Aerial shot of Passaic River Institute.

Why Measure Fecal Indicator Bacteria (FIB)?

The Main Points

  1. FIB measures the potential of water to contain gastrointestinal pathogens and provide indication of drinking and recreational water safety.
  2. Special sampling routines are required to prevent contamination of the sample.

The Details

Who wants to swim in poo?

It sounds like a no-brainer, and so fecal indicator bacteria (FIB) are used to monitor microbial water quality.

The human digestive tract contains a huge range of bacteria. Most are vital for our health and many are largely harmless. Humans can get seriously ill when we are infected with pathogenic bacteria.

Bacteria like enterococci and Escherichia coli (E. coli) are among the most common bacteria in the gut and they not normally considered pathogens. Isolating all pathogens from a water sample is difficult, so the FIB are used as a proxy measurement.

The presence of FIB is therefore a measure of the potential of the water to contain pathogens and not an actual measurement of the pathogen concentration.

The U.S. Environmental Protection Agency has established Recreational Water Quality Criteria for enterococci and Escherichia coli (E. coli). They have also published bacterial analysis methodology to determine the levels of E. coli, total coliform, and enterococci.

Both drinking and recreational waters have standards for the allowable amount of FIB. Testing the waters involves taking a sample to the laboratory and passing it though a very fine filter. FIB become trapped on the filter. The filters are then placed into an incubator. The bacteria grow and multiply to form distinct colonies. These can be counted either with the naked eye or with a low power magnifier. The number of colonies counted at the end of the incubation period gives us the amount of bacteria in the original water sample. A simple test for FIB can be performed in about one day.

This testing procedure has been used for many years and is highly reliable. However it does require both specialized laboratory equipment and time to incubate the bacteria. Bottles and jars must be sterilized prior to collecting the water sample. There are also time limits relating to how soon the sample must be analyzed after arrival in the laboratory. If a sample sits too long, bacteria can begin to grow, which will artificially inflate the number of colonies in the final count. For this reason it is necessary to work with the laboratory staff to schedule the sampling.