Cytotoxic clinical isolates of Pseudomonas aeruginosa identified during the Steroids for Corneal Ulcers Trial show elevated resistance to fluoroquinolones

Abstract

Background: To determine the relationship between type three secretion genotype and fluoroquinolone resistance for P. aeruginosa strains isolated from microbial keratitis during the Steroids for Corneal Ulcers Trial (SCUT) and for two laboratory strains, PA103 and PAO1.

Methods: Confirmed P. aeruginosa isolates from the SCUT were divided into exoU(+) or exoU(-). The exoU(+) strains contained the gene encoding ExoU, a powerful phospholipase toxin delivered into host cells by the type three secretion system. Isolates were then assessed for susceptibility to fluoroquinolone, cephalosporin, and aminoglycoside antibiotics using disk diffusion assays. Etest was used to determine the MIC of moxifloxacin and other fluoroquinolones. Laboratory isolates in which the exoU gene was added or deleted were also tested.

Results: A significantly higher proportion of exoU(+) strains were resistant to ciprofloxacin (p = 0.001), gatifloxacin (p = 0.003), and ofloxacin (p = 0.002) compared to exoU(-) isolates. There was no significant difference between exoU(+) or exoU(-) negative isolates with respect to susceptibility to other antibiotics except gentamicin. Infections involving resistant exoU(+) strains trended towards worse clinical outcome. Deletion or acquisition of exoU in laboratory isolates did not affect fluoroquinolone susceptibility.

Conclusions: Fluoroquinolone susceptibility of P. aeruginosa isolated from the SCUT is consistent with previous studies showing elevated resistance involving exoU encoding (cytotoxic) strains, and suggest worse clinical outcome from infections involving resistant isolates. Determination of exoU expression in clinical isolates of P. aeruginosa may be helpful in directing clinical management of patients with microbial keratitis.

Figure 1

Minimum inhibitory concentration (MIC) of moxifloxacin for P. aeruginosa clinical isolates with or without the exoU gene. The median for each group is represented by the line in the middle of each box. The interquartile range (IQR), the span of the 25^th to 75^th percentiles, is denoted by the lower and upper bounds of each box, respectively. The whiskers extend from the smallest MIC within 1.5 × IQR below the 25^th percentile to the largest MIC within 1.5 × IQR above the 75^th percentile. Individual data points denote values outside this range. Note that 3 additional isolates were identified and included compared to disk diffusion assay data; 2 were exoU(+) and 1 was exoU(−). The P-value displayed was obtained by two-group mean-comparison t-Test of the log, in base 2, of the MIC. The average MIC shown for each subgroup is the mean of the log, in base 2, of the MIC transformed to MIC in micrograms/ml.