“What’s the purpose of heat-fixing bacteria on a slide?”

I check my web server’s logs fairly regularly. Given that right now this is a brand spankin’ new blog with a pitifully small readership (Hi, mom!), seeing a new reader (even just a casual drive-by reader) is interesting to me.

I just noticed someone bounced by the site, having gotten here via a Yahoo search for the question above. Just in case they come back (or anyone else comes by and is interested, for that matter), here’s the answer – at least to the best of my knowledge.

Have you ever thrown a piece of meat into a hot pan or barbeque that wasn’t sufficiently greased? You notice how it sticks and doesn’t want to come off? That’s what heat-fixing is for. It basically “bakes” the bacteria to the surface of the slide, so that when you then soak the slide with stains and rinse it with water and/or alcohol and/or other substances (like the “acid alcohol” stuff used for the “acid-fast” stain for Mycobacteria) they don’t get washed off. This can be an issue, since some of the staining techniques have a whole series of “soak/rinse/soak/rinse/soak/rinse” kind of steps with different kinds of solvents, and it would be very annoying to go through all that work and find out you’ve rinsed the stuff you wanted to look at down the sink in the process. It’s also nice if you want to look at the slide with an oil-immersion lens.

You can’t really “glue” the bacteria to the slide with some sort of chemical, either, since anything you “glue” them with might cover them and interfere with stains that you’re trying to soak them with so you can see them in the microscope.

Actually, it’s probably worth mentioning that since most sources seem to unfortunately assume that “microbiology” just means the tiny fraction of a percent of microbes that cause diseases, a lot of sites will also say that the heat-fixing process is also “to kill the bacteria” (so that if you are overcome by an uncontrollable urge to lick the slide later or rub it on an open wound for good luck or something, you hopefully still won’t get the disease). While it’s true that heat-fixing ought to kill just about any microbe on your slide, I suspect that most of us who are looking at things that aren’t disease-related probably don’t consider this a “purpose” of the heat-fixing process. Still, if you’re answering a question like this on a “General (medical-centric) Microbiology”-type exam, you may want to mention this as well.
(UPDATE 2010-08-25 I dug up some Real Science™ on this “killing the bacteria” idea in “Stir-Fried Stochasticity Episode 4: TuberculosisBurgers“, which is an amateur podcasting project I’m dabbling in. I’d very much appreciate feedback on it!)

Hopefully that information will be of interest or use to someone…

Coming up next – some commentary on the history of staining bacteria, and why it seems like all of the classic techniques of microbiology – most of which seem to still be in common use – seem to have been invented entirely in or near Victorian-era Germany…

Also possibly coming soon: Discussion of the “Everything is Everywhere” concept, making chemicals with bacteria, and (if I can manage to get the thought into some organized form) a simple discussion of the concept of speciation, using science itself as an example. And various other things as I think of them and get time to type them up.

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The Author is (currently) an autodidactic student of Industrial and Environmental microbiology, who is sick of people assuming all microbiology should be medical in nature, and who would really like to be allowed to go to graduate school one of these days now that he's finished his BS in Microbiology (with a bonus AS in Chemistry). He also enjoys exploring the Big Room (the one with the really high blue ceiling and big light that tracks from one side to the other every day) and looking at its contents from unusual mental angles.

17 thoughts on ““What’s the purpose of heat-fixing bacteria on a slide?””

  1. I may be the only one who answers you, but I found it interesting. I think you would make a good teacher of some sort as it appears that you enjoy explaining things. Thanks for the updates. It’s always nice to hear what you are doing.

  2. I keep hoping that Congress reinstates the Office of Technology Assessment (or something like it). I’ve still got just enough navet left in reserve to think that might be a place where an ability to explain scientific and technical matters could Make A Difference…

  3. Thank you for posting such a detailed, yet easily understandable, description of heat-fixing. I was not able to find such a good example of it for my microbiology assignment. I’ll note the credit on the paper.

    Thanks again!

  4. Thanks for the feedback! As always, if by some chance it turns out that I’m incorrect about any of that, please let me know. (Obviously, I BELIEVE I’ve got it right…)

  5. This is one of the most entertaining essays I have ever read on this topic. Thanks for writing such a fun essay!

  6. I hate when people stop by with out leaving a note. Thanks for the info- you helped with my Micro homework 🙂 Oh, and moms that reply to your blog… ARE GOOD ONES!!

  7. Unfortunately I require two purposes for heat fixing, so I’m going to also use the ‘killing the bacteria in case you feel like liking the slide’ reason.

  8. I recently ran into what might be considered a third reason, too – a “Bacteriology” textbook from the late 1950’s suggests that by denaturing the proteins and such, portions of the molecules that make up a bacterium can end up rearranged such that the parts of them that attract certain dye molecules become more available, making the dye work better.

    That’s the first time I’ve run into that one, but it sounds plausible to me…

  9. Hello,
    You offer a good explanation of why we do heat fixing. The question I am researching is why it works. You comparison with meat on a skillet may be a lead. Thanks for taking the time to write.

    Pat O

  10. I’ve been assuming the cooking of bacteria to a slide was analogous to cooking meat to a pan, at least. If you happen to find that it’s something. else, please let me know. Also, thanks for the comment

  11. Hi,
    Was just browsing for an answer on the same. Found another opinion – Prevents autolysis by inactivating the autolytic enzymes. Now how is that important to staining?

  12. Interesting – I’d expect that’s true (that it’d prevent autolysis by denaturing the enzymes) but I’d expect the dessication alone would tend to halt those reactions anyway (or at the very least substantially slow them). A lot of staining procedures seem to involve the use of solvents that I’d expect would have a similar effect, too.

    So, in my OWN opinion, I don’t think that would be very important, and I suspect that prevention of possible autolysis is purely incidental.
    (Personally, is seems like saying that one of the purposes of welding is to get rid of any moisture between the two pieces of metal to be joined in order to prevent corrosion of the weld…)

  13. Very nice blog really helpful,love the way you explained heat fixing.Thanks

  14. Thanks for sharing such useful information buti think heat fixing slide to kill bacteria is not good idea as if heat fixing can kill the bacteria it may dessicate the cell and eventually it will shrink and proper morphology cannot be observed… so heat fixing smear can only adhere bacteria in the slide but donot kill in my opinion.

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