Global emergence of multidrug resistant (MDR) strains limits therapeutic efficacy in Pseudomonas aeruginosa corneal ulcers. Identifying the primary causal factors of resistance shall improve clinical management. In this study, we sought to identify the underlying mechanisms of fluoroquinolone and aminoglycoside resistance in MDR, non-MDR, and drug susceptible P. aeruginosa (n = 19) strains obtained from keratitis patients. Phenotypic assays were performed to study the bacterial growth kinetics, efflux, permeability, and biofilm formation. Mutational alteration of target genes (DNA sequencing), relative expression of efflux system genes (real time PCR), and detection of aminoglycoside modifying enzyme (AME) genes (PCR) were done by molecular methods. We repeatedly found the mutations in quinolone resistance determining region of fluoroquinolone target genes, gyrA and parC, and the presence of AME genes, aph(3″)-I and aph(6)-I, in all MDR isolates. Furthermore, the MDR isolates were largely characterized by slower growth, cytotoxic type III secretion system genotype, better biofilm-forming ability, and the presence of additional AME genes. The non-MDR isolates were resensitized upon inhibition of active efflux or enhancement of membrane permeability. Altogether this study highlights target gene alteration and enzymatic drug modification as the major mechanisms of quinolone and aminoglycoside resistance in P. aeruginosa keratitis isolates.
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