Parallel evolution of linezolid-resistant Staphylococcus aureus in patients with cystic fibrosis

Linezolid is an antibiotic used to treat serious Staphylococcus aureus infections. Resistance to linezolid is considered rare but could emerge with repeated dosing. We recently reported widespread prescription of linezolid for a cohort of patients with cystic fibrosis (CF). The goals of this study were to determine the incidence of linezolid-resistant methicillin-resistant Staphylococcus aureus (MRSA) in CF and identify molecular mechanisms for linezolid resistance. We identified patients who cultured S. aureus resistant to linezolid with minimum inhibitory concentration (MIC) >4 at the University of Iowa CF Center between 2008 and 2018. We obtained isolates from these patients and retested susceptibility to linezolid using broth microdilution. We used whole genome sequencing to perform phylogenetic analysis of linezolid-resistant isolates and examine sequences for mutations or accessory genes that confer linezolid resistance. Between 2008 and 2018, 111 patients received linezolid, and 4 of these patients cultured linezolid-resistant S. aureus. We sequenced 11 resistant and 21 susceptible isolates from these 4 subjects. Phylogenetic analysis indicated that linezolid resistance developed in ST5 or ST105 backgrounds. Three individuals had linezolid-resistant S. aureus with a G2576T mutation in 23S rRNA. One of these subjects additionally had a mutS- mutL- hypermutating S. aureus that produced five resistant isolates with multiple ribosomal subunit mutations. In one subject, the genetic basis for linezolid resistance was unclear. We conclude that linezolid resistant S. aureus can occur through multiple genetic mechanisms in patients with repeated exposure to this antibiotic. IMPORTANCE Patients with cystic fibrosis have persistent lung infections with Staphylococcus aureus that require extensive antibiotic treatments. Linezolid, an antibiotic given by oral or intravenous route, is prescribed repeatedly for patients whose lung disease has progressed. After treatment with linezolid, S. aureus strains can evolve antibiotic resistance through multiple genetic mechanisms. In addition to a common mutation in the 23S ribosomal RNA known to confer linezolid resistance, S. aureus strains can evolve novel resistance based on a combination of mutations affecting the bacterial ribosome. This combination of mutations was observed in a strain that exhibited hypermutation owing to the loss of the DNA repair genes mutS and mutL. In this cohort of patients with cystic fibrosis, linezolid resistance was transient, possibly due to the growth disadvantage of resistant strains. However, ongoing chronic exposure to linezolid may create optimal conditions for the future emergence of resistance to this critical antibiotic.

Impact of elexacaftor/tezacaftor/ivacaftor on vitamin D absorption in cystic fibrosis patients

BACKGROUND

Cystic fibrosis (CF) is a multisystem disorder that results in the buildup of mucus in various organs. Ninety percent of CF patients are classified as pancreatic insufficient, leading to malabsorption of nutrients and fat-soluble vitamins without the assistance of exogenous pancreatic enzymes. This study was designed to determine if serum 25-hydroxyvitamin D concentrations were impacted by initiation of elexacaftor/tezacaftor/ivacaftor (ELX/TEZ/IVA).

METHODS

Serum 25-hydroxyvitamin D concentrations were measured before and 1 year post-ELX/TEZ/IVA initiation. A Wilcoxon signed-rank test was used to compare values.

RESULTS

Seventy-six patients were included in the final analysis. The average age of our population was 25.8 years (SD = 13.2 years) with a majority being male, homozygous F508del, pancreatic insufficient, and not modulator-naive. The median increase of serum vitamin D concentration after initiating ELX/TEZ/IVA was 5 ng/ml (interquartile range = -4, 13; p = .0035).

CONCLUSIONS

We suggest that ELX/TEZ/IVA may improve fat-soluble vitamin absorption, specifically serum 25-hydroxyvitamin D. These results may lead to adjustments in vitamin supplementation in patients receiving cystic fibrosis transmembrane conductance regulator modulator therapy.