Wikipedia's says:

A widely used method for heat sterilization is the autoclave, sometimes called a converter or steam sterilizer. Autoclaves use steam heated to 121–134 °C (250–273 °F) under pressure. To achieve sterility, the article is placed in a chamber and heated by injected steam until the article reaches a temperature and time setpoint. Almost all the air is removed from the chamber, because air is undesired in the moist heat sterilization process (this is one trait that differs from a typical pressure cooker used for food cooking). The article is held at the temperature setpoint for a period of time which varies depending on what bioburden is present on the article being sterilized and its resistance (D-value) to steam sterilization. A general cycle would be anywhere between 3 and 15 minutes, (depending on the generated heat) at 121 °C (250 °F) at 100 kPa (15 psi), which is sufficient to provide a sterility assurance level of 10−4 for a product with a bioburden of 106 and a D-value of 2.0 minutes. Following sterilization, liquids in a pressurized autoclave must be cooled slowly to avoid boiling over when the pressure is released. This may be achieved by gradually depressurizing the sterilization chamber and allowing liquids to evaporate under a negative pressure, while cooling the contents.

Proper autoclave treatment will inactivate all resistant bacterial spores in addition to fungi, bacteria, and viruses, but is not expected to eliminate all prions, which vary in their resistance. For prion elimination, various recommendations state 121–132 °C (250–270 °F) for 60 minutes or 134 °C (273 °F) for at least 18 minutes. The 263K scrapie prion is inactivated relatively quickly by such sterilization procedures; however, other strains of scrapie, and strains of Creutzfeldt-Jakob disease (CKD) and bovine spongiform encephalopathy (BSE) are more resistant. Using mice as test animals, one experiment showed that heating BSE positive brain tissue at 134–138 °C (273–280 °F) for 18 minutes resulted in only a 2.5 log decrease in prion infectivity.

The statistical discussion of "sterility assurance level" and "bioburden" already seems to preclude any possibility of absolute certainty, such as "...all resistant bacterial spores..."

Question: The research cited in Is it known how some heat-resistance Bacillus spores repair their DNA after having been heated to 420 °C? (see below) shows an abrupt transition between 420 and 430 °C. Can we therefore assume that the sentence "Proper autoclave treatment will inactivate all resistant bacterial spores..." is just plain wrong If so, just how widespread are autoclave-resistant bacterial spores?

Related: Why do we use an autoclave at 121°C (250F)? (Origin)

The 2018 Extremophiles paper: Beladjal, L., Gheysens, T., Clegg, J.S. et al. Life from the ashes: survival of dry bacterial spores after very high temperature exposure. Extremophiles 22, 751–759 (2018). https://doi.org/10.1007/s00792-018-1035-6 (also downloadable from researchgate).

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    $\begingroup$ Turns out no, though repeated autoclaving does: here $\endgroup$
    – bob1
    Aug 5 at 1:32
  • $\begingroup$ @bob1 that's excellent, thanks! $\endgroup$
    – uhoh
    Aug 5 at 1:50
  • $\begingroup$ I assume that this question stems from this recent Q&A on Space Exploration? $\endgroup$ Aug 5 at 14:08
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    $\begingroup$ Comment on "The research cited in Is it known how some heat-resistance Bacillus spores repair their DNA after having been heated to 420 °C? (see below) shows an abrupt transition between 420 and 430°C." An important distinction here is that this research was utilizing dry heat, while an autoclave utilizes heat from pressurized steam, which are quite different physical processes. Consider that boiling water and steam can scald human skin almost instantly at 100°C, but a brief exposure to dry heat over 250°C (i.e. reaching into a pizza oven) has almost no noticeable effect. $\endgroup$
    – MikeyC
    Aug 5 at 17:34
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    $\begingroup$ @MikeyC I knew there was something missing from my answer, but couldn't for the life of me work out what it was. If you don't mind I'll edit something in to my answer covering that aspect. $\endgroup$
    – bob1
    Aug 5 at 21:18

Bacterial spores in most contexts are properly called endospores, formed within the bacterial wall and are a survival mechanism, creating resistance to desiccation, heat and cold, with tolerances up to 150 Celsius (300 F).

