Corneal infections caused by Pseudomonas aeruginosa result in severe visual disability, where the drug-sensitive isolates are often associated with worse clinical outcomes. We aimed to analyze the drug tolerance pattern among corneal P. aeruginosa isolates to fluoroquinolone and aminoglycoside antibiotics. Ten antibiotic-sensitive P. aeruginosa keratitis isolates were exposed to 100 µg/ml moxifloxacin or tobramycin to study time-dependent killing and persister formation. The expression of genes associated with persistence, like the type II toxin-antitoxin system (higAB) genes and the homogentisate 1,2-dioxygenase (hmgA) gene, was analyzed by quantitative real-time PCR. Time-dependent killing experiments showed a biphasic kill curve with a fraction of persisters. Moxifloxacin induced 99.9% cell death in four hours and complete bacterial killing between 24 and 48 hours. Tobramycin induced a slower bacterial death, with 99.9% bacterial killing occurring only by 24 hours. While none of the ten isolates survived moxifloxacin treatment, four isolates survived tobramycin treatment and resumed growth after 48h. Notably, the persisters isolated from tobramycin-treated cultures showed a trend towards increased expression of higA and hmgA genes, with considerable variability observed between isolates from different clinical outcomes. Our results show that corneal isolates of P. aeruginosa show increased tolerance to tobramycin with potential alterations in persistence-associated gene expression in response to antibiotic stress.
