Environmental Engineering Reference
In-Depth Information
compounds [6]. As a result of human activities in the extraction, transportation,
refinement, and use of petroleum, these compounds have become serious environ-
mental threats [7]. Due to their simple structure, mineralization of these compounds
is relatively easy and thereby favorable for biodegradation. A number of soil
bacteria, Pseudomonas sp., Ralstonia sp., Burkholderia sp. and Rhizobium sp., for
example, exhibit chemotaxis toward aromatic hydrocarbons (Table 7.2). Chemo-
taxis can play an important role in biodegradation of these compounds by bringing
degrading bacteria closer to sites that are contaminated with these pollutants.
7.5.2 Chemotaxis Toward Polycyclic Aromatic
Hydrocarbons (PAHs)
PAHs are compounds that consist of two or more aromatic rings. Naphthalene,
one of the most prevalent groundwater contaminants at sites contaminated
with PAHs [34], is relatively easily degraded and is often used as a model
compound in degradation studies of PAHs. A number of bacterial strains are
chemotactic toward naphthalene, including P. putida G7, Pseudomonas sp.
strain NCIB 9816-4 and P. putida RK J1 (Table 7.2). The chemotactic responses
in G7 and NCIB 9816-4 species were induced when grown with naphthalene
itself [35], whereas in RK J1 species, the response was induced by growing on
salicylate [36]. The naphthalene chemoreceptor, NahY (an MCP), is encoded
downstream of the naphthalene catabolic genes on the NAH7 plasmid [7].
Pandey and Jain [5] suggest that chemotaxis toward naphthalene and/or sali-
cylate might be due to a change in cellular energy levels due to metabolism of
these compounds and/or because of intracellular receptors that recognize such
contaminants or their degradation intermediates.
7.5.3 Chemotaxis Toward Nitroaromatic Compounds (NACs)
NACs are man-made pollutants and are difficult to degrade. They are generally
used as pesticides, herbicidal dyes, and explosives. Once released into the
environment, NACs undergo complex physical, chemical, and biological
changes resulting in harmful and toxic byproducts [25, 37]. Samanta et al. [25]
isolated Ralstonia sp. SJ98 from pesticide-contaminated soil using the drop
assay technique. Strain SJ98 is chemotactic toward a large number of NACs
and subsequently degrades them by metabolism [22, 25] and co-metabolism [24]
(Table 7.2). Bhushan et al. [33] reported chemotaxis-mediated biodegradation
of cyclic nitramine explosives by an obligate anaerobic bacterium Clostridium
sp. strain EDB2 (Table 7.2), which they isolated from marine sediments.
On the contrary, a number of other compounds such as p-nitroaniline, 2,3-
dinitrotoluene, naphthalene, phenanthrene, and salicylic acid, were neither
biodegradable nor chemoattractants for strain SJ98 [25]. These results further
