A structure known as chlorin e6 (Ce6) could be used in photodynamic inactivation (PDI) therapy to protect patient’s eyes from the dangerous effects of Pseudomonas aeruginosa keratitis, new research has shown.
The paper, which was published on 15th March 2017 in Nature Scientific Reports, showed that when the eyes of mice were infected with multi-drug resistant (MDR) strains of P. aeruginosa and subjected to PDI therapy using Ce6, the number of bacterial cells dropped significantly. However, the data also suggested that multiple rounds of the treatment are needed to successfully clear the infection.
P. aeruginosa is listed as a high priority organism on the WHO’s list of bacteria against which new antibiotics are urgently needed, with the number of MDR strains rapidly increasing worldwide. It’s one of the most common causes of bacterial keratitis, an eye infection which damages the cornea and can rapidly cause visual impairment or even blindness. PDI therapy has previously been used to treat various cancers and conditions such as wet macular degeneration, but more recently its gained attention as a novel antimicrobial therapy. The way it works is by combining light with what’s called a photosensitizer, a molecule which can absorb radiation and produce a reactive oxygen species (ROS). These ROS can damage cellular structures, and are used as part of antimicrobial PDI therapy to attack microbial membranes, killing the bacteria.
Ce6 is an example of such a photosensitizer, often used as part of chemotherapy.
What makes PDI therapy an attractive alternative to traditional antibiotics is the more generalised method by which it kills bacteria; the ‘blunt force’, non-specific style of its attack means that resistance against it is much less likely to develop.
In the study, researchers tested how the number of P. aeruginosa cells (the CFU) in the eyes of mice infected with MDR P. aeruginosa would be affected by 0%, 0.01%, 0.05% or 0.1% Ce6 combined with red light, after 24 hours of infection.
Interestingly, one hour after giving the treatment it was found that using 0.01% and 0.05% Ce6 resulted in the most significantly decreased CFU, lowering the number of bacterial cells by almost 90% as compared to the 0% Ce6 control. By contrast, the 0.1% Ce6 treatment didn’t reduce the CFU of P. aeruginosa by a significant amount, suggesting that lower concentrations of Ce6 are more efficient at clearing the infection in vivo.
However, it was found that 48 hours after PDI treatment, the CFU of P. aeruginosa had shot back up, although this was only a statistically significant increase with 0.1% Ce6, as compared to the control group which lacked Ce6. What this seems to show is that just one application isn’t enough to clear the infection; instead, it’s likely that repeated treatments are needed to treat P aeruginosa keratitis.
The research team also discovered that the regrowth of bacteria they’d observed could be controlled by applying PDI treatment earlier. When they treated the mice with 0.1% Ce6 after only 12 hours of infection, this regrowth was greatly reduced – instead, the CFU levels were comparable to the control group.
The paper shows that using Ce6 as part of PDI therapy to treat MDR P. aeruginosa keratitis is promising in vivo… so what could be next?
One question the authors seem keen on answering is, why does Ce6 seem more efficient at clearing P. aeruginosa at lower concentrations? They suggest that maybe increasing the concentration of the photosensitizer actually interferes with light transmission, meaning that the ROS needed to kill the bacteria aren’t produced as effectively. Alternatively, another theory they put across is that this increased concentration could activate defensive systems in the bacteria, giving them greater protection against the effect of Ce6.
Uncovering the reason behind this finding could help to get a better idea of the best concentration of Ce6 to use, whilst avoiding any potential side effects which could interfere with the molecule’s killing ability.
The authors additionally mention that more studies to determine the mechanisms behind the bacterial regrowth are important, as the double membrane surrounding gram-negative bacteria (like P. aeruginosa) could make them less susceptible to PDI treatment.