Sterility techniques

Question 1

Define sterility.

Answer 1

Absence of viable micro-organisms.

Question 2

What is the sterility assurance level?

Answer 2

The probability of survival of micro-organisms, no more than 1 micro-organism in 10^-6 vials.

Question 3

Give five examples of substances that are required to be sterile.

Answer 3

Injections
Ophthalmic preparations
Implants
Dressings
Surgical ligatures

Question 4

What are the five methods of sterilisation?

Answer 4

Steam sterilisation
Dry heat
Ionising radiation
Gas sterilisation
Filtration

Other methods include formaldehyde use with steam and UV light

Question 5

What are the two types of heat sterilisation?

Answer 5

Dry heat sterilisation and moist heat sterilisation

Question 6

When is dry heat sterilisation usually used?

Answer 6

For glassware, metal surgical equipment and non-aqueous thermostable liquids and
thermostable powders

Question 7

What instrument is used for dry heat sterilisation?

Answer 7

A hot air oven with perforated
shelves to allow for heat & air flow, or heat sterilizing tunnels

Question 8

What temperature and time should the oven reach for sterilisation?

Answer 8

160 degrees for 2 hours
170 degrees for 1 hour
180 degrees for 30 minutes

Question 9

What must you remember when packing items into the oven?

Answer 9

Unscrew any lids to avoid bursting
Pack evenly throughout the oven
Containers should be sealed or covered

Question 10

What are the conditions of the desterilisation of pyogenes?

Answer 10

250 degrees for 45 minutes

Question 11

How does dry heat sterilisation work?

Answer 11

The dry heat sterilization process is accomplished by conduction; that is where heat is absorbed by the exterior surface of an item and then passed inward to the next layer. Eventually, the entire item reaches the proper temperature needed to achieve sterilization.

Question 12

What are two advantages and two disadvantages of dry heat sterilisation?

Answer 12

Advantages:
Good for moisture sensitive products
Less damaging to glass and metal equipment

Disadvantages:
Drastic conditions are not tolerated by certain packaging
material e.g., plastics/rubber
* Not suitable for surgical dressings
* Does not penetrate as well as moist heat
* Natural moisture in fibres vaporises and causes
deterioration

Question 13

How does moist heat sterilisation work?

Answer 13

It destroys micro-organisms by denaturing them with steam under pressure

Question 14

What are the advantages and disadvantages of moist heat sterilisation?

Answer 14

Terminal sterilisation process for aqueous solutions or
suspensions
* Wide safety margin
* Kills bacteria and viruses

Disadvantages:
Only useful for thermostable products
* Can not be used for products with oily bases
* Water is essential

Question 15

What are two gases used in gas sterilisation?

Answer 15

Ethylene oxide (CH2 )2O, and formaldehyde

Question 16

How is the explosive nature of ethylene oxide reduced?

Answer 16

It is presented as a 10% mix with carbon dioxide or 8.6% mix with hydrochlorofluoro carbon) which has replaced fluorinated hydrocarbons (freons).

Question 17

Why is a sterilising humidifier required when using ethylene oxide?

Answer 17

Organisms are more resistant to ethylene oxide in the dried state so there must be a minimum level of moisture.

Question 18

What is the mechanism of action of gas sterilisation?

Answer 18

Gas sterilisation causes the alkylation of sulphydryl, amino, hydroxyl and
carboxyl groups on proteins and imino groups of nucleic acids.

Question 19

What is the procedure for gas sterilisation?

Answer 19

Gas introduced after chamber evacuation
* Humidity required (pulsed steam)
* Gas mix exhausted and rendered harmless

Question 20

When is gas sterilisation normally used?

Answer 20

Re-usable surgical instruments, certain medical,
diagnostic and electrical equipment, and the surface sterilization of powders. Alternative for radiation sterilisation.

Question 21

Describe the sterilisation procedure involving ethylene oxide.

Answer 21

Firstly remove air using a vacuum.
Next add steam to reach the desired moisture (30% RH optimal anti-microbial activity).
Then sterilise by adding ethlyene oxide and set both the time and pressure.
After remove the vacuum and fit the air filter for air flushing.
Total air replacement.

Question 22

What are three advantages and three disadvantages of gas sterilisation?

Answer 22

Few materials damaged minimal heat, minimal moisture
* Good penetration in porous loads will penetrate rubber & plastics
* Effective against all micro-organisms : bacteria, viruses, moulds

Disadvantages:
Gas is toxic and flammable
Costs are higher than dry heat
Residues need disposal

Question 23

What are two advantages and two disadvantages of using radiation sterilisation?

Answer 23

Effective against a large number of bacteria, yeasts, moulds and some
viruses
* Terminal technique
* Used for surgical instruments, sutures, prostheses
* Dry pharmaceutical products (powders)

Disadvantages:
Not effective against some viruses
Cost, side effects

Question 24

How does anti-microbial resistance decreases with ionising radiation?

Answer 24

Resistance decreases with the presence of moisture, dissolved oxygen or with elevated temperatures.

Question 25

Describe the two types of radiation sterilisation.

Answer 25

Ionizing radiation is the use of short wavelength, high-intensity radiation to destroy microorganisms. This radiation can come in the form of gamma or X-rays that react with DNA resulting in a damaged cell. Non-ionizing radiation uses longer wavelength and lower energy. As a result, non-ionizing radiation loses the ability to penetrate substances, and can only be used for sterilizing surfaces. The most common form of non-ionizing radiation is ultraviolet light.

Question 26

What size of membrane filters are normally used for filter sterilisation?

Answer 26

0.2-0.22 µm nominal pore
diameter are mainly used.

Question 27

Describe a depth filter.

Answer 27

Made of asbestos & wood cellulose,
sintered glass, sintered ceramic; and not
absolutely retentive
Multiple layers, trap micro-organisms within
structure (used as pre-filter)
Fibrous material may shed

Question 28

Describe a screen filter.

Answer 28

cellulose acetate
 Screen filters are absolutely retentive
 Suitable integrity tests required
 Before use – check for defects

Question 29

Should you prolong filtration?

Answer 29

No as bacteria may accumulate on filter

Question 30

What bacterium can’t be collected by filters?

Answer 30

• Filter will not remove viruses
• Filter will not remove mycoplasms
• Form of bacteria with no cell wall
• Filters will not remove some G-ve “stunted forms”

Question 31

What are some of the advantages of filter sterilisation?

Answer 31

Remove bacteria dead bodies (endotoxins)
Used for thermolabile products

Question 32

What are some disadvantages of filter sterilisation?

Answer 32

Not suitable for suspensions
Not a terminal process
Sterility tests required so 7-14 day delay
Viruses and mycoplasms not removed