This week, let’s take a look at a group of bacteria called Mycobacteria. They are interesting because they are acid-fast, which will make more sense later, but also because they include two very well known bacteria. Those are Mycobacterium tuberculosis, the cause of tuberculosis, and Mycobacterium leprae, the cause of leprosy. Tuberculosis may conjure up sickly invalids and gunslingers of the late 19th century, but it still causes 2 million deaths annually.

Mycobacteria are shaped like rods, or technically bacilli, and have cell walls with a high concentration of lipids. This is what causes the to be “acid-fast” on staining. There are only two groups that exhibit this property, Mycobacteria and Nocardia, although I have recently seen Nocardia fall under Gram +, as well. So, fact check that one and let us know if you come up with anything different. Hey, we’re all in this together after all, right?

In acid-fast staining, you take the sample, cover it with a red stain called carbolfuchsin and heat it. The heating fixes the bacteria to one place. Then you pour some alcohol on the slide, which removes the stain from anything that isn’t holding onto it too tightly. Throw on a second stain, or counter-stain, of methylene blue and you’re done. The ones that did not lose the first stain when washed with the acid alcohol is considered to be “acid-fast.” Acid-fast organisms are the only ones to have mycosides. We could go into that, but it’s like a whole other article.

Now, Mycobacterium tuberculosis is an obligate aerobe. Makes sense, right? Think about where tuberculosis infects and what is it rich in? The lungs have plenty of oxygen to keep these li’l guys happy. They grow slowly in there, taking up to 6 weeks before they are visible.

An interesting point about Mycobacterium leprae is that it is impossible to grow on artificial media. It has only been grown in the footpads of mice, in armadillos and in monkeys. Approximately 2 million people worldwide are curently infected with leprosy, although only 100 new cases are diagnosed in America every year. We mostly think of leprosy as a skin disease. This is due to the fact that Mycobacterium leprae grows better in cooler environments. So, they migrate close to the surface where they can be coolest. In fact, generally speaking, leprosy is known for sparing the warmer places of our skin – the armpits, groin, and perineum.

Of course, there is so much more to know about both Mycobacterium tuberculosis and Mycobacterium leprae, but we’ll leave it there for now. I sort of feel like we might be doing a whole series on various bacteria soon, so stick around!