Tryptone Bile Agar

Principle And Interpretation

Tryptone Bile Agar was formulated by Anderson and Baird-Parker (1). The International Commission on the Microbiological Specifications for Foods (CMSF) (6) compared the Most Probable Number (MPN) and the Anderson-Baird-Parker Direct Plating Method (DPM) and observed that DPM was superior to MPN for enumeration of Escherichia coli from raw meats. Superiority of DPM method was noticed by the organization on the basis of less variability, better recovery from frozen samples, greater rapidity and the smaller quantity of medium required. The DPM enumerates both anaerogenic and late lactose fermenting strains of E. coli which could be missed by the MPN method (about 10%)(3). This formulation is recommended by ISO committee for the enumeration of E. coli (7). Holbrook et al (5) modified the DPM for detection and enumeration of sublethally damaged cells of E. coli in frozen, dried, heat processed or acid foods and found that resuscitation step reduces the high concentration of sugar present in the inoculum to a level which does not interfere with the production of indole as the synthesis of tryptophanase is inhibited by high sugar concentrations (2). Certain organisms breakdown the amino acid tryptophan with the help of enzymes that mediate the production of indole by hydrolytic activity (10). The indole produced can be detected by either Kovacs or Ehrlichs reagent (4). Indole combines with the aldehyde present in the above reagent to give red colour in the alcohol layer. The alcohol layer extracts and concentrates the red colour complex. The indole positive organisms other than E. coli are inhibited by bile salts and elevated incubation temperature.