From the Guidelines

Lactobacillus colonization frequency is generally transient and not sustained over time.

The relative and absolute abundance of Lactobacillaceae gradually decreases over time, supporting the transient nature of lactobacilli colonization in the infant gut 1.
Supplementation with Lactobacillaceae strains has resulted in comparable increases in the abundance of lactobacilli, but this was not sustained at 2 weeks of age 1.
More persistent increases in the abundance of lactobacilli have been observed with L. reuteri DSM 17938 (1–1.25 × 10^8 CFU) supplementation 1.

The beneficial effects of probiotic strains of Lactobacillaceae on the preterm gut microbiome and infant development are due to different underlying mechanisms than those described for bifidobacteria 1.
Lactobacillus spp, Bifidobacterium spp, and S boulardii combinations have been shown to reduce days to reach full enteral feeds and hospitalization 1.

The use of probiotics containing Lactobacillaceae may be beneficial in preterm infants, but the transient nature of colonization should be considered when making clinical decisions 1.
Further research is needed to generate improved understandings of the role lactobacilli play in the preterm gut and to inform the development of probiotic formulations optimized for the greatest benefit to the preterm population 1.

From the Research

The frequency of Lactobacillus colonization can vary among individuals, with some studies suggesting that the abundance of Lactobacillus species, such as L. reuteri, can decrease with age and is correlated with an increase in inflammatory diseases 2.
Lactobacillus species, including L. reuteri, can colonize various body sites, including the gastrointestinal tract, urinary tract, skin, and breast milk, and have been shown to have beneficial effects on the host immune system and intestinal barrier function 2.
The colonization of Lactobacillus species can be influenced by various factors, including the presence of adhesins, such as the mannose-specific adhesin (Msa), which enables the bacteria to interact with the intestinal epithelium 3.
Some studies have investigated the genomic traits that determine the gut colonization potential of Lactobacillus and Bifidobacterium species, and have identified factors such as bile salt hydrolase and intestinal tissue-anchored elements as important for host adaptation 4.
Oral administration of Lactobacillus species, such as Lactobacillus GG, can result in colonization of the feces and changes in bacterial metabolism, although the effect on bowel function may be less evident 5.
Lactobacillus bacteremia is infrequent, but can occur, particularly in patients with underlying diseases or immune system suppression, and may be associated with probiotic exposure 6.