Differences in antimicrobial resistance between exoU and exoS isolates of Pseudomonas aeruginosa

This study compared antimicrobial resistance between exoU and exoS Pseudomonas aeruginosa strains isolated from microbial keratitis (MK) and examined their resistance genotypes.

The presence of exoU and exoS was determined in 187 MK isolates using PCR. Minimum inhibitory concentrations of ciprofloxacin, levofloxacin, gentamicin, and tobramycin were measured. Whole genome sequencing of 39 isolates was used to identify resistance genes via Resfinder. Mutations in key genes, including DNA gyrase, topoisomerase IV, efflux pumps, and DNA repair systems, were analyzed using Geneious Prime. Functional effects of novel SNPs were predicted using SIFT.

Antibiotic resistance was significantly higher in exoU than exoS: 38.2% vs. 20.5% for ciprofloxacin, 29.1% vs. 12.1% for levofloxacin, 40% vs. 23.5% for gentamicin, and 29.1% vs. 14.4% for tobramycin (all p < 0.05). ExoU isolates exclusively had mutations in GyrA (Thr83Ile) and ParC (Ser87Ile), as well as in efflux pump regulators MexZ (Gly89Ser), NalC (Asp79Glu) and MexS (Val73Ala) (p < 0.01). They also more frequently harbored the acquired resistance genes aph(6)-Id (55% vs. 0%) and aph(3'')-Ib (60% vs. 5.3%) and had higher mutation rates in DNA repair genes mutL (70% vs. 15.8%) and mutS (45% vs. 5.3%) (p < 0.01). Mutations in gyrA, parC, efflux pump (mexB, mexD, mexY) and regulator (mexZ, nalC, mexS) genes correlated with fluoroquinolone resistance (R ≥ 0.33; p ≤ 0.04). Possession of aph(3'')-Ib, aph(6)- Id and SNPs in efflux pump regulators mexZ and parR were associated with aminoglycoside resistance.

ExoU strains exhibited more resistance genes and mutations, contributing to higher resistance to fluoroquinolones and aminoglycosides.