Evaluation of 415-nm Blue Light and Hypochlorous Acid on the Rhesus Macaque (Macaca mulatta) Cranial Implant Microbiota
Cranially-implanted macaques are a crucial model for neuroscience research. Implant complications include abscesses, meningoencephalitis, and implant–tissue margin infections. Antimicrobial overuse has increased bacterial resistance, risking macaque health and complicating treatment of infections. This project aimed to assess the antimicrobial effects of blue light (∼415 nm) and hypochlorous acid (HOCl) treatment of cranial implant margins. Blue light exerts antibacterial effects via the induction of reactive oxygen species. We hypothesized that exposing cranial implant margins to a commercially available blue light device followed by HOCl treatment would improve clinical appearance and decrease bacterial burden as assessed by aerobic/anaerobic culture and tissue margin microbiota analysis (decreased α and β diversity and altered taxonomic composition). Eight rhesus macaques were exposed to 6 min of blue light followed by 0.024% HOCl solution three times weekly for 4 wk. Swabs for microbiota analysis and bacterial cultures were collected before and 24 h after the last treatment session. Control microbiota swabs were collected from a separate implant margin area only exposed to HOCl. All animals tolerated the blue light exposure but had varied improvement in margin clinical appearance. The most common bacteria identified on culture were Staphylococcus aureus (n = 8), β-hemolytic streptococcus (n = 8), and Corynebacterium ulcerans (n = 6). Microbiota analysis of the 16S rRNA V4 gene region demonstrated many anaerobic operational taxonomic units in addition to the aerobic species cultured, highlighting limitations of culture-based methods. All animals had unique microbiota taxonomic profiles with a mean of 84 operational taxonomic units and a median Shannon diversity index of ∼2.6. No significant differences were found between treatment groups, α diversity, or β diversity before and after the study. The effectiveness of blue light therapy likely relates to the device power and depth of penetration into the tissue margin. While safe, future work is needed to optimize the dose and delivery methods of light-based therapies.
(A) Image of the blue light device used for the study displaying the surface shape and array layout. (B) Representative image of application of the blue light device to the cranial implant margin.
Representative photos for 3 macaques of the blue light exposure area over the study duration. The prestudy image for macaque B is unfortunately unavailable. Clinical improvement was most pronounced in macaque D as demonstrated by decreased erythema. Macaque L did not show clinical improvement as demonstrated by a sustained granulation tissue response. Due to the diversity of cranial implant morphologies for animals enrolled on study, we were unable to create a quantitative scoring system.
Relative abundance of the top 20 bacterial genera identified on microbiota analysis for macaques treated with blue light + HOCl compared with HOCl only, before (PRE) and after study (POST). In addition to Staphylococcus, Streptococcus, and Corynebacterium, numerous anaerobic genera are identified, including Fusobacterium, Porphyromonas, Peptoniphilus, Anaerococcus, Parvimonas, and Prevotella. Every macaque demonstrated a unique microbiota profile that was not significantly different posttreatment.
α Diversity as evaluated by the Shannon diversity index of the blue light + HOCl (B) and HOCl only (V) groups at timepoint 1 (prestudy) and timepoint 2 (poststudy). The box-and-whiskers plot displays the inclusive median. No significant difference between groups or timepoints was observed.
β Diversity as visualized as a principal coordinates analysis showing the top 2 axes by variance in microbiota communities explained. Circles represent the blue light + HOCl group, and the triangles represent the HOCl-only group, with each animal designated by a different color. Lines connect sample pairs indicating shifts in microbial community between timepoints 1 (prestudy) and 2 (poststudy). Points are generally most clustered by animal (color), indicating that the individual microbiota has a stronger effect on microbial community than the treatment or timepoint.
Individual assessment of abundance changes for Staphylococcus (6A), Streptococcus (6B), and Corynebacterium (6C) by macaque, treatment and time point. Blue light + HOCl treatment is shown in reddish-orange and HOCl only treatment is shown in teal with pre-treatment abundance on the left (timepoint 1) and post-treatment abundance on the right (timepoint 2). No consistency was observed for changes in abundance for the top three species cultured nullifying our original hypothesis that blue light with HOCl treatment would decrease Staphylococcus and Corynebacterium colonization and increase Streptococcus.
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