During the past few years, an increasing awareness concerning the emission of an unexpected high number of bacterial volatiles has been registered. Humans sense, intensively and continuously, microbial volatiles that are released during food transformation and fermentation, e.g., the aroma of wine and cheese. Recent investigations have clearly demonstrated that bacteria also employ their volatiles during interactions with other organisms in order to influence populations and communities. This review summarizes the presently known bioactive compounds and lists the wide panoply of effects possessed by organisms such as fungi, plants, animals, and bacteria. Because bacteria often emit highly complex volatile mixtures, the determination of biologically relevant volatiles remains in its infancy. Part of the future goal is to unravel the structure of these volatiles and their biosynthesis. Nevertheless, bacterial volatiles represent a source for new natural compounds that are interesting for man, since they can be used, for example, to improve human health or to increase the productivity of agricultural products.

Bacterial antagonists are bacteria that negatively affect the growth of other organisms. Many antagonists inhibit the growth of fungi by various mechanisms, e.g., secretion of lytic enzymes, siderophores and antibiotics. Such inhibition of fungal growth may indirectly support plant growth. Here, we demonstrate that small organic volatile compounds (VOCs) emitted from bacterial antagonists negatively influence the mycelial growth of the soil-borne phytopathogenic fungus Rhizoctonia solani Kühn. Strong inhibitions (99-80%) under the test conditions were observed with Stenotrophomonas maltophilia R3089, Serratia plymuthica HRO-C48, Stenotrophomonas rhizophila P69, Serratia odorifera 4Rx13, Pseudomonas trivialis 3Re2-7, S. plymuthica 3Re4-18 and Bacillus subtilis B2g. Pseudomonas fluorescens L13-6-12 and Burkholderia cepacia 1S18 achieved 30% growth reduction. The VOC profiles of these antagonists, obtained through headspace collection and analysis on GC-MS, show different compositions and complexities ranging from 1 to almost 30 compounds. Most volatiles are species-specific, but overlapping volatile patterns were found for Serratia spp. and Pseudomonas spp. Many of the bacterial VOCs could not be identified for lack of match with mass-spectra of volatiles in the databases.

Volatiles of Stenotrophomonas, Serratia, and Bacillus species inhibited mycelial growth of many fungi and Arabidopsis thaliana (40 to 98%), and volatiles of Pseudomonas species and Burkholderia cepacia retarded the growth to lesser extents. Aspergillus niger and Fusarium species were resistant, and B. cepacia and Staphylococcus epidermidis promoted the growth of Rhizoctonia solani and A. thaliana. Bacterial volatiles provide a new source of compounds with antibiotic and growth-promoting features.

Many interactions between organisms are based on the emission and perception of volatiles. The principle of using volatile metabolites as communication signals for chemo-attractant or repellent for species-specific interactions or mediators for cell-to-cell recognition does not stop at an apparently unsuitable or inappropriate environment. These infochemicals do not only diffuse through the atmosphere to process their actions aboveground, but belowground volatile interactions are similarly complex. This review summarizes various eucaryotes (e.g., plant (roots), invertebrates, fungi) and procaryotes (e.g., rhizobacteria) which are involved in these volatile-mediated interactions. The soil volatiles cannot be neglected anymore, but have to be considered in the future as valuable infochemicals to understand the entire integrity of the ecosystems.

Bacteria emit a wealth of volatiles. The combination of coupled gas chromatography/mass spectrometry (GC/MS) and proton-transfer-reaction mass spectrometry (PTR-MS) analyses provided a most comprehensive profile of volatiles of the rhizobacterium Serratia odorifera 4Rx13. An array of compounds, highly dominated by sodorifen (approximately 50%), a bicyclic oligomethyl octadiene, could be detected. Other volatiles included components of the biogeochemical sulfur cycle such as dimethyl disulfide (DMDS), dimethyl trisulfide and methanethiol, terpenoids, 2-phenylethanol, and other aromatic compounds. The composition of the bouquet of S. odorifera did not change significantly during the different growth intervals. At the beginning of the stationary phase, 60 μg of volatiles per 24 h and 60 easily detectable components were released. Ammonia was also released by S. odorifera, while ethylene, nitric oxide (NO) and hydrogen cyanide (HCN) could not be detected. Dual culture assays proved that 20 μmol DMDS and 2.5 μmol ammonia, individually applied, represent the IC(50) concentrations that cause negative effects on Arabidopsis thaliana.