Environmental Engineering Reference
In-Depth Information
Figure 5.2
Colonies of higher bacteria (actinomycetes).
Most bacteria are heterotrophic; i.e., they derive their nutrition by metab-
olizing simple-complex organic molecules. Bacteria can grow on many sub-
strates, and as a consequence are widely distributed in nature. In decom-
posing dead organic matter or destroying living tissue, they serve as
organisms of deterioration or decay; they are commensal when they live on
the by-products of organisms without causing disease; they are parasitic
when they infect an organism and cause disease.
Bacteria vary in their environmental requirements and tolerances. These
include nutritional substrates, availability of oxygen, moisture levels, pH,
and temperature. Water is required for the growth of individual cells and
for rapid reproduction. Temperature optima vary for different species/types.
Mesophilic bacteria grow well in the temperature range of 20 to 40°C; ther-
mophiles at 40 to 80°C. Bacteria do not tolerate ultraviolet light and may be
killed in a few hours when exposed to direct sunlight.
1.
Bacterial toxins
Bacteria produce a variety of metabolites during growth. In the actinomycete
group, these include antibiotics which limit the growth of other bacteria.
Streptomycin produced by the members of the genus
Streptomyces
is a classic
example.
Bacteria may produce both exotoxins and endotoxins. Exotoxins are
produced within bacterial cells and excreted onto or into substrates. The best
known exotoxin is botulism toxin, produced by
Clostridium botulinum.
Endotoxins are produced in the outer membrane of Gram-negative bac-
teria. The cell wall is covered by a lipopolysaccharide layer that consists of
a complex of lipids, carbohydrates, and proteins. Endotoxins are large mol-
ecules of Lipid-A and a core polysaccharide (2-keto-3-deoxy-
-mannoocta-
losomic acid). They are heat stable and are only released after bacterial walls
have been damaged or destroyed.
D
Escherichia coli
(
E. coli
, found in human
intestines) and the genera
Pseudomonas
and
Salmonella,
are common bacteria
 
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