Environmental Engineering Reference
In-Depth Information
words, they change their tumbling frequency depending on the presence of
chemical stimuli.
Cell communication with their environment consists of complex extracellu-
lar signal reception and intracellular signal transduction mechanisms. A simple
mechanism for Escherichia coli chemotaxis toward amino acids and sugars is
described by Parales and Harwood [7]. E. coli are able to sense and respond to
changes in their environment through surface receptor molecules called methyl-
accepting chemotaxis proteins (MCPs) [8]. These MCPs embedded in the
plasma membrane usually bind the attractant molecules. Upon binding, this
activates intracellular signaling proteins which in turn alter the function of
the effector [13]. This signal transudation mechanism results in a change in
the direction of the flagellar rotation and is manifested as chemotaxis [7]. The
chemotactic machinery of E. coli consists of five MCPs and six chemotactic
proteins [7]. Complex signaling systems are found in many other chemotactic
species with multiple sets of chemotactic genes. For example, 25 or more MCPs
are found in Pseudomonas putida [7]. It is argued that despite these complexities,
the fundamental mechanism of signal reception and transduction is similar in
all species [14].
7.3 Random Motility and Chemotaxis Assays
A number of assays have been developed to evaluate the role of randommotility
and chemotaxis in bacterial transport. These assays can be divided into two
general groups: single-cell and population-scale studies. A comparative study of
bacterial random motility and chemotaxis quantification assays is presented by
Lewus and Ford [15]. In this section, we describe experimental techniques
for measuring and quantifying bacterial random motility and chemotaxis to
specific chemoattractants.
7.3.1 Capillary Assay
This method was first introduced by Adler [16, 17] and has since been repeatedly
modified to enable quantitative evaluation of chemotaxis. A typical capillary
assay consists of a microcapillary tube filled with attractant and placed in a pool
of motile bacteria at one end and sealed at the other (Fig. 7.1A). Motile bacteria
respond to the chemical gradient formed as a result of diffusion into the pool
and swim upgradient into the tube. The tube is removed after a pre-specified
time interval and cells accumulated inside the tube are enumerated to quantify
chemotaxis. A cloud of bacterial accumulation can also be observed micro-
scopically around the mouth of the capillary for qualitative observation.
Random motility can be quantified using similar experiments without a chemi-
cal attractant. Mathematical expressions for deriving quantitative expressions
