Existing Methods of Waterborne Pathogen Detection - Waterborne Pathogens: Detection Methods and Applications - page 43

aerobic and facultative anaerobic Gram-negative, nonsporing, rod-shaped
bacteria that ferment lactose with gas formation within 48 h at 35 °C. Even
though all the coliforms will not come from human feces, the total coliform
count is used as an index of fecal pollution. The representative species of the
fecal group is E. coli , which may be differentiated from members of the non-
fecal group by means of four biochemical tests: indole, methyl red, Voges-
Proskauer and citrate. Coliforms from the intestines are also able to ferment
lactose at an elevated temperature of 44.5 °C with the production of acid and
gas. It is reasoned that if many coliforms are present in a given water sample,
and there is a good likelihood that enteric pathogens may also be present.
Streptococcus fecalis ; Lancefield's group D enterococci, are also normal
inhabitants of the intestines and may be used as an indicator of fecal con-
tamination of water. The ratio of E. coli to Streptococci has been used to
indicate whether the contamination comes from human or animals sources.
One method for measuring coliform numbers (quantity) in water is
the membrane filter technique. This technique involves filtering a known
volume of water through a special sterile membrane filter, with a pore size
of 0.45, thus small enough to retain bacteria. When the water sample is
filtered, bacteria become trapped on the surface. The filter is then placed
on media in a petri dish to allow the bacteria to grow. The use of selective
media allows easy detection of the representative indicator. A wide range
of selective media have been developed and validated for use in membrane
filtration techniques for Coliform and Enterococci/fecal streptococci ana-
lyzes. On m-ENDO plates, coliform colonies will appear pink to dark red
in color with a golden green metallic sheen or luster. On m-FC plates, fecal
coliforms will appear blue. And if necessary, m-enterococcus plates can be
used to detect fecal streptococci colonies, which will appear light pink and
flat or dark red on this media. These colonies can be directly counted and
are typically reported as (colony forming units) cfu 100 mL −1 .
There are a number of official published methods based on the above
approach, notably a series of ISO methods, such as ISO 9308-1 for coli-
forms and E. coli and ISO 7899-2 for enterococci. The US Environmental
Protection Agency (EPA) has also published official microbiological meth-
ods for water testing.
The heterotrophic plate count (HPC) is also another method for moni-
toring overall bacteriological quality of drinking water. Although they do not
directly relate to the safety of water, concentration changes in these counts
can indicate overall changes in the quality of the water. This analysis is per-
formed by mixing a volume, typically 1 mL, of the sample water to a volume