The vast majority of bacterial spores studied are from Gram-positive species, that are medically relevant. These include organisms such as Bacillus anthracis (anthrax) and Clostridium species (colitis (C. difficile), food poisoning (C. perfringens), and tetanus (C. tetani)), which are significant pathogens of humans. The reason that they are studied is that sterility of equipment and items ued in medical settings, as well as in the food industry is a great way to prevent illnesses and not have patients getting sick in the hospital from nosocomial infections.

In the context of autoclaving, the temperatures, times and pressures have been worked out to inactivate the bacterial spores from these medically relevant bacteria, and are suitable for the vast majority of bacteria that people encounter in their every-day lives. These conditions work even for things like Mycobacterium tuberculosis, which are incredibly resistant to many of the ordinary disinfectants used in health-care settings and in the food industry.

However, there are whole classes of bacteria that survive under unusual conditions of high heat and/or high acidity or alkalinity. These organisms are known as extremophiles. It turns out that some of these are quite resistant to autoclaving, as one might expect. It seems that they are resistant to up to 3 standard autoclave cycles and some other pretty extreme conditions:

Cultures of Desulfotomaculum sp. C1A60, D. kuznetsoviiTand D. geothermicum B2T survived triple autoclaving while other related Desulfotomaculum spp. did not, although they did survive pasteurisation. Desulfotomaculum sp. C1A60 and D. kuznetsovii cultures also survived more extreme autoclaving (C1A60, 130 °C for 15 min; D. kuznetsovii, 135 °C for 15 min, maximum of 154 °C reached) and high-temperature conditions in an oil bath (C1A60, 130° for 30 min, D. kuznetsovii 140 °C for 15 min).

While this is interesting, and a real problem for those working with them, none of these sorts of species are medically relevant (as far as I know, I would love to be proved wrong!), so the fact that these are resistant to autoclaving is not a problem for the vast majority of us.

As a note, language is inherently difficult and to be scientifically certain that something is so, is almost impossible. This is particularly so with bacterial species, where it is estimated that there are about a trillion species and we can culture only about 3250 of them, so for the rest we know next to nothing about how they grow, their physiology or anything beyond a genome. This means that we have no way of determining if autoclave resistant bacteria are actually found almost everywhere or if they are as rare as the proverbial hen's teeth.

Basically what I am saying is that the Wikipedia article is incorrect and imprecise in its use of language. If it had said something along the lines of "inactivates all medically relevant spore forming bacteria", then it would have been correct and much more precise.

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    $\begingroup$ "I would love to be proved wrong." I would prefer it if you were right and all medically relevant pathogens could be destroyed by autoclaving. $\endgroup$ Aug 5 at 22:12
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    $\begingroup$ @WaterMolecule - I was talking about these extremophiles being medicallt relevant here, but since you raise it; which medically relevant bacteria aren't sterilized by autoclaving? Note that I deliberately didn't use the term pathogen. I know in terms of pathogens prions aren't generally destroyed, but they aren't bacteria, so not in the scope of this question. $\endgroup$
    – bob1
    Aug 5 at 22:33
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    $\begingroup$ It would be very difficult for extremophiles to be medically relevant, as the extreme conditions that they need to grow/reproduce are not going to exist in the human body. Therefore, even though we have not exhaustively sampled all extremophiles and can't be scientifically certain, we can make a pretty good logical argument that there is no reason for concern. $\endgroup$
    – Cody Gray
    Aug 5 at 23:20
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    $\begingroup$ @CodyGray - my point entirely. Though we do have environments in the body that are highly acidic (stomach) where bacteria such as Helicobacter pylori can thrive. I think you could easily classify H. pylori as an extremophile and pathogen. There's also the gall bladder (alkaline), anaerobic environments (e.g. appendix, prepuce), dry environments (skin) and environments which undergo rapid changes in salinity and humidity (skin) - all of these are colonized by a range of bacteria that may or may not be pathogenic. $\endgroup$
    – bob1
    Aug 6 at 0:27
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    $\begingroup$ @bob1 I never thought about myself as a host for extremophiles (beyond H. Pylori), yikes! $\endgroup$
    – uhoh
    Aug 9 at 23:28

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