Eliminating cell culture contamination for maximum sample protection probably the main challenge facing cell culture laboratories. An automated high temperature sterilization cycle built into a CO2 incubator is a great choice to help protect cultures. However, there are some key points to consider, because not all high temperature cycles are equal.
Choose the tried and true
Dry heat sterilization is a particularly efficient process when it comes to sterilizing hard surfaces. It’s the heat conducting properties of such materials that allow them to be readily sterilized using dry heat processes. This technology is especially suitable for eliminating microorganisms and persistent fungal spores, both of which are a major concern in cell culture laboratories. Because dry heat sterilization is commonly used for medical instruments, the Pharmacopeias of the United States and of the European Union have specified tests to ensure that a sterilization process is validated as such. For dry heat sterilization, both Pharmacopeias require proof of sterilization using at least one million (1 x 106) Bacillus subtilis spores, from a specific strain with well defined heat resistance. They also require that in such cycle, the air be constantly circulated using a fan or blower. The U.S. Pharmacopeia further requires an “overkill” approach where the lethal process, in this case dry heat sterilization, is doubled for an additional 6 logs (106) sterility assurance level (SAL).
CO2 incubators from different manufacturers offer a variety of sterilization temperatures, such that choosing which is best can be confusing. This is why the Pharmacopeias now require proof instead of just listing a given temperature. Part of the problem with establishing a temperature is that often, an incubator will never reach the prescribed temperature. More often, the chamber may reach that temperature in one area, for example the back of the top shelf, but other areas have a much colder maximum temperature, potentially allowing microorganisms in those colder areas to survive the cycle and recolonize the incubator. Thus, look for an automated high temperature sterilization cycle that offers proof of efficacy by eliminating one million (6 logs) of Bacillus subtilis spores, and proof that all areas of the chamber reach the specified temperature.
One other option uses moist heat for sterilization rather than dry heat. The advantage here is that a lower temperature can be used, which can allow sensors to remain in the chamber during the cycle. Water is added just before initiation of the cycle. This is analogous but not identical to using steam sterilization in an autoclave. The water serves to cause heat resistant non-vegetative bacterial spores to germinate, which makes them much less heat resistant. Again, for such an option, look for proof of elimination of one million Bacillus subtilis spores, and proof that the chamber reaches the specified temperature, for example 90°C.
Although various methods have been developed to destroy contaminants over the years, dry heat sterilization remains one of the most reliable and effective sterilization methods. There are a number of considerations to keep in mind when using this method, and selecting an incubator that is built with these in mind will go a long way in ensuring your cultures remain contaminant free.