X-ALPHA-GAL-BASED MEDIUM FOR SIMULTANEOUS ENUMERATION OF BIFIDOBACTERIA AND LACTIC-ACID BACTERIA IN MILK

A new method using the chromogenic substrate 5-bromo-4-chloro-3-indolyl-alpha-D-galactoside was developed for the differential enumeration of Bifidobacterium spp. and lactic acid bacteria used in preparation of fermented and nonfermented dairy products. The methodology is based on the high alpha-galactosidase activity of bifidobacteria as compared to that of Lactobacillus or Streptococcus strains. The X-alpha-Gal-based medium is useful to identify bifidobacteria among Lactobacillus since the enzyme action of alpha-galactosidase splits X-alpha-Gal substrate and releases indol which imparts a blue colour to bifidobacterial colonies on agar plates. The blue colonies can also be assigned to the genus Bifidobacterium using a simple permeabilization procedure with Triton X-100 which allows the reliable determination of fructose-6-phosphate phosphoketolase activity.