Are “Acid-Fast” bacteria Gram-positive or Gram-negative?

I was wondering what today’s post ought to be – but a Google™ search that reached the page posed an interesting question and made it easy.

Someone from a Miami, Florida college got to this site after asking Google:
“Assuming you could stain any cell, would an acid-fast [bacterium] be gram-positive or gram-negative?”

Therefore, today’s post will deal with some Microbial Physiology.

The easy answer is, of course, “no“.

A more useful answer, though, is that it depends on what you mean by “Gram-positive”.

If one takes the “Assuming you could stain any cell” part of the question to mean that you’ve done whatever it takes to get the Crystal Violet/Iodine into the cell wall, and you mean “will the cell still look purple instead of pink at the end of the Gram Stain process”, then I’m pretty sure the answer would be yes, that it would be “Gram Positive”. It actually IS possible to stain “acid-fast” bacteria with the Gram stain. The catch is that it takes 12-24 hours of staining (according to Gram’s original paper) rather than a minute or so. This still counts as Gram-positive, though, and in fact the whole Phylum of Actinobacteria (including the Mycobacterium genus) is considered “High G+C Gram-positive”. (If the query was for an exam or homework problem, this is probably the answer you’re looking for.)

On the other hand, if you mean “does the cell wall structure of an acid-fast bacterium better resemble Gram-positive or Gram-negative bacteria?”, you can make an argument that instead of a nice simple “inner membrane surrounded by a thick peptidoglycan-layer cell wall”, the “acid-fast” bacterial cell wall looks more like a complex gram-negative-type cell wall, having multiple layers, with special proteins that form channels through them to let substances in and out of the cell through the otherwise penetration-resistant outer layer, just as Gram-negative bacteria have through their outer membrane. (On yet another hand, those outer layers are tough like a gram-positive bacterium rather than fragile like a gram-negative bacterium’s outer membrane, and don’t dissolve in alcohol.)

Therefore, if you’re speaking in terms of taxonomy, and by “Gram-positive” you mean firmicutes which, as far as I know at the moment, are really the only class with the simple, officially-gram-positive-type cell wall structures and therefore are usually what is meant when someone says “Gram-positive” (someone please correct me if I’m wrong here), the obvious differences with “acid-fast” type cell walls can at least make a good argument that they are “not ‘Gram Positive’ bacteria”.

But if you put that on your homework or exam answers, don’t blame me if you get marked wrong…

I actually found a pretty nice illustration on the University of Capetown website showing the differences in cell wall structures here, which might help.

So, to summarize – Officially, they’re either “neither” or “Gram-positive”. Unofficially, you can probably argue either way. Hmmm. I should try to work in a post on bacterial taxonomy one of these days.