Microbial coinfection keratitis: clinical spectrum, pathogen distribution, and antimicrobial susceptibility in an 8-year retrospective study.

Background: Coinfection keratitis, involving concurrent infections by bacteria and fungi, poses diagnostic and therapeutic challenges. This study aimed to describe the microbiological and clinical characteristics of keratitis co-infections, including prevalence, predisposing factors, treatment and antimicrobial susceptibility profiles, in patients from a tertiary ophthalmology referral center in Mexico City. Methods: This retrospective and descriptive study analyzed data collected over an 8-year period (2012-2020) following STROBE guidelines. Corneal samples from patients diagnosed with infectious keratitis were cultured. Only cases with confirmed coinfections-defined as the isolation of more than one microorganism-were included. Microbiological identification and antimicrobial susceptibility were analyzed. Clinical data included demographics, risk factors, visual acuity, corneal reepithelialization, and surgical interventions. Results: A total of 306 microorganisms were isolated from 140 patients. Among them, 109 cases (78%) involved bacterial-bacterial coinfections, and 31 cases (22%) were bacterial-fungal coinfections. Staphylococcus epidermidis was the most frequently isolated bacterial species (28%), and Fusarium was the predominant fungal isolate (52%). Resistance to erythromycin (52%), clindamycin (46%), and fluoroquinolones (35%) were observed, particularly among S. epidermidis isolates. Previous ocular surgery (34%) and diabetes mellitus (20%) were the most common risk factors. Similar dominant species were observed among institutional monomicrobial isolates analyzed for contextual reference. Conclusion: This retrospective and descriptive study characterizes microbial coinfections in infectious keratitis and provides contextual data from microbial cases from the same institution. Coinfection often involves multidrug-resistant organisms and occurs in patients with risk factors such as prior ocular surgery, contact lens use, or diabetes. Early identification and targeted therapy remain essential to improve clinical outcomes.