Presence-Absence Broth is used for the detection of coliforms in treated water. The Presence-Absence (P-A) test is a presumptive detection for coliforms in water. The test is a simple modification of the multiple-tube procedure. One test sample (100 mL) is inoculated into a single culture bottle to obtain qualitative information on the presence or absence of coliforms, through the presence or absence of lactose fermentation. This test is based on the principle that coliforms and other pollution indicator organisms should not be present in a 100 mL water sample. Comparative studies with the membrane filter procedure indicate the P-A test may maximize coliform detection in samples containing many organisms that could overgrow coliform colonies and cause problems in detection. The P-A test is described in standard methods for water testing by US EPA. Pancreatic digest of casein, peptone, proteose peptone and beef extract provides nitrogen, vitamins, minerals and amino acids essential for growth. Lactose is the fermentable carbohydrate as carbon and energy source. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Sodium lauryl sulfate is the selective agent used to inhibit organisms other than coliforms. Bromocresol purple is a pH indicator. The potassium salts have a high buffering capacity lactose-fermenting organisms turn the medium from purple to yellow with or without gas production.
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