Taxon of the Week: Pick from a wide range of pathogens

Pick up almost any popular palaeontology book, and you'll be introduced to various 'ages' of life on this planet. We're told that we are currently in the Age of Mammals. The Mesozoic is dubbed the Age of Reptiles. The Devonian was the Age of Fishes, and so on and so forth. The names of all these ages are supposed to reflect the dominant life-form at the time, the pinnacle of evolutionary achievement. They are, of course, complete hooey. Pretty much ever since life even came into existence on this planet, there has been one group of life-forms whose hegemony has never been broken, and almost certainly never will be. There is only one true age - the Age of Bacteria. And with that piece of flowery writing, let me introduce today's Taxon of the Week: the bacterial suborder Corynebacterineae.

Bacterial taxonomy is a particularly murky subject. Though higher classificatory systems have been provided in many cases, very few of them get used in practice. At present, this is probably wise. There are relatively few well-defined clades of prokaryotes, and bacterial phylogeny is almost entirely genetics-based. Some clades of bacteria can be identified by their production of unique biochemicals, such as chlorophyll in Cyanobacteria. In the case of Corynebacterineae, the clade including such well-known bacteria as Corynebacterium and Mycobacterium, the distinguishing feature seems to be their production of mycolic acid (Portevin et al., 2004), the mild and unassuming molecule shown at the top of the page (image from Wikipedia).



Corynebacterineae are Gram-positive*, non-sporulating bacteria that are well-known as pathogens, though this overlooks the wide variety of non-pathogenic species in the clade (the image above shows Corynebacterium diphtheriae, the causative agent for diphtheria, and comes from MicrobeWiki). The clade is diverse in appearance - Corynebacterium and Mycobacterium are rod-shaped, Rhodococcus are spherical, while Nocardia form aerial hyphae. Corynebacterium and Mycobacterium, at least, reproduce by what is known as snapping division - the individual cell increases in length until it divides into two daughter cells which end up at a sharp angle to each other, as shown below in an image from Palaeos.com (though the actual prokaryote shown is not a member of Corynebacterineae, but the thermophilic archaeon Thermoproteus). This gives a distinct angular appearance to the resulting colonies, that has been referred to as "Chinese calligraphy".

*Technically, the structure of the cell envelope (see further on) means that Gram-staining doesn't really work as it should on Corynebacterineae, as they are impermeable to the acid used to wash out the Gram stain - hence they are referred to as acid-fast bacteria. Corynebacterineae are still phylogenetically Gram-positive bacteria.



Now I bet you've been hankering to know just what the significance of the mycolic acid is. The mycolic acid binds to other molecules in the cell wall, and also produces a thick waxy membrane outside the cell wall with other lipids. This waxy membrane greatly reduces the permeability of the bacterium, and makes them highly resistant to penetration by antibiotics (among other things - Portevin et al., 2004). Inability to synthesise mycolates is fatal for Mycobacterium, but appears to be less of an issue for Corynebacterium species - indeed, at least one Corynebacterium, C. amycolatum, does not naturally produce mycolates (Tropis et al., 2005).

REFERENCES

Portevin, D., C. de Sousa-D'Auria, C. Houssin, C. Grimaldi, M. Chami, M. Daffé & C. Guilhot. 2004. A polyketide synthase catalyzes the last condensation step of mycolic acid biosynthesis in mycobacteria and related organisms. Proceedings of the National Academy of Sciences of the USA 101 (1): 314-319.

Tropis, M., X. Meniche, A. Wolf, H. Gebhardt, S. Strelkov, M. Chami, D. Schomburg, R. Krämer, S. Morbach & M. Daffé. 2005. The crucial role of trehalose and structurally related oligosaccharides in the biosynthesis and transfer of mycolic acids in Corynebacterineae. Journal of Biological Chemistry 280 (28): 26573-26585.