The microbial colonisation of the infant gut begins immediately after birth and is essential for the development of the intestine, the immune system and later well-being. Important differences have been reported in the characteristics of such microbiota in different infant population groups. In the present study, we employed an in vitro faecal batch culture model using faeces from different human population groups (adults and full-term breast-fed, full-term formula-fed and preterm infants) to determine the influence that the addition of four bifidobacterial strains and fructo-oligosaccharides (FOS) exerts on the profile of SCFA measured by GC as well as on the levels of some relevant intestinal microbial groups by quantitative PCR during incubation. Differences were found in the levels of SCFA and intestinal microbial groups in the faecal cultures depending on the human group origin of the faecal samples (P< 0·05), this being a predominant factor, compared with bifidobacteria or FOS added, in determining microbiota dynamics. These results exhibit the importance of the initial characteristics of the basal intestinal microbiota in the effect exerted by bifidobacteria or FOS that are added and highlight the need to design probiotics targeting specific human population groups.