Host—microbe molecular signaling

H. P. Spaink, O. Geiger. D. M. Sheeley, A. A. N. Van Brussel, W. S. York, V. N. Reinhold, B. J. J. Lugtenberg, and E. P. Kennedy, The biochemical function of the Rhizobium leguminosarum proteins involved in the production of host-specific signal molecules. Advances in Molecular Genetics of Plant-Microbe Interactions. Vol. 1 (H. Hennecke and D. P. S. Verma, eds.), Kluwer Academic Publishers, Dordrecht, The Netherlands, 1991. 

Odorant molecules are typically small organic molecules with a molecular weight of 26-300. An odor consists of millions of volatile molecules emitted into the fluid media of air or water. The vapor pressure of a compound is a measure that directly relates to its volatility odorants with high vapor pressures in ambient conditions volatilize readily. Inanimate odor sources passively emit odors animate odor sources may passively or actively emit them. The intensity and chemical nature of both passive and active emission are a function of the properties of the compounds present, the temperature, and the atmospheric pressure. Examples of inanimate passive emission would include an inanimate object such as a glass of wine emitting hundreds of compounds from the fluid surface into the headspace in the immediately vicinity of the fluid." Animate passive emission often occurs as a function of a metabolic process. For example, the odors typically associated with rotting meat are produced and emitted by microbes as a byproduct of metabolism." Mammals emit CO2 and lactic acid as a byproduct of metabolism CO2 and lactic acid may be used by insects, such as mosquitos, to locate mammalian hosts." " In active emission, odors are synthesized and emitted by an animate signaler. Examples... 

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