Fundamentals of Biological Intervention - Environmental Biotechnology: Theory and Application - page 55

they carry, which may have been latent over thousands of bacterial generations,
owe their existence to previous exposure over millions of years to an accumu-
lated 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 in Chapter 9. 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 poly aromatic hydrocarbons (PAHs)
although some of these only by cometabolism with a supplied carbon source.
Endocrine disrupters
To date, there are chemicals, including xenobiotics, which still resist degradation
in the environment. This may be due to a dearth at the site of contamination, of
organisms able to degrade them fully or worse, microbial activity which changes
them in such a way that they pose a bigger problem than they did previously. One
such example is taken from synthetic oestrogens such as 17 α -ethinyloestradiol
commonly forming the active ingredient of the birth control pills, and the nat-
ural oestrogens which, of course, are not xenobiotics. Natural oestrogens are
deactivated in humans by glucuronidation, as shown in Figure 3.1, which is a
conjugation of the hormone with UDP-glucuronate making the compound more
polar and easily cleared from the blood by the kidneys. It is in this modified
and inactive form that it is excreted into sewage. However, bacteria present in
the aerobic secondary treatment in sewage treatment plants, have the enzyme,
β-glucuronidase , which removes this modification thus reactivating the hormone.
As an aside, glucuronidation is not confined to hormones but is a process used
to detoxify a number of drugs, toxins and carcinogens in the liver. The enzyme
catalysing this process is induced in response to prolonged exposure to the toxin
thus imparting increased tolerance or even resistance to the chemical.
Returning to the problem of elevated levels of active hormones in the water-
ways, another aspect is that steroids do not occur in bacteria, although they are
present in fungi, and so bacteria lack the necessary pathways to allow com-
plete degradation of these hormones at a rate compatible with the dwell time in
sewage treatment plants. The consequence has been raised levels of reactivated
oestrogen and 17 α -ethinyloestradiol in the waterways leading to disturbances of
the endocrine, or hormonal, system in fauna downstream from sewage treatment
plants. Such disturbances have been monitored by measuring the presence of the
protein vitellogenin (Sole et al ., 2001) which is a precursor to egg yolk protein,
the results of which have indicated feminisation of male fish in many species