Lactobacilli in the intestinal microbiota of Swedish infants.

Lactobacillus colonisation was examined in 112 Swedish infants. Faecal samples obtained at 1, 2, 4 and 8 weeks and at 6, 12 and 18 months of age were cultivated quantitatively on Rogosa agar. Lactobacilli were speciated by PCR and typed to the strain level by randomly amplified polymorphic DNA (RAPD). Lactobacilli reached a peak at 6 months when 45% of the infants were colonised. L. rhamnosus and L. gasseri were the most common species in this period. Colonisation by lactobacilli in general (P < 0.01) and L. rhamnosus in particular (P < 0.05) was more common in breast-fed than in weaned infants at 6 months of age. Lactobacillus isolation reached a nadir of 17% by 12 months (P < 0.0001), but increased to 31% by 18 months of age P < 0.05). The food-related species L. paracasei, L. plantarum, L. acidophilus and L. delbrueckii dominated in this second phase. A single strain persisted for at least 3 weeks in 17% of the infants during the first 6 months, most commonly L. rhamnosus. Lactobacillus population counts in colonised infants increased from 10(6.4) cfu/g at 1 week to 10(8.8) cfu/g at 6 months, and then dropped to 10(5.4) cfu/g faeces at 12 months of age. Lactobacillus colonisation was not significantly related to delivery mode, or to presence of siblings or pets in the household. Our results suggest that certain Lactobacillus species, especially L. rhamnosus, thrive in the intestinal flora of breast-fed infants. After weaning they are replaced by other Lactobacillus species of types found in food.