The aim here was to elucidate the mother-infant association in the gut colonization of 1-6 month-old infants and to establish whether probiotics can influence this process. Fecal samples from 80 mother-infant pairs were analyzed at 1 month (mothers and infants) and 6 months (infants) by real-time polymerase chain reaction to assess bacterial numbers. This double-blind placebo-controlled trial involved 2 different probiotic combinations (1. Lactobacillus rhamnosus + Bifidobacterium longum and 2. Lactobacillus paracasei + Bifidobacterium longum) given to the mothers 2 months prior to and 2 months after delivery. Bifidobacterium bifidum colonization in the mothers significantly increased the infants' probability of being colonized by B. bifidum and their bifidobacterial diversity indexes (DI) and the mother-infant similarity indexes (SI) both at 1 and 6 months of age. The counts of Bifidobacterium genus (at 1 month) and Bifidobacterium longum (at 6 months) correlated between mothers and infants. At 6 months, a significant effect of the probiotic intervention was found in the mother-infant association of fecal bifidobacterial counts but not in the colonization frequencies, DI or SI. In conclusion, a clear association between mother and infant was found in gut colonization by bifidobacteria. Maternal colonization by B. bifidum had the most consistent effects on the infant's bifidobacterial microbiota. Maternal probiotic treatment had little effect on this mother-infant association.