The Membrane Filtration method for fecal coliforms
For quality control of potable water in rural areas of Nicaragua as well as in the cities of Jinotega, León, Matagalpa and Somoto we have used the Membrane Filtration method of to measure fecal coliforms, following the procedure described in "Standard Methods for the Examination of Water and Wastewater" (20th Edition, 1998, American Public Health Association, Washington, DC). The method has many advantages; it is a standard method, recognized world-wide as trustworthy. It can be used in the field as well as in large urban and small rural laboratories. During 21 years of use in Nicaragua, we have found it possible to train workers with a wide range of experience and education in its use and the results obtained have always trustworthy.
Setup for M-FC method, Matagalpa
Principles and Details of the Method.
Escherichia coli is the predominant fecal coliform but the group also includes other enterobacteria such as Citrobacter, Enterobacter and Klebsiella species. Most fecal coliform bacteria are not themselves pathogenic but are measured because their presence in water reliably indicates the presence of fecal contamination of either human or animal origin. The group is distinguished from other bacteria by the ability to ferment lactose and to grow in the presence of detergents. Thus the coliform-selective medium M-FC contains detergents to inhibit the growth of other bacteria. Lactose and a colored indicator of lactose fermentation are also present. The fecal coliforms are distinguished from other coliforms by incubation at an elevated temperature.
Water samples of at least 120 ml are collected in "Whirlpak" sterile sample bags and within 6 hours each sample is analyzed by passing two volumes (usually 10 & 100 ml) through sterile filters, which retain any bacteria in the sample. Each filter is placed in a Petri dish over an absorbent paper disc soaked in the M-FC medium. The plates are sealed and incubated for 16 to 24 hours at a temperature of 44.5 ± 0.2oC. The number of blue colonies on each plate are then counted as fecal coliforms. Overcrowded plates are not counted. The average of the countable plates is recorded as "fecal coliforms per 100 ml of water". If there are colonies that are not blue growing on the plates, their presence is recorded as "Other types of bacteria (OTB)," without counting them.
The desired result for safe water is zero fecal coliforms per 100 ml. A result of more than 5 or 10 fecal coliforms per 100 ml of water signifies the presence of serious contamination. with the presence of pathogenic parasites. bacteria and viruses almost certain.