Ocular surface unfriendly to pathogenic bacteria

Seattle-Whether or not the ocular surface hosts a live microbiome has been an unresolved question, but findings of a study presented at ARVO 2016 provide further insight on this issue.

Presented by Suzanne M. Fleiszig, OD, PhD, and colleagues, the research was conducted in a mouse model and evaluated whether stable bacterial colonization of the ocular surface could be established after deliberate inoculation.

Using a novel method to detect live bacteria, the results indicated that the ocular surface does not host a stable, metabolically active microbiome, noted Dr. Fleiszig, professor of optometry and vision science, Infectious Diseases and Immunity, and Microbiology, Berkeley Optometry, University of California.

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The investigators acknowledged, however, that the study does not rule out the possibility that non-metabolically active bacteria were present on the ocular surface.

In designing their experiment, the researchers noted that findings from previous studies using high-throughput sequencing of the microbial 16s ribosomal gene indicated an abundance of microbes resides on the ocular surface.

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However, that particular analytical method does not distinguish between living and non-viable organisms because it is based on the detection of bacterial nucleic acid.

To overcome that limitation, the investigators used alkyne functionalized D-alanine as a novel marker to identify live microbes because it incorporates into the cell wall of only metabolically active bacteria.

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After the performance of this novel labelling technique was confirmed, animal eyes were inoculated with bacteria, including Pseudomonas aeruginosa, Staphylococcus aureus, and a gram-positive commensal organism isolated from the mouse dermis.

Then, groups of animals were sacrificed after 24 or 72 hours for analyses to detect metabolically active bacteria in tear fluid, cornea, and conjunctiva.

Whether inoculated with a high or low bacterial load (~10⁸ or ~10⁴ CFU, respectively), the ocular surface was found to be an inhospitable environment for supporting bacterial colonization, either by the pathogenic organisms or the commensal bacterium.

In eyes assessed at 24 hours after inoculation, clearance rates were >99.99% when the inoculum contained ~10⁸ CFU and >99.75% with the smaller inoculum.

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The results were similar when the analyses were conducted 3 days post-inoculation, and metabolically active bacteria were sparsely found on the cornea (mean 0.50 bacteria/image), they reported.

Investigators from Touro University, Vallejo, CA, and Stanford University, Stanford, CA, collaborated in the project.