Pressure cooker to sterilize

[gerry shown]

I clearly remember my high school science teacher running across the room with a pressure cooker in hand making a beeline to the sink. After some sizzling when cold water hits its hot surface, my teacher finally opened it. What were we up to? Making agar plates of course!

Years later, research published in PLoS One puts the pressure cooker to a test: can these tools be used for sterilization when a steam autoclave is not available?

This autoclave looks a bit like a giant pressure cooker!
Steam heating to 121°C at 15 PSI for ~20 minutes per liter of liquid is standard practice to consider something sterile. Lower temperatures (115°C) may be adequate at a longer sterilization time.

However, autoclaves can be prohibitively expensive for many facilities and schools. They also have a large physical footprint and are difficult for field researchers to sterilize equipment as needed.

Enter, the pressure cooker. These researchers tested four different 8-quart electric pressure cookers (GoWISE, CORSORI, Gormia, and Instant Pot) for their utility as lab sterilization tools.

They first tested whether or not the pressure cookers could sterilize 1.5 L of nutrient broth or agar medium. For all pressure cookers, 15 minutes of sterilization time was enough to sterilize the medium. But growth medium freshly prepared is homogenous and unlikely to contain a lot of microbes.

Thus, the researchers turned to soil mixture containing small particles and many more microbes than media would. They resuspended 20 g of soil per liter of water and tested sterilization times. Unlike the nutrient agar, the soil water mixture took 45 minutes to completely sterilize.

For dry items, such as a spatula or scissors, the researchers dunked them into soil water and then placed them into the pressure cooker for different sterilization times. Afterwards, they submerged them into liquid medium for a week to see if things grew. What they found was that sterilization times differ for different instruments. Spatulas and probes were sterilized after 15 minutes but the scissors required 30 minutes.

While these tests seem to indicate that the pressure cooker may be up for the task at hand, they were still all done with samples with low bacterial densities (103 – 104 CFU/mL in soil water) when compared to densities seen in laboratory grown bacteria (>107 CFU/mL in laboratory cultures or disease samples). They tested a diverse set of microbes at these densities and found that 15 minutes was required to inhibit growth of bacterial strains but one hour was needed to inhibit fungal growth. The pressure cookers used can inactivate up to 1010 CFU/mL bacteria and 107 CFU/mL fungi.

The ultimate test for a sterilization device uses the most heat-resistant organisms, commonly Geobacillus stearothermophilus endospores. If the hardiest of microorganisms is killed, than all other microbes should die as well. An ampoule containing 106 G. stearothermophilus endospores is autoclaved and then the spores are plated on growth media. Failure to grow means the autoclave is working properly. As for the pressure cookers in this study, only the Instant Pot was able to inactivate the endospores (but required 150 minutes), making it the best choice for a laboratory pressure cooker.