To determine the amount of dissimilarity (distance) between any pair of bacterial communities, we used the UniFrac metric (10, 22, 23). UniFrac distances are based on the fraction of branch length shared between two communities within a phylogenetic tree constructed from the 16S rRNA gene sequences from all communities being compared. A relatively small UniFrac distance implies that two communities are compositionally similar, harboring lineages sharing a common evolutionary history. In unweighted UniFrac, only the presence or absence of lineages is considered. In weighted UniFrac, branch lengths are weighted based on the relative abundances of lineages within communities. We used the analysis of similarities (ANOSIM) (24) function in the program PRIMER (25) to test for differences in community composition among groups of samples.