Tonight’s post will be a bit short, but I’ll try to make up for it tomorrow. (Three more days of Just Science week to go…)
First, a quick unsolicited plug for someone else’s site: Aetiology is doing a series of posts on “Normal Flora” – that is, the microbes that normally live on and in healthy people (as opposed to microbes that just cause disease). Since that’s an area where my own interests overlap with the more conventional “medical” microbiology, it seems appropriate to mention it here. As of right now, there is a Part 1 and a Part 2. Interesting and informative stuff.
Second, a small addendum to the previous post – I mention that I think when people say “Gram-Positive” they generally really mean Firmicutes, but I just realized there’s one exception to that. The Firmicutes actually include Mycoplasma and related bacteria – which have no cell wall at all. I would guess their ancestors were normal “Gram-Positive”-type firmicutes but somewhere along the way “lost” function of a key gene involved in making the thick cell wall. (There’s a similar group of Archaea – the Thermoplasmata. ) These don’t stain Gram-positive (and perhaps don’t stain gram-negative, either – seems like such fragile things would be destroyed by the staining procedure). In order to see these in the microscope, the standard method seems to be to use a chemical that actually stains DNA instead. Instead of staining the outer surface of the cell like most of the classical stains, this stains the inside of the bacteria (which tend to have their DNA spread more or less throughout the entire cell in one form or another, since they have no nucleus to pack it into) with a fluorescent material, which you can then see in the microscope with the right kind of light. You can also use this kind of technique for other bacteria, too. The “Live/Dead” stain I previously mentioned works this way, if I remember correctly.
Since you probably don’t want to try heat-fixing Mycoplasma, you have to use a “fixative” (a preservative made of alcohol and pure acetic acid) and then air-dry the slides.
This leads to one last correction – in a previous post I suggested that it was unlikely that you could “glue” your smear to the slide rather than heat-fix (assuming that the “glue” would interfere with the subsequent staining and viewing of the sample) – this is actually not completely correct. I heard today of a protocol for doing endospore and acid-fast stains which called for mixing the culture sample with serum on the slide to make it stick, so there are at least a few ways to “stick” cells to the slide and still look at them.