Xenobiotics and Other Problematic Chemicals
The word is derived from the Greek ‘xenos’ meaning foreign. Throughout this book the definition used is that xenobiotics are compounds which are not produced by a biological procedure and for which no equivalent exists in nature. They present a particular hazard if they are subject to bioaccumulation especially so if they are fat soluble since that enables them to be stored in the body fat of organisms providing an obvious route into the food chain. Despite the fact that these chemicals are man made, they may still be degraded by micro-organisms if they fit into one of the following regimes; gratuitous degradation, a process whereby the xenobiot resembles a natural compound sufficiently closely that it is recognised by the organism’s enzymes and may be used as a food source, or cometabolism where the xenobiot is degraded again by virtue of being recog-nised by the organism’s enzymes but in this case its catabolism does not provide energy and so cannot be the sole carbon source. Consequently, cometabolism may be sustained only if a carbon source is supplied to the organism. The ability of a single compound to be degraded can be affected by the presence of other contaminants. For example, heavy metals can affect the ability of organisms to grow, the most susceptible being Gram positive bacteria, then Gram negative. Fungi are the most resistant and actinomycetes are somewhere in the middle. This being the case, model studies predicting the rate of contaminant degrada-tion may be skewed in the field where the composition of the contamination may invalidate the study in that application. Soil micro-organisms in particular are very versatile and may quickly adapt to a new food source by virtue of the transmission of catabolic plasmids. Of all soil bacteria, Pseudomonads seem to have the most highly developed ability to adapt quickly to new carbon sources. In bacteria, the genes coding for degradative enzymes are often arranged in clus-ters, or operons, which usually are carried on a plasmid. This leads to very fast transfer from one bacterium to another especially in the case of Pseudomonas where many of the plasmids are self-transmissible. The speed of adaptation is due in part to the exchange of plasmids but in the case of the archaeans particularly, the pathways they carry, which may have been latent over thousands of bacterial generations, owe their existence to previous exposure over millions of years to an accumulated vast range of organic molecules. It is suggested that, unless there has been evolutionary pressure to the contrary, these latent pathways are retained to a large extent requiring little modification if any to utilise new xenobiotics. Even so, bioremediation may require that organisms are altered in some way to make them more suitable for the task and this topic is addressed. Briefly, the pathways may be expanded by adaptation to the new molecule, or very much less commonly, wholescale insertion of ‘foreign’ genes may occur by genetic manipulation. There have been several cases reported where catabolic pathways have been expanded in the laboratory. Hedlund and Staley (2001) isolated a strain of Vibrio cyclotrophicus from marine sediments contaminated with creosote. By supplying the bacteria with only phenanthrene as a carbon and energy source, the bacteria were trained to degrade several PAHs although some of these only by cometabolism with a supplied carbon source.