Describe the Construction and Operation of a Laboratory Autoclave Giving the Respective Sterilization Controls

DISCUSS THE CONSTRUCTION AND OPERATION OF AN AUTOCLAVE IN THE FOLLOWING ASPECTS:

A.    Types of Autoclaves

An autoclave is a pressure chamber that is used to sterilize equipments and supllies by subjecting them to high pressure saturated steam at 121 degrees Celsius for around 15-20 minutes depending on the size of the load and contents.

Three different types of autoclaves are available;

·         Gravity

·         Pre-vacuum

·         Isotherm

i.                    Gravity autoclaves are used in the sterilization of empty and or non-porous containers which are stable to both heat and moisture. Common examples of these items include; liquids, media and solids (i.e. biomedical waste)

ii.                  Pre-vacuum autoclaves draw out a significant portion of the air within the autoclave chamber improving the speed and efficiency of the sterilization process. The removal of air from the chamber allows for the penetration of porous materials which are both heat and moisture stable. Common examples of these items include bench-coat and animal bedding.

iii.                An isothermal autoclave uses a lower temperature in the sterilization of materials which are either heat sensitive or congealable. This sterilization process is commonly used in the pasteurization of items such as media.

B.     Articles  sterilized and precautions

The vast majority of items which are used to store liquids within a typical molecular or microbiology laboratory are comprised of autoclavable materials. However, some materials are manufactured out of materials which cannot be autoclaved either due to their inability to withstand high temperatures or their release of toxic gases when sterilized. Therefore, you must determine if the vessel can be autoclaved or not prior to treatment.

Articles sterilized include;

·         Culture media – bacterial, viral and fungal cultures

·         Contaminated items such as petri dishes, tips, tubes, gloves etc

·         Glassware, media, aqueous solutions and specific equipment

·         Culture dishes and related items

Precautions

i.                    Articles should not be tightly packed, do not overload the autoclave

ii.                  Air discharge must be complete leaving no residual air trapped inside.

iii.                Caps of bottles and flasks should not be tightly screwed on.

iv.                The autoclave must not be opened until the pressure has fallen or else the contents will boil over.

v.                  Articles must be paper to prevent drenching, bottles must not be overfilled.

C.    The Sterilization Process

A simple autoclave has vertical or horizontal cylindrical body with a heating element, a perforated try to keep the articles, a lid that can be fastened by screw clamps, a pressure gauge, a safety valve and a discharge tap. The articles to be sterilized must not be tightly packed. The screw caps and cotton plugs must be loosely fitted. The lid is closed but the discharge tap is kept open and the water heated. As the water starts boiling, the steam drives air out of the discharge tap. When all the air is displaced and steam start appearing through the discharge cap, the tap is closed.

The pressure inside is allowed to rise up to 15 lbs per square inch. At this pressure the articles are held for 15 minutes, after which the heating is stopped and the autoclave is allowed to cool. Once the pressure gauge shows the pressure equal to atmospheric pressure, the discharge tap is opened to let the air in. The lid is then opened and articles removed.

D.    Sterilization Control

I.                   Physical method includes automatic process control, thermocouple and temperature chart recorder.

II.                Chemical method includes Browne’s tube No. 1 (black spot) and succinic acid (whose melting point is 121̊C) and Browie Dick tape is applied to articles being autoclaved. If the process has been satisfactory, dark brown stripes will appear across the tape.

III.             Biological method includes a paper strip containing 106 spores of Geobacillus stearothermophilus.

E.     Advantages

1.      Very effective way of sterilization.

2.      Quicker than hot air oven.

F.     Disadvantages

1.      Drenching and wetting of articles may occur

2.      Takes long time to cool

3.      Trapped air may reduce the efficacy

References;

1.      Seymour Stanton Block (2001). Disinfection, Sterilization and Preservation. Lippincott Williams & wilkins. ISBN 978-0-683-30740-5. Retrieved 19 January 2013

2.      Le, R.N., et al (2005), Autoclave Testing in a University setting. Applied Biosafety, 10(4), 248-252.