CONTINUOUS-CULTURE SELECTION OF BIFIDOBACTERIA AND LACTOBACILLI FROM HUMAN FECAL SAMPLES USING FRUCTOOLIGOSACCHARIDE AS SELECTIVE SUBSTRATE

The human large intestine contains a large and diverse population of b acteria. Certain genera, namely Bifidobacterium and Lactobacillus, are thought to exert health-promoting effects. Prebiotics such as fructoo ligosaccharides (FOS) have been shown to stimulate the growth of endog enous bifidobacteria. In this study, changes of lactic acid producing bacteria in continuous culture fermenters (semi-defined, anaerobic med ium containing 5 g l(-1) FOS, dilution rate of 0.1 h(-1), pH 5.5) were followed over a 21 d period after inoculation with blended human faec es from four healthy adults. Samples were also taken every 3 d for inf luent/effluent FOS, short chain fatty acid (SCFA), lactate and microbi ological analyses. Results showed that SCFA concentrations decreased a bruptly 1 d after inoculation while lactate concentrations increased. Classical methods of enumeration using selective media showed that the proportion of total culturable count represented by bifidobacteria an d lactobacilli increased from 11.9% on day 1 to 98.1% on day 21. Howev er, molecular methods using genus-specific 16S rRNA oligonucleotide pr obes indicated that the bifidobacterial population maintained a level between 10 and 20% of total 16S rRNA during the first 6 d and disappea red rapidly when the maximum concentration of lactate was reached. Lac tobacilli, which were initially present in low numbers, increased unti l day 9 and remained at high levels (20-42% of total 16S rRNA) to day 21, with the exception of day 18. Although FOS has usually been regard ed as a selective substrate for bifidobacteria, these observations sug gest that: (1) lactobacilli are also able to use FOS, (2) lactobacilli can out-compete bifidobacteria in continuous culture at pH 5.2-5.4 wh en FOS is the primary carbon and energy source, and (3) bifidobacteria can grow faster on FOS than lactobacilli under controlled conditions.