The human gut is the natural habitat for a large and dynamic bacterial community 
that has a great relevance for health. Metagenomics is increasing our knowledge 
of gene content as well as of functional and genetic variability in this 
microbiome. However, little is known about the active bacteria and their 
function(s) in the gastrointestinal tract. We performed a metatranscriptomic 
study on ten healthy volunteers to elucidate the active members of the gut 
microbiome and their functionality under conditions of health. First, the 
microbial cDNAs obtained from each sample were sequenced using 454 technology. 
The analysis of 16S transcripts showed the phylogenetic structure of the active 
microbial community. Lachnospiraceae, Ruminococcaceae, Bacteroidaceae, 
Prevotellaceae, and Rickenellaceae were the predominant families detected in the 
active microbiota. The characterization of mRNAs revealed a uniform functional 
pattern in healthy individuals. The main functional roles of the gut microbiota 
were carbohydrate metabolism, energy production and synthesis of cellular 
components. In contrast, housekeeping activities such as amino acid and lipid 
metabolism were underrepresented in the metatranscriptome. Our results provide 
new insights into the functionality of the complex gut microbiota in healthy 
individuals. In this RNA-based survey, we also detected small RNAs, which are 
important regulatory elements in prokaryotic physiology and pathogenicity.