Infectious keratitis is caused by a wide variety of microorganisms and typically treated with broad-spectrum antimicrobials, typically either cephalosporin/aminoglycoside dual therapy or fluoroquinolone monotherapy. Depending on the abundance of or interaction with causative organisms such as fungi, parasites and bacteria, incidences of susceptibility and resistance vary around the world. To date, researchers report that there’s little data on the incidence of infectious keratitis in the United Kingdom. The Nottingham Infectious Keratitis Study aimed to fill this gap and provide a comprehensive analysis of incidences, microbiological profiles and information on susceptibility and resistance to infectious keratitis.

The retrospective study included all patients who were diagnosed with infectious keratitis and underwent corneal scraping over a 12-year period at a single tertiary referral center in the United Kingdom. The researchers collected and analyzed demographic data, microbiological profiles and in vitro antibiotic susceptibility of infectious keratitis.

From the 1,333 corneal scrapes, the researchers estimated the incidence of infectious keratitis to be 34.7 per 100,000 people per year. A total of 502 (37.7%) of these scrapes were culture-positive, and the researchers identified 572 causative microorganisms, 60 (4.5%) of which were polymicrobial in origin (caused by at least two different microorganisms). The most commonly isolated microorganism was gram-positive bacteria (308, 53.8%), followed by gram-negative bacteria (223, 39%), Acanthamoeba (24, 4.2%) and fungi (17, 3%). Pseudomonas aeruginosa (135, 23.6%) was the single most common organism among the cultures. The researchers also noted a significant increase in Moraxella spp. and a significant decrease in Klebsiella spp. over time.

The researchers reported strong in vitro susceptibilities of common broad-spectrum antibiotics of gram-positive and gram-negative bacteria (Table 1). They noted an increase in resistance against penicillin in gram-positive organisms from 3.5% to 12.7% and in gram-negative organisms from 52.6% to 65.4%.

“Antimicrobial resistance (AMR) is emerging as a global health threat of the 21st century,” the researchers wrote in their article. “AMR has been increasingly reported in both systemic and ocular infections. In our study, we observed a substantial increase in penicillin resistance in both gram-positive (12.7%) and gram-negative bacteria (65.4%). However, most of the bacterial isolates, including the most common organisms, were susceptible to the current broad-spectrum antibiotics (i.e., cephalosporin/aminoglycoside dual therapy and fluoroquinolone monotherapy), which was similarly reported in other parts of the United Kingdom. Reassuringly, there were only four (0.3%) multidrug-resistant isolates and one (0.07%) methicillin-resistant Staphylococcus aureus identified in our study.”

The investigators concluded that infectious keratitis is a relatively common and persistent burden in the United Kingdom, with a likely underestimated incidence. They noted that while current broad-spectrum antimicrobial treatment provides good coverage for infectious keratitis, antimicrobial resistance and polymicrobial infection may present some challenges.