In Vitro Study of Microbial Growth in Artificial Tears Using a Novel Kinetic and Culture-Based Model

Artificial tears (ATs) are widely used to relieve ocular symptoms of irritation, yet their diverse formulations containing varying viscosity agents, electrolytes, osmoprotectants, oils, and surfactants may differentially support microbial growth. This study compared the growth of Staphylococcus aureus and Pseudomonas aeruginosa in commercially available ATs to identify formulation-dependent differences in susceptibility to contamination.

Laboratory investigation.

Clinical and reference isolates of methicillin-sensitive S. aureus (MSSA), methicillin-resistant S. aureus (MRSA), and P. aeruginosa were recovered from patient sources (conjunctiva [n = 2], contact lens [n = 1], cornea [n = 5], corneal button [n = 1], eyelids [n = 6], lacrimal sac [n = 2], and sclera [n = 1]) and inoculated into ten ATs categorized as preservative-containing multidose bottles (MDBs), preservative-free (PF)- MDBs, or PF single-dose vials (SDVs). Turbidity of the samples was recorded for 24 hours and growth kinetics, lag time (λ), specific growth rate (µ), and area under the curve (AUC), were derived using the Gompertz equation. Samples were cultured to quantify microbial growth.

Microbial growth.

PF ATs supported significantly greater microbial proliferation than preserved formulations across all organisms (P < .001). Among PF ATs, SDVs exhibited the highest AUCs, particularly for P. aeruginosa, which showed delayed but more robust growth compared with S. aureus strains. Preserved ATs demonstrated uniform bactericidal activity irrespective of preservative type, whereas PF formulations were largely bacteriostatic or permissive to growth. Linear mixed-effects modeling indicated that AT formulation explained most AUC variability (57%), exceeding strain-level effects (9%).

Microbial proliferation in ATs is strongly formulation dependent. PF formulations pose the greatest contamination risk, whereas preserved ATs provide broad-spectrum inhibition. These findings emphasize the need for formulation-specific evaluation of ATs and future optimization strategies that balance antimicrobial protection with ocular surface safety.