9/30/2020

Chemical Sterilization.

 

Introduction:

  • Definition: Sterilization is defined as complete removal of microorganisms from an object, surface or a product.

  • Chemical Sterilization Causes of removal of microorganisms by the use of chemical bactericidal agents.

  • Although the physical methods of sterilization are more appropriate it is not possible to use them in certain conditions e.g. in case of thermolabile materials, in such cases Chemical sterilization is preferred.

  • However compatibility between chemical agent and substance to be sterilized must be first taken into the consideration.

  • It is important to follow safety rules in the workplace safety during the use of chemical agents.

  • The chemical method of sterilization can be classified as,

    •  liquid sterilization.

    • gaseous sterilization

Gaseous Sterilization:

Ethylene oxide:

  • Ethylene oxide (EO) gas is a common gas used for chemical sterilization because of its wide range of compatibility.

  • This method is a widespread method used for almost 70% of all sterilizations and around 50% for disposable medical devices.

  • Mechanism of antimicrobial action: 

    • Causes alkylation of sulfhydryl, amino, hydroxyl, and carboxyl groups on proteins and imino groups of nucleic acids.

  • EO sterilization is usually conducted at the temperature range of 30-60°C for several hours which aids in the activity of the gas.

  • The efficacy of the gas depends on the concentration of gas available for each article which is greatly assisted by the good penetrating nature of the gas.

  • Ethylene oxide kills all known microorganisms, such as bacteria (including spores), viruses, and fungi (including yeasts and molds), and is compatible with almost all materials even when repeatedly applied.

  • Drawback:

    •   Gas gets absorbed by the material, and the treated articles need to undergo a process of desorption to remove the toxic residual wastes.

    • Organisms are more resistant to ethylene oxide treatment in a dried state.




Formaldehyde:

  • Formaldehyde is another important highly reactive gas which is used for sterilization.

  • This gas is obtained by heating formalin (37%w/v) to a temperature of 70-80°C.

  • It possesses broad-spectrum biocidal activity and is used in the sterilization of reusable surgical instruments, specific medical, diagnostic and electrical equipment, and the surface sterilization of powders.

  • Mechanism of antimicrobial action: 

    • Causes alkylation of sulfhydryl, amino, hydroxyl, and carboxyl groups on proteins and imino groups of nucleic acids.

  • Has poor penetration power as compared to the ethylene oxide.

  • As a result of the low penetrating power, its use is often limited to paper and cotton fabrics.

  • Formaldehyde can generally be detected by smell at concentrations lower than those permitted in the atmosphere and thus can be detected during leakage or other such accidents.

Nitrogen dioxide (NO2):

  • Nitrogen dioxide is a rapid and effective sterilizing agent that can be used for the removal of common bacteria, fungi, and even spores.

  • NO2 has a low boiling point (20°C) which allows a high vapor pressure at standard temperature.

  • This property of NO2 enables the use of the gas at standard temperature and pressure.

  • Mechanism of Action: 

    • The biocidal action of this gas involves the degradation of DNA by the nitration of phosphate backbone.

  • No toxic residue formation.

Ozone:

  • Ozone is a highly reactive gas that is commonly used to sterilize air and water and as a disinfectant for surfaces.

  • Ozone is a potent oxidizing agent.

  • High reactivity of ozone allows the removal of waste ozone by converting the ozone into oxygen by passing it through a simple catalyst.

  • However, because ozone is an unstable and reactive gas, it has to be produced on-site, which limits the use of ozone in different settings.

  • It is also very hazardous and thus only be used at a concentration of 5ppm, which is 160 times less than that of ethylene oxide.

Liquid Sterilization:

  • Liquid sterilization is the process of sterilization which involves the dipping of equipment in the liquid agent to kill all microorganisms and their spores.

  • Although liquid sterilization is not as effective as gaseous sterilization, it is appropriate in conditions where a low level of contamination is present.

  • Various liquid chemicals used for liquid sterilization includes the following:

Hydrogen peroxide:

  • Hydrogen peroxide is a liquid chemical sterilizing agent which is a strong oxidant.

  • It is useful in the sterilization of heat or temperature-sensitive equipment.

  • H2O2 has a short sterilization cycle time as these cycles are as short as 28 minutes where ethylene oxide has cycles that are as long as 10-12 hours.

  • Drawbacks:

    • low material compatibility, 

    • lower penetration, 

    • associated health risks.

  • Vaporized hydrogen peroxide (VHP) is used to sterilize largely enclosed and sealed areas, such as entire rooms and aircraft interiors.

Glutaraldehyde:

  • Glutaraldehyde is an accepted liquid sterilizing agent which requires comparatively long immersion time

  • For the removal of all spores, it requires as long as 22 hours of immersion time.

  • The presence of solid particles further increases the immersion time.

  • The penetration power is also lower as it takes hours to penetrate a block of tissues.

  • The use of glutaraldehyde is thus limited to certain surfaces with less contamination.


Hypochlorite:

  • Hypochlorite solution, which is also called liquid bleach, is another liquid chemical that can be used as a disinfectant.

  • Submerging devices for a short period in liquid bleach might kill some pathogenic organisms but to reach sterilization submersion for 20-24 hours is required.

