Environmental Engineering Reference
In-Depth Information
of carbon and/or energy are termed chemoattractants for those specific bac-
teria. Similarly, chemorepellents repel certain bacteria and are often toxic [5].
Recently, it has been reported that chemical contaminants that are readily
degraded at low concentrations are toxic to bacteria above certain concentra-
tion thresholds [9, 10]. In such cases, a chemical can act both as a chemoat-
tractant (at low concentrations) and a chemorepellent (at high concentrations)
for the same bacterial strain.
7.2.1 Why Microorganisms Pose Chemotaxis
Responding to changes in the environment is a fundamental property of living
cells and is of prime importance to single-cell microorganisms as they interact
with their changing environment [5]. Through evolution, microorganisms have
developed mechanisms that help them regulate their cellular mechanisms to
changing environments [11]. Chemotaxis is an advantageous behavior selected
by bacteria that has probably evolved as a result of bacterial pursuit of energy
sources. It has been observed that numerous bacteria develop the ability to
respond chemotactically to certain chemicals when they are grown under spe-
cific carbon and energy source conditions. Childers et al. [12] report that growth
of flagella and pili, appendages responsible for chemotactic movement of
bacteria, takes place on the surface of Geobacter metallireducens cells only
when cells are grown under insoluble electron acceptor conditions. In contrast,
cells grown in the presence of soluble substrates are neither chemotactic nor do
they develop flagella and pili. They concluded that chemotaxis enables bacteria
to establish contact with insoluble electron acceptors. Pandey and Jain [5]
suggest that bacteria developed taxis, including chemotaxis, aerotaxis and
phototaxis, over time as naturally available energy sources became limited.
Introduction of anthropogenic pollutants into the environment, along with
increased competition for natural resources, may have given bacteria the ability
to respond to and degrade these chemical pollutants as sources of carbon and
energy.
7.2.2 Molecular Mechanisms of Chemotaxis
In a uniform environment, bacteria swim in a randommigration, which consists
of relatively straight swimming, interrupted by tumbling events that change the
swimming direction. This alternating series of runs and tumbles is governed by
the direction of flagellar rotation. The overall bacterial motion results in a 3D
random walk which is somewhat analogous to diffusion of gas molecules [8]. In
the presence of chemical gradients in the surrounding medium, bacteria lengthen/
shorten their run length between two consecutive tumbling movements; in other
