Ethan Clarke
Plastic Petri dishes are commonly used in laboratories for experiments, cell growth observation, and sample storage. However, reusing plastic Petri dishes can increase the risk of cross-contamination due to remnants from previous experiments. To avoid this, proper sterilisation techniques must be employed. This paragraph will discuss how to autoclave plastic Petri dishes to ensure effective sterilisation and prevent contamination.
| Characteristics | Values |
|---|---|
| Purpose | To avoid cross-contamination that could compromise research outcomes |
| Materials | Petri dishes made of heat-proof materials such as polypropylene or polystyrene |
| Pre-sterilization | Petri dishes should be pre-sterilized by the manufacturer |
| Collection | Collect used Petri dishes containing agar and cap the dishes to prevent microbial proliferation |
| Loading | Load used dishes into a perforated sterilization basket, with lids arranged to minimize contamination |
| Setup | Place a stand inside a solid bucket and fill it with water to prevent media from solidifying |
| Autoclaving | Place the perforated basket with disassembled dishes on the stand inside the autoclave and run a normal sterilization cycle for at least 20 minutes |
| Unloading | Remove the basket after the cycle and allow the autoclave to cool, being mindful of hot steam, liquids, and media |
| Alternative methods | UV light, chemical sterilization, microwave ovens |
| Precautions | Handle under sterile conditions with gloves and/or tools, cover and store in a sterile environment |
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What You'll Learn
Check the plastic type
Autoclaving is a sterilization method that uses high temperatures and pressure to kill microorganisms. It is a highly effective method but due to the high temperatures involved, not all plastics are suitable for autoclaving. Some plastics will melt, deform, or become weak. Therefore, it is important to check the plastic type before autoclaving.
Plastics that can withstand higher temperatures are ideal for autoclaving. Polypropylene (PP) is a low-cost plastic that can withstand autoclave temperatures and can be repeatedly autoclaved without losing strength. Polypropylene is used for plastic pans, trays, bags, and wipes. Polypropylene copolymer products can also be repeatedly autoclaved without losing strength. Polycarbonate (PC) is another plastic that can withstand high temperatures. It is optically clear and more expensive than polypropylene. Polycarbonate is often used for bottles, desiccators, and vacuum chambers. However, polycarbonate can only be autoclaved for a certain number of cycles as its strength decreases with each cycle.
Other plastics that are safe for autoclaving include polytetrafluoroethylene (PTFE or Teflon), polyfluoroalkyl (PFA), and polymethylpentene (PMP or TPX). These plastics can withstand higher temperatures and can be autoclaved without problems. However, it is always recommended to check with the manufacturer for specific product recommendations.
Some plastics are not suitable for autoclaving due to their sensitivity to heat or moisture. These include polyethylene (PE) and its variants low-density polyethylene (LDPE) and high-density polyethylene (HDPE), polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETG), polymethyl methacrylate (PMMA or acrylic), polystyrene (PS), and polyvinyl chloride (PVC). These plastics may be damaged by autoclaving but can be sterilized using other methods such as ethylene oxide formaldehyde, a gentler approach.
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Use chemical alternatives
While autoclaving is the most reliable method for sterilizing Petri dishes, chemical alternatives can be used in certain cases. These methods are generally less effective than autoclaving, which combines high heat, moisture, and pressure to achieve complete sterilization. However, chemical alternatives may be considered when autoclaving is not feasible due to the type of plastic used in the Petri dishes.
One commonly used chemical for sterilizing plastic Petri dishes is ethanol. Ethanol is a safe and effective option for sterilization as it does not subject the plastic to excessive heat, which could deform the material and lead to incomplete sterilization. Another chemical that can be used is hydrogen peroxide. Hydrogen peroxide is a strong oxidizing agent and has antimicrobial properties, making it effective for sterilizing surfaces. Chloroform vapors are also utilized for sterilizing Petri dishes. Chloroform is a volatile organic compound with potent anesthetic and antiseptic properties, making it suitable for surface sterilization.
In addition to these chemicals, UV sterilization is another alternative method. UV light, specifically UV-C light, has been shown to be effective in surface disinfection. It can achieve a high level of bacterial and viral surface contamination sterilization, including against challenging microorganisms like SARS-CoV-2. However, it is important to note that while these chemical and UV sterilization methods are safer for plastics than microwave ovens, they may not achieve the same level of complete sterilization as autoclaving.
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Prepare the autoclave
To prepare the autoclave, start by collecting all the used Petri dishes containing agar. It is important to cap the dishes to prevent microbial proliferation and contamination. Next, load the used Petri dishes into a perforated sterilisation basket. This basket will facilitate uniform steam penetration during the sterilisation process.
