Drigalski Selective Agar

Drigalski Selective Agar is recommended for the selective isolation of Enterobacteria from urine stool and other clinical samples on the basis of their ability to ferment lactose for the detection of enteric pathogens. Bromothymol blue is the pH indicator in the medium. Lactose fermentors produce acid and thus change the colour to yellow with yellow zones. Lactose non-fermenters develop blue colonies on the medium due to alkalization. Non lactose fermenting gram-negative (enteric) pathogens (Salmonella, Shigella, Proteus, Pseudomonas) form blue to green colonies whereas lactose fermenting coliform organisms (Escherichia coli, Klebsiella , Enterobacter) form yellow colonies due to acid production and decrease in pH.