Fundamentals of Biological Intervention 55be used to convert cellulose into industrially useful substances. A note of caution
is that cellulose is a major product of photosynthesis and, being the most abundant
biopolymer on this planet, has a vital role to play in the carbon cycle. Large-scale
disturbance of this balance may have consequences to the environment even less
welcome than the technologies they seek to replace. However, judicious use of
this biotechnology could reap rewards at many levels.
Bacteria have also adapted to degrade man-made organics called xenobiotics.
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 pro-
duced 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 ofPseudomonas
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