Sugar fermentation in probiotic bacteria – an in vitro study

Objectives: Food supplemented with probiotic bacteria may be an alternative way to combat oral infections but before introduction, their sugar fermenting capacity must be established. The aim was to evaluate the metabolic properties of selected probiotic strains suggested as candidates for oral use.

Methods: Six lactobacillus strains (L. plantarum 299v and 931; L. rhamnosus GG and LB21; L. paracasei F19; L. reuteri PTA5289) were cultivated anaerobically at 37°C on MRS agar (48h) or broth (16h). After centrifugation of broth cultures, the cells were washed and resuspended in sterile PBS to a final optical density of 1.0 and immediately subjected to a fermentation assay with 12 different carbohydrates (9 sugars and 3 sugar alcohols). The assay was performed in microtiter-plates with bromcresol purple as pH indicator. The plates were examined for colour changes after 24, 48 and 72h of incubation under aerobic and anaerobic conditions. Three scores were used: negative (pH>6.8); weak (pH 5.2-6.8) and positive (pH<5.2). The strains were characterized with the API 50 CH system to confirm their identity.

Results: L. plantarum fermented all sugars except for melibios, raffinose, xylitol and sorbitol. Both L. rhamnosus strains were generally less active although L. rhamnosus GG was slightly more active than strain LB21 in the aerobic setting. The latter strain exhibited negative reactions for sucrose, maltose, arabinose, melbios, raffinose, sorbitol and xylitol under anaerobic conditions. The assay with L. paracasei and L. reuteri resulted in negative or weak reactions for all tested sugars under both aerobic and anaerobic conditions.

Conclusion: The metabolic capacity differed significantly between the various probiotic strains. It was demonstrated that the L. plantarum strains were most active in fermenting carbohydrates while L. paracasei and L. reuteri were least active under the present conditions.