Page 453 - Corrosion Engineering Principles and Practice

420 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 421

microorganisms in terms of total cells present, viable cell numbers,
and metabolic activity. Table 10.9 also identifies assays that can be
used to establish the presence of biomass in a field sample, identify
the organisms present, and assess the activity of enzymes, such as
hydrogenase, that are thought to accelerate corrosion through
cathodic depolarization. A more detailed description of these assays
and other techniques is provided in the following sections.
Direct Inspection Direct inspection is best suited for the enumeration
of planktonic organisms suspended in relatively clean water. In liquid
suspensions, cell densities greater than 107 cells cm may cause the
−3
sample to appear turbid. Quantitative enumerations using phase
contrast microscopy can be done quickly using a counting chamber,
which holds a known volume of fluid in a thin layer and visualization
of microorganisms can be enhanced by fluorescent dyes that cause
cells to light up under ultraviolet radiation [19].
Cells separated by filtration from large aliquots of water can be visu-
alized and counted on a 0.25 µm filter using the epifluorescent technique
and a stain such as acridine orange. Stains such as fluorescein diacetate,
5-cyano-2,3-ditolyltetrazolium chloride or p-iodonitrotetrazolium violet
indicate active metabolism by the formation of fluorescent products [9].
Antibody fluorescence microscopy is similar to general fluorescent
microscopy, except that the fluorescent dye used is bound to antibodies
specific to SRBs. Only bacteria recognized by the antibodies fluoresce.
Results can be analyzed within two hours. The technique detects viable
and nonviable bacteria but it is limited to the type of SRB used in
the manufacture of these antibodies [19].
Identification of organisms can be accomplished by use of
antibodies generated as an immune response to the injection of
microbial cells into an animal, typically a rabbit. These antibodies
will selectively bind to the target organism in a field sample. A second
antibody tagged with a fluorescent dye is then used to light up the
rabbit antibody bound to the target cells. In effect the staining
procedure can selectively light up target organisms in a mixed
population or in difficult soil, coating, or oily emulsion samples [20].
Such techniques can provide insight into the location, growth rate,
and activity of specific kinds of organisms in mixed biofilm
populations.
Growth Assays A widely used method to assess microbial populations
in water samples is through growth tests using commercially available
growth media for groups of organisms most commonly associated
with industrial problems. These are packaged in a convenient form
suitable for use in the field. Serial dilutions of suspended samples are
grown on solid agar or liquid media. Based on the growth observed
for each dilution, estimates of viable cells, most probable number