A corrosive bacterium that hunts in packs and smells like rain

Myxobacteria are a strange subset of bacteria known for their ability to glide across solid surfaces (leaving behind trails of slime) and clump together into multicellular swarms that prowl about searching for food. They're pretty cool. Nannocystis exedens (see number 29) is a myxobacterium commonly found in a wide range of soils all over the world.

N. exedens obtains food by corroding the cells of bacteria and fungi, causing them to rupture and spill out their nutritious guts (exedens is Latin for "to eat away"). If you grow the bacterium on the surface of a gel-like agar-based growth medium, it will etch holes, tunnels, and caves into the agar as it glides about.

Clumps of N. exedens etching holes into an agar plate (Source)

Myxobacteria are known for their ability to inhibit the growth of soil-based plant pathogens and likely have an important ecological role in regulating their populations. Among the fungi attacked by N. exedens are the moulds Aspergillus flavus and Aspergillus parasiticus, which grow on cultivated plants and are capable of producing highly toxic aflatoxins that can harm people and livestock who consume mouldy crops. This has raised the possibility of using N. exedens to control the growth of aflatoxin-producing fungi.

Like members of the phylum Actinobacteria, myxobacteria are known for their ability to produce unique secondary metabolites, some of which are under investigation as potential drugs (e.g. anticancer epothilones). In particular, N. exedens makes and releases a bunch of interesting compounds:

• Geosmin, which is partly responsible for the wholesome earthy smell that lingers after a rainstorm (petrichor)
• Phenylnannolone A, which has an usual chemical structure and may someday be used as a drug to target cancer cells and reduce their resistance to anticancer drugs
• 2-Methylisoborneol, which is to blame for the off-putting musty smell sometimes associated with drinking water
• Nannochelins, which are capable of binding iron (an essential nutrient that can often be difficult for microbes to acquire) and inhibiting the growth of bacteria and fungi
• Carotenoids, which are pigments also found in plants and fungi that confer a bright orange-yellow or red-violet colour to swarms of N. exedens

References

Dickschat JS, Nawrath T, Thiel V, Kunze B, Müller R, Schulz S. 2007. Biosynthesis of the off-flavor 2-methylisoborneol by the myxobacterium Nannocystis exedens. Angewandte Chemie International Edition 46(43):8287-8290.

Kunze B, Trowitzsch-Kienast W, Höfle G, Reichenbach H. 1992. Nannochelins A, B and C, new iron-chelating compounds from Nannocystis exedens (myxobacteria). Production, isolation, physico-chemical and biological properties. Journal of Antibiotics 45(2):147-150. [Full text]

Ohlendorf B, Leyers S, Krick A, Kehraus S, Wiese M, König GM. 2008. Phenylnannolones A-C: Biosynthesis of new secondary metabolites from the myxobacterium Nannocystis exedens. Chembiochem 9(18):2997-3003.

Reichenbach H. 1970. Nannocystis exedens gen. nov., spec. nov., a new myxobacterium of the family Sorangiaceae. Archives of Microbiology 70(2):119-138. [First two pages]

Taylor WJ, Draughon FA. 2001. Nannocystis exedens: A potential biocompetitive agent against Aspergillus flavus and Aspergillus parasiticus. Journal of Food Protection 64(7):1030-1034.

Trowitzsch W, Witte L, Reichenbach H. 1981. Geosmin from earthy smelling cultures of Nannocystis exedens (Myxobacterales). FEMS Microbiology Letters 12(3):257-260.