Agriculture Reference
In-Depth Information
are used to assess the effi cacy of a treatment process (e.g., drinking water treatment),
while fecal indicators indicate the presence of fecal contamination. An index (or
model) organism represents the presence and behavior of a pathogen in a given envi-
ronment. These indicators have been used to judge the safety of drinking and recre-
ational waters, as well as indicating the success of treatment processes. However, how
useful these indicators are for assessment of irrigation is currently not known.
The coliform group, which includes the genus
Escherichia
,
Citrobacter
,
Enterobacter
, and
Klebsiella
, is relatively easy to detect. Specifi cally, this group
includes all aerobic and facultatively anaerobic, Gram - negative, non - spore - forming,
rod-shaped bacteria that produce gas upon lactose fermentation in prescribed culture
media within 48 hours at 35 °C. The coliform group has been used as the standard for
assessing fecal contamination of drinking waters for most of the last century. However,
they can originate from certain plants and are capable of growth in the environment
under certain conditions. They appear to be very common in large numbers in irriga-
tion waters in Arizona not directly impacted by fecal contamination (Kayed 2004).
The die-off rate of coliform bacteria depends on the amount and type of organic matter
in the water and its temperature. If the water contains signifi cant concentrations of
organic matter and is at an elevated temperature, the bacteria may increase in numbers.
This phenomenon has been observed in eutrophic tropical waters, waters receiving
pulp and paper mill effl uents, wastewater, aquatic sediments, and organically enriched
soil (i.e., sewage sludge-amended) after periods of heavy rainfall (Zaleski and others
2005; Carrillo and others 1985).
Fecal coliforms, which include the genera
Escherichia
and
Klebsiella
, are differ-
entiated in the laboratory by their ability to ferment lactose with the production of
acid and gas at 44.5 °C within 24 hours. The frequent occurrence of coliform and fecal
coliform bacteria in unpolluted tropical waters, and their ability to survive for consid-
erable periods of time outside the intestine in these waters, have suggested that these
organisms occur naturally in tropical waters (Solo-Gabriele and others 2000) and that
new indicators for these waters need to be developed.
E. coli
is more commonly being used as an indicator because it can easily be dis-
tinguished from other members of the fecal coliform group (e.g., absence of urease
and presence of
-
glucuronidase) and more likely to indicate fecal pollution. Fecal
coliforms also have some of the same limitations in use as the coliform bacteria, i.e.,
regrowth and less resistance to water treatment than viruses and protozoa.
The enterococci have been suggested as useful indicators of risk of gastroenteritis
for recreational bathers, and standards have been recommended (Cabbelli 1981).
However, they may not be useful in tropical waters (Byappanahalli and Fujioka 2004;
Fujioka and others 1999) and are also common in warm irrigation waters used in
Arizona (Kayed 2004 ).
β
Standards and Criteria for Indicators
The use of microbial standards also requires the development of standard methods
and quality assurance or quality control plans for the laboratories that will do the
monitoring. Knowledge of how to sample and how often to sample is also important.
All this information is usually defi ned in the regulations when a standard is set. For
