sugary arabinogalactan. They can be classified under the gram positive bacteria category but they do not stain like them
Yeah, staining them is a PITA.
Mycobacterium only flowers in rich media and multiplies every 16-20 hours, which is sluggish as compared to other bacteria
The slow growth rate has been postulated as to why it took so long for them to be isolated. A lot of plates are incubated for 24 to 72 hours. For a lot of bugs, this is a ton of doubling times (as in your E. coli example, this is 72 to 216 doublings - far enough time for visible colonies. For Mycobacterium, that could be as few as 6 doublings in about a week. We now think that earlier bacteriologists may have done everything right for isolation, but just simply assumed nothing was growing.
Thanks for the detailed info. They do grow very slowly huh. I bet this caused a lot of hassle especially to diagnose tuberculosis especially many years ago. Staining works but I don't think the sensitivity rate is as high as culturing. Furthermore, we need the culture results for the antibiotics susceptibility testings as some mutated strains are resistant to certain common anti-tb drugs.