Then, set up a stand inside a solid bucket and fill it with water. This will prevent melted media from solidifying inside the bucket. Place the loaded basket on the stand, ensuring that the agar in the dishes remains uncovered, but the lids are arranged to minimise contamination.
Now, you can place the water-filled bucket inside the autoclave. It is important to ensure that the autoclave is compatible with plastic Petri dishes and that the dishes themselves are made of heat-proof materials, such as polypropylene or polystyrene.
Finally, close the autoclave and run a normal sterilisation cycle. The duration of the cycle may vary depending on the number of Petri dishes being sterilised.
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Run a cycle
To run an autoclave cycle for plastic petri dishes, follow these steps:
First, ensure that the plastic petri dishes are made of a heat-proof material such as polypropylene, as not all plastics are autoclave-friendly. Polystyrene, for example, will melt if heated above 50 degrees Celsius. If your plastic petri dishes are not made of a heat-proof material, consider using alternative sterilization methods such as UV light or chemical sterilization.
Assuming your plastic petri dishes are made of a heat-proof material, collect the used dishes containing agar. Cap the dishes to prevent microbial proliferation and minimize the risk of contamination. Next, load the dishes into a perforated sterilization basket, ensuring that the lids are arranged to minimize contamination while allowing for uniform steam penetration. Place the loaded basket on a stand inside a water-filled bucket, which is then placed inside the autoclave.
Close the autoclave and run a normal sterilization cycle for at least 20 minutes. Note that the sterilization time may differ depending on the number of plastic petri dishes being sterilized.
After the cycle is complete, allow the autoclave temperature to cool down to a safe level before removing the basket. Be cautious of hot steam, liquids, and media when unloading the basket. The plastic petri dishes and their lids will be melted and re-forged into a chunk, and some of the agar material may be trapped within the plastic. Avoid touching any exposed agar to prevent a mess.
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Post-cycle handling
Once the autoclave cycle is complete, it is important to allow the autoclave to cool to a safe temperature before removing the sterilised Petri dishes. The dishes will be soaked in liquid nitrogen, so caution should be taken to avoid any spillages.
After removing the Petri dishes, it is important to handle them in a sterile environment to avoid contamination. Wear appropriate personal protective equipment, such as gloves and a lab coat, and work in a laminar flow hood or clean bench if possible. Always handle the dishes with sterilised hands or sterile forceps, and avoid touching the inside of the dish, especially the agar surface.
If you intend to reuse the Petri dishes, ensure they are stored in a controlled environment, such as a refrigerator or storage cabinet, to maintain their integrity. Seal the dishes with parafilm or suitable sealing materials to create an airtight seal and minimise the risk of contamination.
If you are disposing of the Petri dishes, ensure they are decontaminated to eliminate any potential biohazards. This can be done by autoclaving for a further 15-30 minutes at 121°C, or by using another suitable decontamination method recommended by your laboratory's biosafety guidelines. Place the decontaminated dishes in biohazard bags or containers designated for biological waste disposal, clearly labelled with appropriate biohazard symbols. Finally, follow any local regulations and guidelines regarding the disposal of biological waste, contacting your local waste management authorities for specific instructions if needed.
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Frequently asked questions
Yes, but not all types of plastic can withstand the process. Only heat-proof materials such as polypropylene can be autoclaved. Most plastic petri dishes are made of polystyrene, which will melt if heated above 50°C.
First, collect the used petri dishes and cap them to prevent microbial proliferation. Then, load the dishes into a perforated sterilisation basket and screw on the loading basket to keep the leaky lids in place. Next, set up a stand inside a solid bucket and fill it with water. Place the basket on the stand and then put the bucket inside the autoclave. Run a normal sterilisation cycle for at least 20 minutes. Finally, remove the basket after the cycle is complete and the autoclave has cooled to a safe temperature.
Alternative methods include using a microwave oven, UV light, or chemical sterilisation. However, these methods may not kill all bacteria and are less effective than autoclaving.
To sterilise plastic petri dishes with agar, you can use a bleach solution. First, mix one part bleach with nine parts warm water. Then, clean the dishes with a soft, non-abrasive cloth, antibacterial dish soap, and warm water. Place the dishes in the bleach solution for two minutes, then remove them with sterile lab tongs. Allow them to air dry before placing them in a sterile area until their next use.
It is generally not recommended to reuse plastic petri dishes, especially if your experiment involves live pathogens as there is a high risk of cross-contamination. However, some types of plastic such as PC or PP plastic can be sterilised and reused.
