freezedrying)
Substance is rapidly frozen and sealed in a vacuumSubstance may also be turned into a powderUsed for longterm preservation of microbial culturesPrevents formation of damaging ice crystals
Dessication and Lyophilization
The use of dessication as a means of preserving apricots
Lyophilization
Ionizing radiation: if the radiation ejects orbital electrons from an atom causing ions to form
Nonionizing radiation: excites atoms by raising them to a higher energy state but does not ionize them
l Radiation as a Microbial Control Agent
Ionizing Radiation
Gamma radiation produced by Cobalt60 source
Powerful sterilizing agent; penetrates and damages both DNA and protein; effective against both vegetative cells and spores
Often used for sterilizing disposable plastic labware, e.g. petri dishes; as well as antibiotics, hormones, sutures, and other heatsensitive materials
Also can be used for sterilization of food; has been approved but has not been widely adopted by the food industry
Mode of action
Radiation can cause both ionization and excitation and their absorption is not affected by structure of the molecule. It may act directly or indirectly.
Direct action: Every microorganism and living cell having target region which is radiation sensitive, a single ionization of radiation in this sensitive zone or region will kill the microorganism.
Indirect effect: Absorption of radiation by water, within or surrounding living cell produces free radicals. these are powerful oxidizing and reducing agent capable of damaging essential molecule and therefore causing death.
Sterilization with Ionization; Radiation machine which uses Cobalt 60 as a Gamma radiation to sterilize fruits, veg, fish, meat, etc..
Applications
Sterilization of disposable surgical materials and equipments ex. Plastic syringes, catheters, hypodermic needle and scalpel blade.
Sterilization of adhesive dressings.
Sterilization of single application capsule of eye ointment.
Sterilization of catgut
Sterilization of packaging materials such as plastic films and aluminium foil.
Sterilization of bacterial and viral vaccines.
Used in radiotherapy of cancer.
Increased shelf life of food achieved by Ionizing Radiation
Ultraviolet Radiation
DNA absorbs ultraviolet radiation at 260 nm wavelength
This causes damage to DNA in the form of thymine dimer mutations
Useful for continuous disinfection of work surfaces, e.g. in biological safety cabinets
NonIonizing Radiation
Ultraviolet Radiations
Method: The effectiveness of the method depends on the wavelength absorbed by the particular region or component of microbial cell.
260nm is the wavelength giving 100% effectiveness because it is the adjacent wavelength strongly absorb by nucleoprotein.
Applications
Irradiation of incoming and internal air of sterile filling area of antibiotic plants.
Sterilization of thermolabile products.
Improvement of bacteriological water used to manufacture non sterile pharmaceuticals.
As aid to asepsis in manufacturing houses and hospitals.
To prevent cross infection in hospitals and schools
Modes of Action of Ionizing Vs Nonionizing Radiation
Filters are used to sterilize these heatlabile solutions.
Filters simply remove contaminating microorganisms fromsolutions rather than directly destroying them.
The filters are of two types:
(a) Depth filters
(b) Membrane filters.
Filtration
Consist of fibrous or granular materials that have been bonded into a thick layer filled with twisting channels of small diameter.
The solution containing microorganisms is sucked in through this layer under vacuum and microbial cells are removed by physical screening or entrapment and also by adsorption to the surface ofthe filter material.
Depth filters
Depth filters are of the following types:
1. Candle filters:
These are made up of
(a) diatomaceous earth
(b) unglazed porcelain
They are available in different grades of porosity and are used widely for purification of water for drinking and industrial uses.
2. Asbestos filters
Made of asbestos such as magnesium silicate.
Eg: Seitz and Sterimat filters
These are disposable and singleuse discs available in different grades.
They have high adsorbing capacity and tend to alkalinize the filtered fluid.
Their use is limited by the carcinogenic potential of asbestos.
3.Sintered glass filters
These are made up of finely powdered glass particles, which are fused together.
They have low absorbing property and are available in different pore sizes.
These filters, although can be cleaned easily, are brittle and expensive.
Membrane Filters
Porous membranes with defined pore sizes that remove microorganisms primarily by physical screening.
These filters are circular porous membranes and are usually 0.1 mm thick.
Although a wide variety of pore sizes (0.015–12μm) are available, membranes with pores about 0.2μm are used, because the poresize is smaller than the size of bacteria.
This has replaced Depth Filters.
Membrane Filter Sterilisation
Filtration equipment used for microbial control
The role of HEPA filters in Biological Safety Cabinets
HighEfficiency Particulate Arresting (HEPA) air filters are used in medical facilities, automobiles, aircraft, and homes.
The filter must remove 99.97% of all particles greater than 0.3 micrometer from the air that passes through.