Species-level quantification of Faecalibacterium spp. in faeces of healthy Japanese adults

<i>Faecalibacterium prausnitzii</i> has been considered one of the predominant microbes in the gut microbiota of healthy human adults. Moreover, due to its beneficial metabolites and its reduced population in patients with various disorders, this organism has been regarded as one of the key gut microbes in human health. However, following recent revisions in the taxonomy of the genus <i>Faecalibacterium</i> and <i>F. prausnitzii</i>, the reported population distribution and health benefits of this species have become unclear. In the present study, the population of nine species-level taxonomic groups (hereafter referred to as species) within <i>Faecalibacterium</i> was quantified at the species level in the faeces of healthy Japanese adults (<i>n</i>=88). qPCR, combined with <i>rpoA</i>-based species-specific primers, showed that <i>Faecalibacterium taiwanense</i> had the highest detection rate (prevalence) and copy number among <i>Faecalibacterium</i> spp., followed by <i>Faecalibacterium longum</i>, <i>Faecalibacterium duncaniae</i> and <i>F. prausnitzii</i>, while the remaining five species were detected only occasionally. The population of <i>F. duncaniae</i> varied significantly between age groups, being higher in individuals in their 40s and 50s compared to those in their 20s (<i>P</i>=0.047 and 0.002, respectively). The present study indicates that <i>F. prausnitzii</i> is not the predominant <i>Faecalibacterium</i> species in the healthy Japanese adults included in the present study. Future studies will shed light on the health benefits of the dominant <i>Faecalibacterium</i> spp.