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We have recently moved into a lab space where we share an autoclave with other people.

We generally autoclave empty beakers, flasks, etc., before running experiments, and we autoclave waste (Petri dishes, pipette tips) in autoclave bags after our investigations.

Reading up on the proper use of an autoclave, I came across this quote:

Do not mix autoclave bags and other items to be autoclaved in the same autoclave cycle. Liquid media requires a shorter cycle, often 15- 20 minutes, while autoclavable medical waste requires a minimum of 30 minutes in order to be effectively sterilized

But my understanding was that whatever comes out of the autoclave would be sterile as long as you pick the proper cycle.

To be clear, is it ok to autoclave waste and equipment in the same cycle? For example, liquid waste and liquid 7H9 on a liquid cycle or solid waste and empty beakers on a gravity cycle.

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    $\begingroup$ You generally don't want to cook your media any long than necessary to sterilize it, depending on how sensitive your application is. Excessive autoclave times can degrade media's ability to support certain organisms. It might be less of a problem with lab-adapted strains than wild-type isolates, but still not in line with what I would consider best practices. $\endgroup$
    – MikeyC
    Nov 3, 2022 at 20:39

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Short answer: no.

The reason is that waste volumes are typically variable (e.g. one beaker with 50 ml, but another with 2 litres) and often have high biological loads compared to media or similar materials. If you were to put both into the same autoclave, and sterilized the waster properly, your media would be over-autoclaved (results in sugars being caramelized, damage to other nutrients too).

As your quote suggests, wastes take longer to decontaminate because you need to be certain that the waste is decontaminated before it can be discarded into normal waste streams (landfill or incineration etc.). If the waste is not decontaminated properly, you run the risk of accidental release of unwanted or restricted organisms into the environment. You may not care if your M. smegmatis contains a genetic modification and gets out of containment, but I bet your local biological safety officer/committee do, as do the local laws surrounding this topic.

Long story short - use the appropriate cycle for differing loads.

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  • $\begingroup$ Thank you bob. How about solid waste such as Petri dishes in autoclave bags, plus empty beakers/flasks etc? Our autoclave is large enough that we could put the flasks and beakers on a separate shelf from the waste bags. $\endgroup$
    – Johan Wikström
    Nov 14, 2022 at 16:08
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    $\begingroup$ @JohanWikström this may be OK, but generally you want your glassware to remain clean - sterile isn't the same as clean! In addition "dry" goods cycles usually have a drying process built in to remove most of the water from the items being autoclaved. If you run dry goods through a waste cycle you are likely to end up with a lot of water in your glassware. $\endgroup$
    – bob1
    Nov 14, 2022 at 22:27

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