All microbiological waste is required to be sterilised using a pressure cooker or autoclave before disposal. This process of sterilisation using steam under pressure is carried out at a temperature of 121oC for 15-20 minutes at 15psi pressure. Plastic Petri dishes are commonly made from clear polystyrene plastic which are heat resistant up to 80oC1. They are regarded as a use once disposable item and under sterilisation conditions in an autoclave or pressure cooker they will deform.
The best way to sterilise Petri dishes containing agar inoculated with microorganisms in an autoclave or pressure cooker is by placing them into a bag that will withstand the sterilisation conditions and contain the treated contents. Schools have the option of using two different types of bags:
- Autoclavable biohazard bags which are available from scientific suppliers, see the Science ASSIST School science suppliers list. These are made from a heavy duty plastic e.g. polypropylene marked with the international biohazard symbol and usually have the word autoclavable written on them. They are available in a variety of sizes.
- Oven bags which can be purchased from supermarkets. These are also available in different sizes.
If you are unsure if your biohazard bags are suitable for autoclaving we suggest that you contact the supplier for advice.
Procedure for using an autoclavable biohazard or oven bag for sterilising inoculated agar in plastic Petri dishes:
- Loosely pack agar plates into bags to no more than 2/3 full. This will ensure that the steam during sterilisation will penetrate the entire load. Bags that are tightly filled to capacity will not allow effective steam penetration and the contents will not be sterilised even if all sterilisation parameters are met.
- Make sure there are no sharp objects present that may puncture the bag.
- Loosely tape shut the bag leaving an opening of about 5-6cm to allow good steam penetration. This can be done with autoclave tape or a rubber band. Never tightly close the bags as they are impervious to steam and therefore the temperature of the inside of the bag will not be sufficient for sterilisation.
- It is advisable to place the bag into a secondary container within the steriliser to prevent any leakage into the steriliser should the bag rupture. The container must be able to withstand the autoclaving conditions.
- Do not overload the steriliser with too many bags as this may block steam circulation.
- Use a sterility compliance strip to indicate if the correct time, temperature and pressure have been reached during the run time. These are available from scientific suppliers.
- Sterilise at 15psi, 121oC for 15-20 minutes.
- After sterilisation has been verified, the autoclave or oven bag containing waste items should be disposed of by placing it into a sturdy garbage bag which is sealed for immediate disposal in industrial bins.
- Wear heat protective gloves when removing waste from the steriliser.
- Sterilisation of these items ideally takes place as soon as possible after completion of a practical activity and occurs within the science laboratory or prep room area.
Science ASSIST has developed the following resources with related information:
1'Petri dishes, plastic', Marienfeld-Superior website. http://www.marienfeld-superior.com/index.php/petri-dishes-accessoires/articles/petri-dishes-plastic.html (Accessed November 2016)
'Autoclave Use', Environmental Health and Safety Princetown University website. https://ehs.princeton.edu/book/export/html/380 (Accessed November 2016)
'Procedures for Decontamination by Autoclaving' Virginia Polytechnic Institute and State University website. http://www.ehss.vt.edu/programs/BiosafetyDocuments/AutoclavingProcedures_4-18-08.pdf (Accessed November 2016)
'Requirements for Decontamination by Autoclaving', Environmental Health & Safety University of Virginia website, http://ehs.virginia.edu/biosafety/bio.documents/Autoclaving_Guidelines.pdf (Accessed November 2016)
'Properties of laboratory plastics', The Lab Depot website, http://www.labdepotinc.com/articles/laboratory-plastics.html (Accessed November 2016)