Aspect Techniques

Aseptic Techniques

Before inoculation with the desired microorganisms, microbiological media and all materials coming into contact with it must be sterile. During any subsequent handling of thebacterial cultures, unwanted or contaminant organisms must be excluded employing aseptic techniques.

Sterilisation implies the complete destruction of all microorganisms including spores, this isaccomplished by the use of heat, chemicals, radiation, filtration.


Heat
Denatures and coagulates vital proteins. There are various forms of heat sterilisation.

Red Heat
Inoculating wires orloops are sterilised by holding them in a Bunsen flame until they are red hot.

Moist Heat
Bacteria are more readily destroyed by moist heat (steam) than dry heat. Usually used for the sterilisationof culture media, aqueous solutions and the destruction of discarded cultures. Air must first be removed in order to achieve the 121 °C necessary for successful sterilisation. This is accomplished bythe use of an autoclave (the technical version of a pressure cooker), which follows automatic cycles of heating under pressure for the required time.

Dry Heat
Usually employed for materials whichcould either be corroded by steam or must remain dry before use. These include metal instruments, glass petri dishes, flasks and pipettes and cotton wool. In practice, dry heat sterilisation requireslonger time intervals and higher temperatures than steam sterilisation, e.g.steam sterilisation 121°C for 15mins or dry heat sterilisation 160°C for 120 minutes.


Chemical
Usually employed fordelicate equipment such as optical instruments and electrical devices which would be damaged by heat. Due to the toxicity of the chemicals used, this is not the most popular form of sterilisation.Chemicals employed include: gaseous ethylene oxide, which alkylates amino, sulfhydryl, carboxyl and hydroxyl groups of microbial cell compounds; formaldehyde, used as a fumigant; and hydrogen peroxide...