The Human Microbiome Project, stepchild of the earlier and grander Human Genome Project, is fi nally starting to make a name for itself. Beyond the remarkable success of bacteriotherapy for C. difficile
Mapping the Microbiome
The microbial communities that inhabit our bodies are far more extensive than previously appreciated, outnumbering our own cells by an estimated factor of at least 10 to one.
In 2008, the National Institutes of Health launched the Human Microbiome Project with the goal of mapping the diversity of microbial species that cohabit the human body, and in doing so laid the groundwork for understanding the importance of these communities in human health.
Studies from the Human Microbiome Project demonstrate a great deal of variation between individuals for each of the body areas examined. In addition, the microbial footprint within a given body habitat, or the diversity and abundance of different types of organisms, appears to have some predictive value when viewed in relation to various diseases.
What About the Eye?
Studies from the Human Microbiome Project gave rise to the question of whether mucosal membranes such as those at the ocular surface also possess a resident microbiota and, if so, do these too play a role in disease physiology?
The first study documenting the presence of microbes on the ocular surface dates back to 1930, when researchers utilized conventional culture techniques to identify microorganisms.
Additional challenges to characterizing the ocular biome result from the physiological differences between the eye and other sites on the human body. Lid wiper function and bactericidal peptides in the tear film exert a constant disinfecting pressure on the ocular surface. Recent estimates project the number of bacteria on the ocular surface to be much less than that on other mucosal surfaces; the tear film may harbor ~100 colonyforming units per ml, while the oral or gastrointestinal mucosa contains between 107 and 108 CFUs/ml.
Despite these challenges, recent studies have set out to identify an ocular surface microbiota that represents a persistent and stable consortium of viable organisms on the ocular surface, and to assess how this population varies in health and disease. One of the first examples of these efforts dates to a 2002 study showing that both the number of CFUs and diversity of bacterial species are increased in individuals who wear contacts.
In 2009, scientists at the Bascom Palmer Eye Institute initiated the Ocular Microbiome Project, and in 2010, they published initial results classifying species from four healthy subjects to the phylum level by utilizing 16s rRNA gene-based sequencing.
Leveraging the Microbiome
Based upon the lessons from other microbiomes, a key feature seems to be the population diversity that’s associated with disease states. Patients with chronic rhinosinusitis, for example, show the same types of nasal bacteria as unaffected patients, but their populations are altered such that diversity of species is reduced.
An alternative example of the potential importance of the ocular biome comes from a 2002 study of the role of commensal bacteria in mucin metabolism.
How might identification of an ocular biome affect future therapies? One recent study explored the possibility of microbiome-based therapies for the treatment of ocular disorders through the use of probiotic eye drops.
Untila core ocular flora is defi ned with greater precision, it will be diffi – cult to identify bacteria as symbiotics, commensals, pathogens or some combination of these.
Dr. Abelson is a clinical professor of ophthalmology at Harvard Medical School. Mr. Lane is director of Research and Development at Ora Inc. Dr. Slocum is a medical writer at Ora Inc.