Background: Infectious keratitis (IK) is a sight-threatening corneal infection that causes inflammation, pain, and in severe cases blindness. Risk factors include contact lens use, trauma, surgery, and systemic disease. A recent outbreak of IK highlighted the potential danger of contaminated artificial tears. This incident involved a specific strain of drug-resistant Pseudomonas aeruginosa transmitted to patients. This critical event led to Food and Drug Administration (FDA) recalls of various artificial tear products. The need to understand microbial growth in artificial tears is clear. While preservatives are used to control contamination, their effectiveness varies, and the underlying mechanisms remain unclear. Understanding how the composition of artificial tears influences the growth rate of microbes causing IK is crucial. This study investigates the in vitro growth kinetics of IK isolates within artificial tears.

Methods: Twenty different artificial tears were chosen for this study to cover a broad range of common ingredients (i.e. viscosity enhancing agents, electrolytes, buffers, osmoprotectants, etc.). These tears fell into three categories: preservative-free single-dose vials, preservative-free multidose bottles, and preservative-containing tears. Artificial tears were inoculated with a variety of microbes isolated from patients with IK including: bacterial (Staphylococcus spp., Mycobacterium spp., Corynebacterium spp., Serratia spp., Pseudomonas spp.), fungal (Candida spp., Fusarium spp., Aspergillus spp.), and protozoal (Acanthamoeba spp.) isolates. Artificial tear solutions were aliquoted in quadruplicate into a 96-well microplate and inoculated with microbial suspension. Microbial controls in tryptic soy broth and saline, as well as saline-only controls, were included. The turbidity of the samples was recorded periodically for 24 hours in a microplate reader at 37°C (Varioskan, ThermoFisher Scientific) to determine growth kinetics. Cell counts for each group were also performed using serial dilutions on Muller Hinton agar.

Results: Microbial growth varied across artificial tear formulations and microbe types. Preservative-free solutions generally supported higher bacterial growth compared to preservative-containing tears. Cell count data confirmed the trends observed through turbidity measurements. Artificial tear only and saline only samples did not show any microbial growth.

Conclusion: Preservative-free formulations appear to be particularly susceptible to supporting bacterial growth. This in vitro data highlights the importance of understanding how artificial tear composition influences microbial proliferation. Further research is warranted to explore the specific mechanisms by which tear ingredients promote or inhibit microbial growth. These findings can inform the development of safer artificial tear formulations with improved resistance to microbial contamination, ultimately contributing to the prevention and management of IK.