NTA Level 5 • Semester 1 • PST05103

Sterilization Methods and Criteria for Selection

Pharmaceutical Microbiology • Source Session/Topic 44

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Session 44: Sterilization Methods and Criteria for Selection

Total Session Time: 120 minutes

Pre-requisites

• Human anatomy and physiology

Students Learning Tasks

By the end of this session students are expected to be able to:

• List methods used for sterilization

• Explain (dry and moist) heat sterilization

• Explain gaseous sterilization

• Explain radiation sterilization

• Explain sterilization by filtration

• List the criteria for selecting sterilization method

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

• Handout 48.1: Sterilization Methods

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Introduction to Sterilization (15 minutes)

• Microorganisms cause contamination, infection, and decay

• In order to prevent contamination of pharmaceuticals and infection, it

is necessary to remove microorganisms

• Sterilization is the process of killing or removing all viable

organisms by by physical, chemical and mechanical means.

• Physical sterilization employs the use of heat and radiation to kill

the microorganisms

• Chemical sterilization uses chemical agents in form of liquid or gases

to kill the microorganisms

• Mechanical methods involve removal of microorganisms by filtration

• In sterilization all micro-organisms both pathogenic and non-

pathogenic including spores are killed or removed.

• All sterilization methods must be validated and monitored e.g.

bacillus stearothermophilus is used to monitor moist heat

sterilization process

STEP 3: Heat Sterilization (30 minutes)

Heat Sterilization

• Heat sterilization is the killing of all organisms from an item by the

use of heat.

• Heat sterilization is the safest and most common method of

sterilization

• Types of heat sterilization

o Dry heat sterilization

o Moist heat sterilization

Dry Heat Sterilization

• Dry heat sterilization involves use of dry heat to kill

microorganisms. Organisms are killed by oxidation effects

• Dry heat sterilization is divided into flaming, incineration and hot

air sterilization

o Flaming

▪ It is the oldest and the simplest method of sterilization

▪ The item to be sterilized is held in an open flame.

▪ Example of sterilization by flaming is the heating of a loop

wire to a red glow on a Bunsen flame before using it to

transfer microorganisms

[pic]

▪ Advantages of Flaming Method

• Flaming is a convenient way of killing many, but not

necessarily all, microorganisms

• It can be used to kill all microorganisms in meat

▪ Disadvantages of Flaming

• Does not necessarily kill all microorganisms

o Incineration

▪ This is an excellent method of destroying or disposing of

materials such as heavily contaminated cloth, animal carcasses

and pathological materials

[pic]

▪ Advantages

• Incineration destroys 100% of microorganisms and reduces

the volume of waste significantly.

• It's also the ideal way to prevent contaminated items from

being reuses

▪ Disadvantages

• Incineration completely the object/item being incinerated

• It is expensive method (repair, maintenance and operation)

o Hot air sterilization (Hot air oven)

▪ Is a sterilization method in which organisms are killed by

oxidation effects

▪ In this process, the various items need to be sterilized are

duly kept in an electric oven, preferably with a stainless-

steel chamber inside, and duly maintained at 170°C for a

duration of approximately 2 hours (to ensure complete

sterilization)

▪ Dry heat sterilization requires raising the temperature of an

object to a controlled 170° C for approximately 2 hours to kill

all microorganisms

▪ The items to be sterilized is exposed at higher temperature

for prolonged duration of time

▪ Dry heat is the ideal way to kill microorganisms attached to

fats, oils and powders, which can't be sterilized in an

autoclave because they either resist water or are destroyed by

it.

▪ The Hot Air Oven is the equipment used for this type of

sterilization. This oven has;

• A thermostat controlling the temperature

• Double walled insulation keeps the heat in and conserves

energy

▪ Hot air oven is used for sterilization of;

• Medical equipment resistant to heat e.g. forceps,

scissors, scalpels, swabs

• Thermal stable and moisture-labile pharmaceuticals

products

o Nonaqueous Liquids (Liquid paraffin, fats and grease

o Solids (powders)

Moist Heat Sterilization

• This is a sterilization method that uses heat in moisture to kill

microorganisms

• In this method microorganisms are killed by coagulating or

denaturation of their proteins

• Moist heat sterilization methods include boiling, autoclaving and

pasteurization

• Boiling

o Boiling at 100°C at 760 mm atmospheric pressure kills particularly

several varieties of vegetative states of microbial strains, many

viruses and fungi usually within a period of 10 minutes

o This method cannot destroy endospores and certain viruses

o This method is mainly used for making food products safer for human

consumption

• Autoclaving

o This is a sterilization method that is operated in a special

equipment called Autoclave

o The process of moist heat sterilization is also known as

Autoclaving

o Items to be sterilized are heated and maintained at temperatures of

120 ± 2°C and high positive pressure in an autoclave

o Autoclaving is used for items/products that are not sensitive to

moisture and thermostable

An autoclave

[pic]

Schematic diagram of an autoclave

[pic]

o Uses of autoclave

▪ To sterilize culture media for the identification and

propagation of pure strains of microorganisms and yeasts.

▪ To sterilize various surgical stainless steel instruments

that are required for most of the surgical procedures, dental

procedures, obstretrics etc.

▪ To sterilize various types of surgical dressings, gauzes,

sutures etc.

