SAMPLE QUESTIONS: You should be able
to answer these questions at the conclusion of this laboratory. NAME, ID #:_______________________________________________.
Mankind and water borne diseases have been on intimate terms throughout evolutionary history. However, serious water borne disease (WBD) problems really didn't begin until cities were established. A fixed population of humans CONFINED in a compact area quickly results in a HIGH LOCAL level of human fecal pollution. This pollution rapidly finds its way into the nearest water supply which is usually the same one the humans use for drinking and food preparation. Although man has recognized for over #5,000 years that grossly contaminated water is undesirable, only in the last 150 years have we understood the relationship between polluted water and the transmission of human disease. A number of pathogenic microbes have evolved to take advantage of water as a vehicle of distribution between hosts. These include cholera, typhoid fever, amebiasis, hepatitis A, salmonellosis, infantile & traveler's diarrhea, campylobacteriosis, a number of viruses, and H. phylori. Altogether, these diseases take a terrible toll of life every year; approximately 15,000,000 children die every year from WBD. Even today, most of the world's people lack a safe, clean water supply.
Shortly after the relationship of polluted water and WBD was recognized people began demanding fecal-free water. However, the problem was how do you measure "fecal pollution"? The "nose test" only detects pollution in gross amounts and the taste test isn't recommended. Studies on the bacterial content of the human feces soon showed that several bacteria were common to man and many other animals whose feces routinely find their way into the water supply. It was reasoned that if one found: (a) a bacterium that resided, in nature, MAINLY IN THE GUT of humans that; (b) this organism could be used as an INDICATOR of likely fecal pollution. One such organism that fit these qualifications was a G- bacterium that was eventually named Escherichia coli. E. coli has a number of characteristics that make it a suitable #FECAL INDICATOR.
Because of these characteristics E. coli was chosen as the indicator of fecal pollution of water. Other bacteria have been found that also serve this purpose. European nations use a gut-Streptococci, rather than E. coli as a fecal indicator. Soon diagnostic/selective media were developed that enrich for E. coli and make its detection easier and faster. Because of its role as a fecal indicator, every microbiology lab has the media and expertise to grow E. coli. Therefore, E. coli has become the most thoroughly studied bacterium on the planet and in 1998 its entire genome was sequenced.
Two procedures are used to detect E. coli in water. These are the membrane filter technique (MFT) and the most probable number assay (MPN). The MFT requires that 100 ml of a water sample be filtered through a membrane that RETAINS THE BACTERIA. This membrane is then aseptically placed onto a differential/selective medium and incubated. Following incubation the number of E. coli colonies on the membrane are counted to determine the extent of the contamination.
The MPN assay involves adding aliquots of the water sample to LACTOSE broth tubes which contain small inverted tubes for trapping any gas formed. Following a 24 hour at 37oC incubation the number of tubes containing at least 10% gas in the inverted tubes are counted. Using statistical techniques the range of E. coli numbers present in a given sample can be estimated. Further tests are usually done on the presumed E. coli to verify their identification.
A FAQ is "I read in the paper that the residents of a town were told to boil their water because 'coliforms' had been found in the water supply. What does this mean?"
Answer: A common term for E. coli and related fecal bacteria is "coliforms", which of course comes from the combination of "coli" & "form". Thus in advising a community to boil it's water, the city officials are telling the people that their water has indications of fecal pollution and is probably is unsafe. I guess "coliforms" is less upsetting than telling people that their water probably has poop in it. What do you think?
The instructor will explain which of these two (or both) procedures to carry out.
SAMPLE QUESTIONS: You should be able
to answer these questions at the conclusion of this laboratory. Copyright © Dr. R. E. Hurlbert, 1999.
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