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Three Common Methods of Incubator Sterilization Explained

By Aimee O'Driscoll, 09 May 2018

The information in this article is courtesy of Alexander Cranston at Sheldon Manufacturing.

There are many methods provided by manufacturers to facilitate incubator self-decontamination. Three of the most common methods are:

  1. UV sterilization
  2. Moist heat sterilization
  3. Dry heat sterilization

This article will examine these three types of sterilization in detail.

1. UV Sterilization

DNA nucleotides harbor the kinds of conjugated bonds that absorb UV light. UV sterilization generates an antimicrobial effect by the damage it causes to a microorganism’s DNA when aromatic nucleotides absorb high energy photons. This can make UV sterilization an effective solution to reduce contamination in an incubator chamber.

However, there are significant drawbacks. Your light source would need to have unrestricted access to all surfaces of the incubator chamber, shelves, and shelf mounting hardware. Shadowed regions will not be decontaminated by a UV light. Also, a common method for microorganisms to survive UV exposure is through enhanced DNA repair mechanisms. In this case, survivors of a UV cycle will be more likely to survive repeat treatments.

Plus, UV light is generally not effective in destroying endospores. Microorganisms which survive the UV decontamination process will potentially have the opportunity to form monocultures and increase their likelihood to reach quorum. This is unless UV sterilization is combined with other methods of incubator decontamination, such as a tear-down and washing of all surfaces, a dispersed chemical treatment, or an effective high-heat cycle.

2. Moist Heat Sterilization

Moist heat decontamination is often employed on incubators that are not designed to safely reach the high temperatures needed for an effective dry-heat sterilization regime. This may be due to a risk of damage to internal components or the risk of overheating the incubator’s outer body.

Traditional autoclaves operate by heating to ~121°C and applying elevated steam pressure to increase rates of thermal transmission to targeted contaminant organisms. A moist heat decontamination cycle performed above but close to 100°C and at ambient pressure is guaranteed to be less effective than an autoclave, and does not meet any medical organizational criteria for SAL6 sterilization. SAL6 represents a Sterility Assurance Level of 10-6 meaning you get a Log6 reduction of microorganisms.

Interestingly, the archaea Geogemma barossii, better known as “Strain 121,” is a species of microorganism that has been shown to grow and reproduce successfully in a pressurized autoclave at 121°C.

3. Dry Heat Sterilization

Dry heat sterilization at 160-180°C for several hours satisfies several international guidelines for SAL6 sterilization. High temperature dry heat causes cell death through denaturing of all proteins, destruction of nucleic acids, and desiccation. 180°C for 4 hours will kill all known microorganisms including toughened endospores from bacteria such as Anthrax.

The incubator below features a dry heat decontamination cycle which maintains a temperature of 180°C for two hours.

A SHEL LAB SCO6AD Decontamination CO2 Incubator.

A SHEL LAB SCO6AD Decontamination CO2 Incubator.

An incubator designed to handle dry-heat sterilization is akin to an oven with a self-cleaning cycle. Both are designed to handle much higher heat loads than typical operating conditions without experiencing damage or presenting hazards, and so are manufactured to be tougher and meet a higher standard of quality than non-high heat units.

High heat dry sterilization will kill bacteria in the chamber that may be located in shadowed crevices such as the undersides of shelves and within shelf mounting hardware, and can serve as a substitute for having to remove all your shelving to autoclave it.

These Methods Complement (Rather Than Replace) Proper Laboratory Procedures

No self-decontamination routine is a true substitute for maintaining proper laboratory procedures, including regular cleaning of your incubator chamber and enforcement of hygienic practices. It’s always worth remembering these two things:

  1. If your petri dishes are experiencing contamination, and you remove them to disinfect the chamber and put them back in again afterward, then you just reintroduced the contamination to your chamber.
  2. The biggest source of contamination for your incubator is “You” since you’re covered head-to-toe in a complex skin microbiota. A fair amount of the dust in your home is actually dead skin, and the dead skin and dust that falls off your hands/arms/head when you reach inside the chamber is quite effective for contaminating an incubator.

Conclusion

The type of sterilization method best suited for your incubator will depend on the design and specifications of your machine. However, in order to minimize contamination, you must also be acutely aware of how improper laboratory practices can introduce unnecessary contamination to your incubator.