We developed a novel selective medium, modified-rhamnose-2,3,5-triphenyltetrazolium chloride-LBS-vancomycin agar (M-RTLV agar), that utilizes the fermentability of l -rhamnose to distinguish Lactobacillus casei and Lactobacillus paracasei from Lactobacillus rhamnosus . Whereas L. casei and L. paracasei formed red colonies on the M-RTLV agar, L. rhamnosus formed either pink-toned colonies or white colonies with a red spot. An intervention study was conducted to confirm the capability of M-RTLV agar to detect ingested L. casei when recovered from human feces. Subjects consumed one bottle daily of a fermented milk product (Yakult ® or Yakult ® Light, which contains L. casei strain Shirota; LcS) for 7 days. Diluents of the fecal samples were cultivated on M-RTLV agar. We were able to enumerate circular medium-sized red colonies, which were morphologically similar to L. casei / L. paracasei but clearly distinguishable from the remaining colonies owing to the color difference. These colonies were then subjected to enzyme-linked immunosorbent assay in order to identify the LcS. The viable counts of LcS were ± log 10 CFU/g feces after intake of Yakult ® and ± log 10 CFU/g feces after intake of Yakult ® Light (mean ± SD).
Informed consent was obtained from the volunteers who provided the faecal samples used. The Ethical Committee of the Yakult Central Institute provided ethical clearance for this microbiological research study in accordance with the Helsinki Declaration. The study population comprised 11 healthy volunteers (age range, 23–59 years; mean ± SD, 32·8 ± 10·3 years) who ingested a commercially available fermented milk product (Bifine S™; Yakult Hansha Co. Ltd, Tokyo, Japan), containing 10·7 log CFU BbrY, once daily for 10 days. Faeces excreted before and after drinking the fermented milk product were collected in individual sterile Faeces Containers (Sarstedt), refrigerated and taken to the laboratory within 4 h. No subject ingested probiotic products, including the study product, during the 3 weeks before drinking the BbrY-containing fermented milk product for this study.