▪ To sterilize a host of IV applicators, equipment, solutions,

and syringes

▪ To sterilize transfusion equipment(s) and a large number of

other allied items that can conveniently withstand high

pressures and temperatures

|Activity: Take home Assignment (10 minutes) |

| |

|DIVIDE students in groups or individual. |

| |

|ASK the students to work on the following assignment |

| |

|List the advantages and disadvantages of; |

|Moist heat sterilization |

|Dry heat sterilization |

| |

|ALLOCATE time for students to do the assignment and submit |

| |

|REFER students to recommended references |

STEP 4: Gas Sterilization (20 minutes)

• Gaseous sterilization is the use of reactive gases to kill all

microorganisms

• The reactive gases used are commonly ethylene oxide and formaldehyde

o Ethylene oxide is more common than formaldehyde

• Uses of gas sterilization

o Sterilization of reusable surgical equipment

o Some medical diagnostic and electrical equipment

o Surface sterilization of powders

• Ethylene oxide sterilization

o Recognized by pharmacopoeia e.g. the BP

o It is supplied as 10% mixture with CO2 to reduce possibility of

explosion

o The gas kill microorganisms by alkylation reaction

▪ The reaction takes place in a gas sterilizer

[pic]

o Disadvantages of Ethylene oxide

o low penetrating power

o pungent

o inflammable

o toxic

o carcinogenic

o irritation of skin and mucous membrane e.g. conjunctiva and nasal

mucosa

• Formaldehyde

o This gas is obtained by heating formalin to a temperature of 70 to

75°C

o Has similar toxicity to ethylene oxide

o Has lower absorption to material

o In its sterilizer, it operates at sub-atmosperic pressure steam

STEP 5: Radiation Sterilization (15 minutes)

• This is a method of sterilization that uses radiation energy to kill

microorganisms by disrupting their genomes

• Radiations used in sterilization are ionizing radiation and non

-onizing radiation

• Ionizing radiation

o This employ γ-rays, X-rays and high energy electron beams

▪ High energy electron beams are used in sterilization of

pharmaceuticals, disposable dental materials and disposable

medical supplies (e.g. plastic syringes, catheters, surgical

gloves and suturing materials)

• Non-ionizing radiation

o Employs UV light which is useful to control microorganisms in the

air and in drinking water for homes, hospitals and public places.

STEP 6: Sterilization by Filtration (15 minutes)

• Sterilization by filtration is the process of removing particles from

a solution by allowing the liquid to pass through a membrane or other

particle barrier

• Sterilization filtration removes bacteria from heat labile liquids

such as sera and solutions of sugar and antibiotics

• Filters used in sterilization include;

o Asbestos filters

▪ used for purification of water

o Sintered glass filter

o Membrane filters

▪ used for water purification and analysis

▪ used in sterilization and sterility testing

▪ used in preparation of solutions for parenteral use

• High-Efficiency Particulate Air (HEPA) Filters

o These are filters are mostly used to remove practically all

microbes that happen to be larger than 0.3 μm in diameter in the

air

o HEPA filters are used in;

▪ Intensive care units (ICU) in specialized hospitals treating

severe cases e.g. severe burns

▪ In sterile zones of antibiotic preparations, packaging, IV

injections, and sensitive sterile preparation

STEP 7: Criteria for Selecting Sterilization Method (10 minutes)

• No one sterilization method is suitable in every circumstances

• So methods must be selected based on the following criteria;

o Purpose of sterilization

▪ Lab sterilization, hospital sterilization, pharmaceutical

sterilization all have different requirements

o The items/products to be sterilized

▪ Heat sensitive, moisture labile, sensitivity to gas, or

chemicals are all different for different items

o Nature of contaminating microorganisms that need to be removed or

killed

▪ Bacteria, bacterial spores, viruses, fungi all have different

resistance to the method being chosen

o Time

▪ Some methods need longer time than others to complete

sterilization

o Safety

▪ Some methods are not safe in certain circumstances e.g. the

use of Ethylene oxide in pharmaceutical products

o Budget

▪ Cost of sterilization method differ from one to the other

method

• [pic]REFER Students to Handout 48.1: Sterilization Methods for

reference

STEP 8: Key Points (5 minutes)

• Sterilization is the absence of all form of life. It involves killing

or removing all microorganisms from an item or product.

• Sterilization methods are grouped into physical, chemical and

mechanical methods

• Physical methods are heat and radiation sterilization

• Chemical methods include gas sterilization while mechanical

sterilization include filtration method.

• Choice of a method for sterilization depends on a number of factors or

criteria

• Criteria for selection of sterilization method include type of

contaminants, nature of the item/product to be sterilized and purpose

of the sterilization process

STEP 9: Evaluation (5 minutes)

• What is a sterilization?

• What are the uses of radiation sterilization?

• What are disadvantages of sterilization by filtration?

•

References

Hugo and Russell (2011), Pharmaceutical Microbiology 8th Edition, Willey-

Blackwel publications

Karen C. Carroll et al (2013); Jawetz, Melnick and Adelberg’s Medical

Microbiology 26th Ed. McGraw Hill Co. Inc.

Greenwood et al (2012); Medical Microbiology, 18th edition Churchill

Livingstone

|[pic] |Handout 48.1: Sterilization Methods |

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