Environmental Engineering Reference
In-Depth Information
present-day human population is the increasing number of infections caused by
bacteria not normally considered highly virulent. These organisms, sometimes
considered secondary pathogens, are opportunistic bacteria that, under certain
conditions, can cause infections through contact in some way with water.
31
Cer-
tain groups of people notably, infants, elderly, immunocompromised, transplant
recipients, and convalescents, are at greatest risk of susceptibility to infection by
these organisms. A summary of some important opportunistic bacterial pathogens
appears in Table 1.5. Several of the bacterial species listed in the table are rela-
tively newly discovered and responsible for specific pathological problems. Two
such organisms are
Helicobacter pylori
and
Legionella pneumophila
.
Gastrointestinal disturbances are so commonplace in the human experience in
wealthy countries that they are essentially an accepted fact of life, hence, usu-
ally receive little medical attention and go unreported. However, in undeveloped
lands, gastrointestinal diseases are a ravishing scourge that accounts for numer-
ous deaths, especially, among children. Of an estimated 2.2 million deaths from
diarrheal-type diseases, 1.8 million of these involve children under five years
of age.
32
To grasp the importance of safe drinking water on reduction of child
mortality in various countries of the world, examine the comparative data in
Figure 1.2.
33
The occurrence of a large number of diarrheal cases indicates that
there has been a breakdown in hygiene or in the sanitary control of water or food
and may forewarn impending cases of salmonellosis, typhoid fever, dysentery,
or other illness.
Bacteria are prokaryotic, microscopic organisms, typically unicellular with
morphologies described as coccoidal (ovoid), bacillary (rodlike), spiral (vibroid
or helical), and filamentous. Typical eubacterial single-cell dimensions average
0.5to1
m in length. Bacterial physiologies are
more varied among the species than those of any group of microorganisms that
supports the notion that plant and animal life on earth as we know it would
not be possible without the bacteria. Unfortunately, the typical notoriety that
bacteria in general have among the uninformed is that bacteria are “germs” and,
therefore, are synonymous with disease.
Rickettsias
are obligate, intracellular
parasitic bacteria not cultivatable outside host cells. Unlike viruses, they are
retained by the Berkefield filter.
34
Their sizes average 0.3 to 0.7
µ
mby1to2
µ
m.
Viruses are submicroscopic, genetic parasitic elements consisting of a nucleic
acid (DNA or RNA) core surrounded by a protein coat, fall in the size range of
10 to 100 nm, pass through filters that retain bacteria, are visible only with the
aid of an electron microscope, and can replicate only following invasion of living
(host) cells. Viruses responsible for diseases transmitted by the water route are all
RNA viruses, and most are geometrically icosahedral (ovoid) and small (about
30 nm) in size. Virus particles (virions) maintain infectiousness outside the host.
Although all viruses require a host for sustaining replication of virions, expression
of a clinical disease does not always take place. Animal enteric viruses do not
appear to be readily transmissible to humans, although hepatitis A virus has been
shown to pass from chimpanzees to humans. There are more than 100 types of
human enteric viruses excreted in large numbers from the gastrointestinal tract.
µ
mindiameterby1to5
µ
