Angewandte
Chemie
DOI: 10.1002/anie.200702496
Bacterial Scents
Biosynthesis of the Off-Flavor 2-Methylisoborneol by the
Myxobacterium Nannocystis exedens**
Jeroen S. Dickschat, Thorben Nawrath, Verena Thiel, Brigitte Kunze, Rolf Müller, and
Stefan Schulz*
Bacteria are generally not regarded as organisms that
contribute significantly to odors of our environment. Never-
theless, many bacteria produce volatiles of different substance
classes, such as fatty acid derivatives, aromatic compounds,
nitrogen and sulfur compounds, or terpenoids.[1] These odors
can play important ecological roles, such as influencing the
behavior of humans and other organisms. Highly complex
profiles of volatiles are emitted by streptomycetes and
myxobacteria,[2–7] and the frequently found musty or earthy
smelling terpenoids 2-methylisoborneol (1) and geosmin (2)
Especially cyanobacteria can cause contaminations of
freshwater with the undesirable flavor (off-flavor) 1, that can
result in significant economic losses in fishery.[13] Further-
more, low concentrations of 1 (10 ngLÀ1) cause malodorous
drinking water that is strongly rejected by humans.[14] Such
concentrations are not toxic, but the offensive odor may lead
to psychosomatic effects, such as headaches, stomach upsets,
or stress.[15]
Interestingly, all known bacterial producers of the wide-
spread terpenoids 1 and 2, that is, myxobacteria, actino-
mycetes, and cyanobacteria, exhibit complex life cycles
including the formation of multicellular complexes.[16] The
inevitably coordinate behavior of single cells in the under-
lying processes to multicellular stages requires cell–cell
communication, as has been demonstrated in the myxobacte-
rium Stigmatella aurantiaca where fruiting body formation is
initiated by the volatile pheromone stigmolone (3).[17] The
terpenoids 1 and 2 might play a role in bacterial communi-
cation, but this remains speculative. The biosynthesis of the
sesquiterpenoid 2 has recently been reported in myxobac-
teria[18] and streptomycetes,[19] whereas only little was known
about the biosynthesis of 1 prior to this study. Early radio-
labeling experiments suggested that 1 is a methylated mono-
terpene, the additional methyl-group being derived from S-
adenosylmethionine (SAM).[20] Herein we report the unique
biosynthesis of 1 in the myxobacterium Nannocystis exedens
and the odor bouquet of this species.
have extremely low odor thresholds. Compound 1, a homo-
monoterpene, was first identified in Streptomyces lavendu-
lae,[8] and later in several other actinomycetes,[6,9] cyanobac-
teria,[10] fungi,[11] and a liverwort, where it occurs as the (À)-
enantiomer.[12]
Odor analysis of bacteria can be conveniently performed
by headspace collection techniques from agar plate cultures,
followed by GC/MS analysis.[3] Biosynthetic studies are thus
possible with milligram amounts of labeled precursors added
to the medium.[4] The volatiles of N. exedens (Na e485,
Na eB37) and N. exedens subsp. cinnabarina (Na c29) were
investigated using these techniques. Their complex bouquets
are composed of up to 31 different compounds (Supporting
Information). The main component of all strains is 1
accompanied by 2 and minor amounts of its biosynthetic
precursor, (1(10)E,5E)-germacradien-11-ol (4). Further com-
pounds can be classified as esters, lactones, aromatic com-
pounds, terpenoids, fatty alcohols, or pyrazines; 2,5-diisopro-
pylpyrazine being especially prominent in one strain. To
clarify the absolute configuration of 1, both enantiomers were
synthesized from d-(+)- and l-(À)-camphor by CeCl3-cata-
lyzed addition of MeMgCl (Supporting Information).[21] GC
on a chiral stationary phase established that only (À)-1 is
released by N. exedens. This is the same enantiomer as in the
liverwort Lophocolea heterophylla,[12] whereas the absolute
configuration of 1 produced by other bacteria is unknown.
[*] Dr. J. S. Dickschat, Dipl.-Chem. T. Nawrath, Dipl.-Chem. V. Thiel,
Prof. Dr. S. Schulz
Institut für Organische Chemie
Technische Universität Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
Fax: (+49)531-391-5272
E-mail: stefan.schulz@tu-bs.de
Dr. B. Kunze
Helmholtz-Zentrum für Infektionsforschung
Mascheroder Weg 1, 38124 Braunschweig (Germany)
Dr. J. S. Dickschat, Prof. Dr. R. Müller
Institut für Pharmazeutische Biotechnologie
Universität des Saarlandes
Im Stadtwald, 66123 Saarbrücken (Germany)
[**] This work was supported by fellowships of the Verband der
Chemischen Industrie and the Deutsche Akademie der Natur-
forscher Leopoldina to J.S.D. We thank Birte Engelhardt
(Braunschweig) for technical assistance and Petra Holba-Schulz
(Braunschweig) for recording NMR spectra. Off-flavor: An unde-
sirable flavor imparted on a food product.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2007, 46, 8287 –8290
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8287