PA HiVeg™ Broth

Principle and Interpretation

This medium is prepared by completely replacing animal based peptones with vegetable peptones which makes the medium free of BSE/TSE risks. A simple procedure was proposed by Weiss and Hunter for the bacteriological examination of treated water that should be free of polluting organisms (1). Later on the PA (Presence Absence) test was developed as a simplified version of the test based on the principle that coliforms and other bacterial indicators of pollution should not be found in 100 ml samples of treated water (2). Other aspects of PA test were studied by Clark et al (3). PA Broth has been included as a tentative standard in the Standard Methods for the Examination of Water and Wastewater (4) justified on the theory that a 100 ml sample of drinking water should not contain any coliforms. PA HiVeg Broth is the modification of this medium by using vegetable peptones and serves the same purpose.

The medium contains HiVeg peptone, HiVeg extract, HiVeg hydrolysate No. 1 which supply nitrogenous growth factors and trace ingredients to the coliforms. Lactose serves as the fermentable carbohydrate and /or energy source for bacterial metabolism. Lactose-fermenting organisms form acid which is identified by the pH indicator bromocresol purple as a colour change from purple to yellow. Dibasic and monobasic potassium salt phosphates provide buffering action while sodium lauryl sulphate inhibits many organisms other than coliforms. Bromo cresol purple is the pH indicator which turns yellow at acidic pH.