For a long time colonic microflora has been considered to play an important role in the maintenance of the health and well-being of the host . In addition to promote normal gastrointestinal functions and protecting against pathogenic bacteria, the microflora exerts beneficial effects on systemic metabolism and immune system .
The ability to control the growth and the pathogenic potential of these bacteria depends on the proper function of the microflora .
Imbalance in the colonic microflora with relative predominance of aggressive bacteria and insufficient concentration of protective species has been associated with colonic inflammation [4, 5] and pouchitis.
Intake of probiotics (living micro-organisms), prebiotics (non-digestible oligosaccharides) and synbiotics (mixture of probiotics and prebiotics) has been demonstrated to modify the composition of the microflora, restore the microbial balance and therefore have the potential to provide health benefits [7–9]. However it has only been during the last few years that well designed clinical studies have provided clear evidence of health promoting effects, such as prevention of antibiotic-associated diarrhoea , treatment of acute diarrhoea, inflammatory bowel disease, eradication of C difficile infection and enhancement of intestinal immunity [14, 15].
The current state of evidence suggests that probiotic effects are strain-specific and even strains belonging to the same species may have marked or no probiotic effect .
As a result, in recent years there has been an increasing demand to select, by means of in vitro and in vivo tests, new strains with potential superior probiotic effects . There is a general consensus that probiotic strains should be of human origin, as these bacteria have a greater chance of competing with resident bacteria, and of becoming numerically predominant after short intake and to persist in the colonic environment for some time after discontinuation of use.
Prebiotic substances are non digestible food ingredients which could be fermented by selected groups of beneficial bacteria; their positive influence on intestinal flora has been assed by a number of studies (for a review see ) The use of probiotic strains together with prebiotic substances will provide a combined effect, named "synbiotic" .
A large number of new lactobacilli strains have been previously isolated from faecal samples of newborns ; they were identified, by means of genetic analysis, as naturally persisting in the same subjects for several following days . Following phenotypic characterisation and in vitro evaluation, three new lactobacilli strains (L. paracasei strain B 21060, L. paracasei B21070 and L. gasseri strain B21090) have finally selected in view of their use as probiotics.
A synbiotic formulation, consisting of a mixture of the above selected strains, oligosaccharides as prebiotic ingredients, glutamin, vitamin B6 and zinc, has been developed. The rationale of this formulation is to exploit a complementary probiotic action resulting from the different intrinsic properties of each individual strain and the promotion of bifidobacterial growth due to oligosaccharides. This formulation has been here assessed in a nutritional trial aimed at evaluating the ability of the selected strains to survive, grow and persist along the gastrointestinal tract and its efficacy and safety in various gastrointestinal disorders when administered in the final pharmaceutical formulation, in order to follow the recently issued FAO/WHO guidelines .
The primary aim of this study was to evaluate the ability of the probiotic strains delivered by the synbiotic preparation to survive following passage through the gastrointestinal tract and to persist in the stools after discontinuation of the intake in healthy volunteers.
The secondary aim was to evaluate the effects of the synbiotic formulation on some members of the indigenous flora.