  • It is an oxidizing agent and thus acts by oxidizing organic compounds which results in the modification of proteins in microbes which might ultimately lead to death.

  • Appropriate concentrations of hypochlorite can be used for the disinfection of workstations and even surfaces to clean blood spills and other liquids.


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9/29/2020

Sterilization by Filtration.

 

Introduction:

  • Definition: Sterilization is defined as complete removal of microorganisms from an object, surface or a product.

  • Sterilization by filtration is the preferred method of sterilizing thermolabile liquid and gases. 

  • It simply removes microbes instead of killing them. 

  • It is the method of choice for sterilizing antibiotic solutions, toxic chemicals, radioisotopes, vaccines, and carbohydrates, which are all heat-sensitive.

Mechanism of Filtration Sterilization:

  • Filters work by physically trapping particles larger than the pore size and by retaining somewhat smaller particles via electrostatic attraction of the particles to the filters. 

  • Besides porosity, other factors also influence the efficiency of filtration, they are:

    • electric charge of the filter

    • electric charge carried by the organisms

    • nature of the fluid being filtered

Biological indicator: 

  • Micromonospora  diminuta.

Procedure:

The solutions to be sterilized is passed through the filter and collected in the sterile receiver by the application of positive pressure to the nonsterile compartment or negative pressure to the sterile side.

Precautions to be taken:

  1. Avoid excessive positive or negative pressure.

  2. Prolonged filtration must be avoided to prevent the growth of a contaminant through the filter media and entry into the sterile solution.

  3. Filter elements of porcelain or of sintered materials those are used repeatedly should be tested for cracks or leaks prior to each use.

  4. Fiber shedding filters such as asbestos filters should be avoided. If, however, the use of this type of filters is unavoidable, a fiber-retaining filter should be used downstream of the asbestos filter.

  5. The effective pore size of the sterilizing filters should be confirmed before use and the continued integrity of the filters should be confirmed after use.

  6. The process of sterilization must be validated by monitoring the microbial load in the solution to be filtered.

Advantages of sterilization by filtration:

  1. Thermolabile solutions can be sterilized.

  2. It removes all the living microorganisms.

Disadvantages of sterilization by filtration:

  1. Filters may break down suddenly or gradually on use.

  2. Sterility testing is compulsory on the filtered solution.

  3. Filter media may be absorbed on the filter surface.

  4. Viruses are not removed by filtration.

MEMBRANE FILTERS

  • They are made of cellulose-derivative (acetate or nitrate). 

  • They are 150μm thick.

  • They are very fine and are fixed in some suitable holders.

  • Nominal pore size is 0. 22  plus, minus 0. 02μm or less is required.

  • The membranes are brittle when dry, in this condition they can be stored for years together. 

  • They become very tough when dipped in water.

  • They are sterilized by autoclaving or by ethylene oxide gas.

  • They cannot be sterilized by dry heat as they decompose above 120°C.

  • They are suitable for sterilizing aqueous and oily solutions.

  • Not suitable for organic solvents such as alcohol, chloroform etc.

  • Membrane filters are generally blocked by dirt particles and organisms.

  • Pre-filtration (through glass-fibre paper prefilter) reduces the risks of blockage of the final filter.

  • Examples of membrane filters:

    • MF-Millipore – it is a mixture of cellulose esters

    • Sartorius Regular ¬¬– it is made of cellulose nitrate

    • Gelmen Triacetate Metricel – cellulose triacetate

5) Suspensions and oils cannot be sterilized by this method due to their heavy load of particulate matters  and viscosity.



SINTERED (or FRITTED) GLASS FILTERS:

  • Also called “Fritted Glass Filter” or “Morton filters”.

  • Borosilicate glass is finely powdered in a ball-mill and the particles of required size are separated. 

  • This is packed into disc mounted and heated till the particles get fused. 

  • The discs thus made have a pore size of 2 micrometer and are used for filtration.

  • They are cleaned with the help of sulfuric acid.

  • Low absorptive properties hence good.

  • Brittle and very expensive.

Seitz Filter (Asbestos Filter):

  • Disposable, single use discs made up of Asbestos (Magnesium trisilicate).

  • Pore size ranges from 0.01 to 5 microns.

  • It is supported on a perforated metal disc inside a metal funnel.

  • After single use the filter is discarded and then the entire assembly is sterilized.


Filter Candles:

  • Made up of porous porcelain or kieselguhr.

  • Usually available as cylindrical candles with thick walls.

  • They were fitted to the filter assembly and placed in a mantle.

  • The liquid to be filtered is then poured into the mantle.

  • The vacuum pushes the liquid through the filter.

  • Filtrate is collected after removal of candles.

  • Gets easily clogged, requires pressure for filtration.



HEPA filter:

  • High Efficiency Particulate Air Filter.

  • A typical HEPA filter is a single sheet of borosilicate glass fiber that has been treated with a water-repellent binder

  • The filter, folded to increase the overall surface area, is mounted inside a rigid, supportive frame. 

  • HEPA filters come in various shapes and sizes, from several square centimeters for vacuum cleaners to several square meters for biological containment hoods and room air systems.

  • HEPA filters typically remove 0.3 μm test particles with an efficiency of at least 99.97% including most microorganisms, from the airstream.

  • Used to maintain aseptic areas and clean rooms.


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