X-α-Gal-based medium for simultaneous enumeration of bifidobacteria and lactic acid bacteria in milk

Abstract

A new method using the chromogenic substrate 5-bromo-4-chloro-3-indolyl-α- 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 α-galactosidase activity of bifidobacteria as compared to that of Lactobacillus or Streptococcus strains. The X-α-Gal-based medium is useful to identify bifidobacteria among Lactobacillus since the enzyme action of α-galactosidase splits X-α-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.