Defined substrate (DS) is an alternative technique to monitoring the water quality based on species-specific enzyme activity. Although more sensitive and more specific than traditional media, there is some controversy over use in the warmer waters of tropical and subtropical environments, rich in organic matter and microorganism groups capable of interfering with results. The aim of this study was to test the specificity of DS method (Colilert, IDEXX) for detection of coliforms and Escherichia coli in stormwater seawater samples from a coastal city (Fortaleza, Brazil) compared to findings obtained with the multiple tube fermentation (MTF) method. The samples were collected from stormsewers and adjacent seashore locations. The most probable number (MPN) of total coliforms (TC), thermotolerant coliforms (TtC) and E. coli was determined and the selectivity of the enzymatic substrate medium in the seawater samples was tested. The MTF method showed samples from sampling points 1, 2 and 3 to be 13.3, 13.3 and 46.7%, respectively, above the legal cut-off value for coastal balneability. With the DS method, the corresponding figures were 60, 53.3 and 80% for E. coli. Overall, coliform levels were higher with the DS medium. Vibrios and aeromonads were isolated from E. coli-positive DS tubes.

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