Microbial volatile compounds in health and disease conditions

Abstract

Microbial cultures and/or microbial associated diseases often have a characteristic smell. Volatile organic compounds (VOCs) are produced by all microorganisms as part of their normal metabolism. The types and classes of VOC produced is wide, including fatty acids and their derivatives (e.g. hydrocarbons, aliphatic alcohols and ketones), aromatic compounds, nitrogen containing compounds, and volatile sulfur compounds. A diversity of ecological niches exist in the human body which can support a polymicrobial community, with the exact VOC profile of a given anatomical site being dependent on that produced by both the host component and the microbial species present. The detection of VOCs is of interest to various disciplines, hence numerous analytical approaches have been developed to accurately characterize and measure VOCs in the laboratory, often from patient derived samples. Using these technological advancements it is evident that VOCs are indicative of both health and disease states. Many of these techniques are still largely confined to the research laboratory, but it is envisaged that in future bedside 'VOC profiling' will enable rapid characterization of microbial associated disease, providing vital information to healthcare practitioners.

Figures

Figure 1.

Composition and microbial hydrolysis pathways for complex feedstock. The majority of carbon-energy nutrient is in the form of soluble or insoluble polymers.

Figure 2.

Formation of indolic (1) and phenolic (2 and 3) volatile compounds. (Adapted from Macfarlane and Macfarlane .)

Figure 3.

Microbial mechanisms in the generation of oral malodour gases from a ‘generalized’ bacterial cell.

Figure 4.

Main VOCs from fermentation in the gastrointestinal tract (large colon).

Figure 5.

Classes of physical in vitro model for measuring VOC production rates from microbes by gas analysis (GA). (a) Planktonic batch culture; microbial headspace can be sampled from closed accumulative models. (b) Batch culture biofilm; a supporting substratum (SS) is provided to enable surface microbial growth. (c) Planktonic continuous culture; enabling steady state growth conditions, as nutrients (growth medium, M) are continuously added and waste products (W) taken away. (d) Continuous flow biofilms using impermeable substratum; nutrients (growth medium, M) are continuously added and waste products (W) taken away, enabling dynamic quasi-steady state microbial biofilms to form. (e) Continuous flow biofilms using highly permeable matrix perfusion substratum; all cells are close to nutrient micro-channels and are served with growth-limiting nutrient at approximately the same rate. More homogeneous environment with all cells growing at nutrient supply rate (i.e. µ proportional to flow rate) forming near true steady state biofilms under controlled conditions.