![]() Using a sterile pipette add 10 mL of water to 5 tubes containing 10 mL double strength medium.Take 5 tubes of double strength and 10 tubes of single strength for each water sample to be tested.Sterilize by autoclaving at 15 lbs pressure (121☌) for 15 minutes.Examine the tubes to make sure that the inner vial is full of liquid with no air bubbles.Dispense the double strength medium in 5 tubes (10mL in each tube) and 50 mL single strength medium in 1 bottle and add a Durham tube in an inverted position.Dispense the double strength medium in 10 tubes (10mL in each tube) and single strength medium in 5 tubes (10 mL in each tube)and add a Durham tube in an inverted position.Prepare medium (either MacConkey broth or lactose broth) in single and double strength concentrations.Others: Sterile pipettes Preparation of the Medium.Glasswares: Test tubes of various capacities (20ml, 10ml, 5ml), Durham tube.Medium: Lactose broth or MacConkey broth or Lauryl tryptose (lactose) broth.The method of the presumptive test varies for treated and untreated water. ![]() If, however, any tube in the series shows acid and gas, the water is considered unsafe and the confirmed test is performed on the tube displaying a positive reaction. If the presumptive test is negative, no further testing is performed, and the water source is considered microbiologically safe. ![]() The presumptive test is a screening test to sample water for the presence of coliform organisms. The number of total coliforms is determined by counting the number of tubes giving positive reaction (i.e both color change and gas production) and comparing the pattern of positive results (the number of tubes showing growth at each dilution) with standard statistical tables. The presence of acid is indicated by the color change of the medium and the presence of gas is detected as gas bubbles collected in the inverted Durham tube present in the medium. Water to be tested is diluted serially and inoculated in lactose broth, coliforms if present in water utilizes the lactose present in the medium to produce acid and gas.
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