Autotrophic Mycobacterium spp. - whatever next?!

One of the many genera within the Bacteria that I have worked on is Mycobacterium. When I tell people this, they often say “Oh, you work on TB?” and the answer is no. I’ve never worked on the tubercle bacillus, as TB stands, though I did apply to do a Ph.D on TB at the National Institute of Medical Research - I even went and had my TB vaccination done because I’d never been vaccinated (growing up in the Styx, it wasn’t necessary so we never had it done). The Mycobacterium strain that I worked on was called (by my own invention) DSQ3 and I’ll say a bit more about it in a minute. I read an article this past week in Frontiers in Microbial Physiology and Metabolism, our newest microbial physiology journal concerning autotrophic growth of two Mycobacterium spp. at the expense of elemental sulfur. This is very exciting as although autotrophy at the expense of hydrogen has been found in Mycobacterium spp. before (Lukins & Foster, 1963), the prospect of them using sulfur opens up a whole new set of doors…but I’m not really that surprised. You see - Mycobacterium is one of those genera (like Bacillus) that is just full of surprises and metabolic diversity (usually by way of pinching genes from other bugs) - it never ceases to amaze me. I myself discovered methylated amine use as a sole carbon and nitrogen source in Mycobacterium sp. DSQ3 (Boden et al. 2008) so I’ve seen first hand just how adaptable these bugs can be.

The genus Mycobacterium has been known about for over 100 years now, but diseases caused by some of its members have been around for far longer than that. Leprosy and tuberculosis are caused by M. leprae and M. tuberculosis (in man anyway, M. bovis in cattle) respectively and are probably two diseases that are immediately associated with particular periods of world history or particular parts of the globe. M. tuberculosis is the type species of the genus though M. leprae is an excellent example of an important organism that we can easily identify in its environment (living organisms!) but can’t actually grow in the lab (outside of a laboratory animal). The names of these species are rather boringly just Latinisations of “of [disease name]” but Mycobacterium itself tells you a lot about the genus. It very literally means “fungal rodlet” and that’s pretty much what the little swines look like - small rods and there’s something distinctly fungal about them. When they’re growing on solid media in the lab their colonies look very much more fungal than bacterial and the produce floating mats on complex broths which look a lot like fungi.

Now, the main thing about the Mycobacterium genus is that it’s stupidly large as genera go. There are at present 154 species of Mycobacterium, which is frankly idiotic and there are numerous non-Code divisions within the genus such as “fast”/”slow” and different “complexes” relating to different disease causing groups. What it really needs is a good spring-clean and a good sort out but, even as a taxonomist, I ain’t touching it. It’s way too far gone to get any real sense out of without moving some clinically important strains into different genera, which won’t help with diagnostics and, although it’ll make things systematically “right”, it could complicate things more in other ways. The species that I worked with was M. fluoranthenivorans, the type strain of which degrades fluoranthene (a polyaromatic hydrocarbon from coal tar), though my strain was never tested on it - mine grows pretty well on dimethylamine hydrochloride though (Hormisch et al., 2004; Boden et al., 2008) and came from the sediments of the River Thames.

The strains that have just been shown to grow on elemental sulfur are from the species M. cosmeticum and M. pallens and were isolated from sandstone at the Angkor Wat in Cambodia (Kasumi et al., 2011). The former was originally isolated from a sink in a nail salon in Atlanta and also from an infection under the skin of a Venezuelan woman who had undergone a minor invasive cosmetic procedure (Cooksey et al., 2004) - the name “cosmeticum” refers to cosmetics in general. The latter species came from soil in Hawaii and takes the name “pallens” from its pale yellow colour (Hennessee et al., 2009). In the original studies on these species, there is nothing to really make it obvious that the whole species could grow on sulfur - in fact, it could very well be just the strains from Angkor that can do it - but I can’t help wondering - how widespread is autotrophy in the genus Mycobacterium? What about these ones we can’t grow? What about M. leprae? I’d be willing to bet money on the fact that no one has tried these “obligate” pathogens on substrates such as sulfur or ammonium for autotrophic growth or even on things like methanol and things other than complex medical-microbiology broths. 

The human body is an environment like any other and yet medically-related organisms are usually treated and handled differently to how an “environmental” microbiologist would work. I think there’s probably a lot to gain by doing a bit of swapping of strains and trying some of these organisms that have “no sink in the environment” and are “obligate” pathogens or have to have a host on some of the less mainstream substrate combinations to see if they will grow at all. I’d be willing to bet that out of 154 species of Mycobacterium, more than two exhibit some degree of lithotrophy.

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Boden et al., 2008. Environ. Microbiol. 10, 3225-3236.

Cooksey et al., 2004. Int. J. Syst. Evol. Microbiol. 54: 2385-2391.

Hennessee et al., 2009. Int. J. Syst. Evol. Microbiol. 59: 378-387.

Hormisch et al., 2006. Syst. Appl. Microbiol. 27, 653-660.

Kasumi et al., 2011. Frontiers Microbiol. Physiol. 2, 104. 

Lukins & Foster, 1963. Z. Allg. Mikrobiol. 3, 251–264.