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Images: Testing the Waters and Closing Beaches

To test water safety at swimming beaches, workers take water samples, filter them to collect the bacterial cells, and place them on a culture medium that promotes the growth of only enterococcus bacteria—an indicator of contamination. Single cells can grow explosivley into colonies (blue because of a pH-indicator dye in the medium). Massachusetts bathing beach regulations set the limit for enterococcus cells in beach waters at 104 "colony forming units" in 100 millimeters. (Courtesy of Bethany Sadlowski, Barnstable County Department of Health and Environment)

Regular  testing leads to beach closures if high levels of bacteria are found in the water. But the time needed to grow and count bacterial colonies inserts a lag time of days between sampling and when signs are posted. Beaches may be closed after bacteria have dispersed, or kept open when bacteria are present but not yet counted. New methods of detection based on identifying bacterial DNA one day may significantly shorten the time between sampling and results and let managers make more timely decisions about beaches. (Courtesy of Elizabeth Halliday, Woods Hole Oceanographic Institution)

Elizabeth Halliday, a graduate student in the MIT'WHOI Joint Program, takes samples of sand at Wood Neck Beach in Falmouth, Mass., to analyze in the laboratory. Bacteria that are indicators of fecal contamination can live and grow in beach sand, and the DNA from microbes living within this sand will be used to find out how many indicator bacteria are present. (Courtesy of Elizabeth Halliday, Woods Hole Oceanographic Institution)