New research published in Antibiotics has identified a naturally-occurring material, hydroquinine, as a potential disinfectant for contact lenses in combination with multipurpose solutions (MPSs) to prevent one of the most common bacteria linked to contact lens-related microbial keratitis (CLMK): Pseudomonas (P.) aeruginosa.
This bacteria sounds familiar …
It should. An antibiotic-resistant form of P. aeruginosa—carbapenemase-producing, carbapenem-resistant P. aeruginosa (CP-CRPA)—was most recently the focal point of an investigation conducted by the Centers for Disease Control and Prevention (CDC) regarding its link to a multistate outbreak of artificial tear-associated patient cases reported in 2023.
That outbreak resulted in 81 cases across 18 states, with four deaths, 14 reports of vision loss, and four reports of enucleation.
Give me a brief rundown on it.
Accounting for 55-59% of CLMK-related cases, P. aeruginosa, on a broader scale, most often can lead to infections in humans, including in the blood, lungs (pneumonia), and other locations throughout the body after surgery.
Per the CDC, this bacteria resides in the environment and can spread to individuals in healthcare settings when they are exposed to water or soil containing P. aeruginosa, including via contaminated hands, equipment, and surfaces.
And on the ocular side?
Researchers have linked P. aeruginosa to a higher degree of ocular severity (such as corneal perforation within 72 hours). In fact, the World Health Organization (WHO) reported that corneal blindness resulting from microbial keratitis (MK) has been emerging as a prominent cause of visual disability.
The bacteria’s multidrug resistant (MDR) classification makes it “one of the most concerning pathogens by the WHO,” according to investigators.
Let’s zero in on MK.
The conditions for MK to occur might be when the cornea is susceptible to infection by a particular bacteria, often due to improper care or cleaning of contact lenses (as we mentioned; CLRMK) or injury to the cornea itself.
Biofilm formation can occur when these conditions are met, where bacteria clusters irreversibly attach and grow on a surface in a moist environment (such as the ocular surface) and begin to reproduce within compromised tissue.
In MK, biofilm formation is considered the predominant damaging factor associated with an increased severity of the condition and potential risk for vision loss.
Give me an example.
Let’s take P. aeruginosa biofilm formation—this could promote antibiotic treatment resistance. Subsequently, bacterial biofilms such as this may contribute to prolonged and continued contamination of contact lenses materials, research has found.
The investigators stated, “it has been demonstrated that the annualized incidence of CLMK significantly increases with overnight and/or extended lens wear compared to daily wear (from a 5- to 10-fold increase).”
Gotcha. So did they find a “magic bullet” to combat MK?
The researchers analyzed the disinfecting properties of hydroquinine, a naturally occurring substance (organic compound) found in tree bark that has previously demonstrated antimicrobial as well as broader antibacterial, antifungal, and antiproliferative properties.
More importantly: another study proposed that hydroquinine has the potential as an antimicrobial agent to target MDR strains of P. aeruginosa.
And what did they test?
They evaluated the effect of hydroquinine (1.25 mg/mL and 2.50 mg/mL) in reducing “adhesion-related genes” of P. aeruginosa as well as when incorporated into contact lens MPS to mitigate contamination.
Hydroquinine-formulated MPSs were compared against MPS alone based on the ISO 14729 stand-alone antimicrobial activity test using two strains of P. aeruginosa.
Which MPS were used?
- OPTI-FREE Replenish (0.001% POLYQUAD and 0.005% ALDOX; Alcon)
- Q-eye MPS (0.001% polyhexamethylene biguanide [PHMB], Maxim)
- Phosphate buffer saline (untreated control)
And their findings?
Investigators found that hydroquinine 1.25 mg/mL reduced adhesion-related gene (mRNA) expression and anti-biofilm formation in P. aeruginosa.
To note, both hydroquinine concentrations had anti-adhesion that was greater than 50% for both strains of P. aeruginosa.
Based on ISO testing, hydroquinine-based MPS had a >99.9% killing at disinfection time for both drug-sensitive and multidrug-resistant P. aeruginosa. Further, the solutions were also successful in “inhibiting P. aeruginosa adhesion and destroying preexisting biofilms,” the authors stated.
Meaning…
Based on the findings, they concluded that, “hydroquinine might be used as a disinfecting contact lens solution like MPSs for inhibiting bacterial growth.”
And the significance of this research?
As the authors noted, this latest study into P. aeruginosa and contact lenses is the first to demonstrate strong evidence that MPS combined with hydroquinine can “inhibit P. aeruginosa adhesion and prevent biofilm formation on contact lens surfaces.”
Lastly, what do they recommend for avoiding CLMK?
They suggest that, “soaking contact lenses in MPS containing hydroquinine is possibly helpful in decreasing bacterial adhesion, preventing biofilm formation, and removing the existing biofilm mass,” and recommend additional testing to assess the safety of MPS formulations combined with hydroquinine to minimize risk for adverse ocular effects.