 Gene abundances - The investigators will compute the relative abundances of genes in metagenome samples by applying a similar approach to that above for deriving relative abundances of bacteria. To this end, rather than mapping the reads to the reference bacterial genome database, the investigators will map them to a reference database of bacterial genes which was recently extended and that collectively contain over 3 million distinct bacterial genes. The derived gene abundance vectors are complementary to the bacteria abundance vectors, with the advantage being mapping to genes whose embedding bacterial genome is unknown and thus allowing more metagenome sequencing reads to be mapped, and with the disadvantage of producing a much larger feature vector. 