A closer look at the genomes of microbial communities in the human mouth The incredible, variable bacteria living in your mouth. Micrograph showing Rothia cells (light blue) in their native habitat, a bacterial biofilm scraped from the human tongue. CREDIT Photo credit: Jessica Mark Welch, Marine Biological Laboratory. Some bacteria are abundant in specific locations while absent from others. But, how did the bacteria get into the wrong place? How do we add the good bacteria into the right place when the biogeography has gotten ‘out of whack’? Bacterias are so tiny and small that it is difficult to characterize which subgroups of bacteria live and what genes or metabolic abilities allow them to thrive in these ‘wrong’ places.