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Analytical Methods

The OWML provides water-quality data on three major groups of microorganisms of public-health significance in the United States— protozoa, bacteria, viruses.

  • Protozoa are one-cell animals. They are the largest in size and most complex and diverse among the three groups. The cells of protozoa have true nuclei that contains DNA organized into chromosomes. They range in size from 2 to 200 microns in diameter.
  • Bacteria are simpler in structure and smaller than protozoa. In bacteria, genetic information is encoded in a single molecule of DNA that is in a free state within the cell. They range in size from 0.1 to 50 microns.
  • Viruses are the smallest in size (0.02 to 0.3 microns) and simplest in structure among the three groups and are not considered to be living organisms; they are sometimes referred to “submicroscopic particles” or “genetic elements.” They consist of DNA or RNA and a protein coat. They must take over the machinery of a living cell to replicate.

Surface water, ground water, and sediments are collected and analyzed for these microorganisms. For sample-collection methods for water and sediment and for special processing steps for sediments, refer to the USGS National Field Manual.

Because pathogenic (disease-causing) microorganisms may appear intermittently and in low concentrations and methods to detect pathogens are often costly and time consuming, bacterial and viral indicator organisms are often used to assess the microbiological quality of water. Indicator organisms are present in the feces of warmblooded animals and provide information on the possible presence of pathogens.

  • Total coliforms, fecal coliforms, Escherichia coli (E. coli), enterococci, and Clostridium perfringens are bacterial indicators.
  • Coliphage are viral indicators.

Find out more about the analytical methods used by the OWML for indicators and pathogens.

other bacteria

Actinomycetes And Cyanobacteria



protozoan pathogens

Protozoan Pathogens

microbial source tracking methods

Microbial Source Tracking Methods