A continuation of the section How hot? -- on bacteria that grow at "ultra-high" temperatures.
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These continuing discoveries of organisms with ever higher Tmax, maybe even up to the common operating temperature of an autoclave, raise some questions:
* Will higher Tmax be found? Probably. Why not? A graph of reported Tmax vs year of discovery does seem to be slowing down, but there is no reason to believe it has reached a true maximum. Stay tuned.
* Is there a theoretical limit to Tmax? If only we knew what the relevant theory was! Various values have been suggested for Tmax over the years; some have already been exceeded. The hyperthermophiles discussed above are already growing at temperatures we would have thought impossible only a few decades ago. For example, they are growing under conditions where we would have thought that DNA could not be maintained in its double stranded form. Now there are suggestions that above about 150° C common bio-molecules would be too unstable for life. I suppose that is the theoretical limit -- for now. I would be cautious; Mother Nature tends to win these arguments.
* Is there a danger to human health now that we know some organisms can grow under conditions commonly thought to kill all life? Probably not, but that answer deserves some elaboration. First, it is extremely unlikely that any organism that could grow in our body could also grow as a hyperthermophile. Most bacteria can grow only over a temperature range of thirty degrees or so. (Why? Protein flexibility is probably one key issue. If proteins are rigid enough to work well at, say, 100° C, they will be too rigid to work at 37° C.) The common use of pasteurization reflects the idea that bacteria of concern to us cannot grow above some moderate temperature. The use of autoclaving to "sterilize" material (to remove "all" life) is aimed at special non-growing (dormant) forms of bacteria known as spores. These dormant spores are more resistant to killing by heat than actively growing ("vegetative") cells. Indeed, standard autoclaving procedures are designed to kill the most heat-resistant bacterial spores known. So the finding of vegetative cells that can grow under autoclave conditions does not appear to have any particular implication for human health. All of the points made here are subject to being shown to be wrong by real data, but one might suggest that if there are organisms of concern that survive autoclaving, we would have found them by now. Well, the prions might be one example -- but that is another story: see the Prions (BSE, CJD, etc) page for Biotechnology in the News (BITN).
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Last update: August 27, 2019