Intra-species analysis of pulsed field gel electrophoresis (PFGE) on human fecal Bifidobacterium longum isolates revealed that a majority of 12 Japanese subjects harbored strains of unique PFGE types or subtypes over a 68-week period, suggesting that "indigenous"Bifidobacterium strains remain stable for a considerable time in each individual intestinal microbiota.

Pulsed-field gel electrophoresis and lectin blotting were performed on a total of 41 B. longum strains isolated from 27 human fecal samples and 14 probiotic products in order to characterize their clonality and variation in expression of exopolysacchrides (EPSs; cell-bound polysaccharides). The probiotic isolates formed several distinct clonal clusters, and most of them shared the same lectin blotting profile. Almost all human fecal isolates were distinct from the probiotic isolates not only clonally but also in terms of EPS expression. Most of the fecal isolates within each clonal cluster presented different lectin blotting profiles, suggesting that the structures of EPSs from B. longum were of a labile nature independent of their clonality. Lectin blotting was also performed on a total 38 B. longum strains that had been isolated periodically (at the 1st, 23 rd, and 68th weeks) from fecal samples of 12 human subjects in order to evaluate whether EPS expression was host-specific. B. longum strains that had been isolated from fecal samples of the same individual at different times presented identical or almost identical lectin blotting profiles in 9 out of 12 subjects despite being clonally distant from each other. These findings suggest that variations in EPS expression are related to the hosts but not to the strains' clonality. This in turn suggests that human hosts can discriminate indigenous strains from exogenous ones based on EPS expression profiles.