Improving Antibiotic Strategies To Combat Respiratory Infections In Cystic Fibrosis Patients

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Cystic fibrosis (CF) is an inherited, progressive disease caused by mutations in the cystic fibrosis transmembrane regulator (CFTR) leading to persistent lung infections. Production of thick, viscous mucus within the lungs prevents cilia from beating on normal epithelia, allowing bacterial species to become entrapped.

Respiratory infections are the most common cause of morbidity and mortality in CF patients. Pathogens frequently associated with this disease are Pseudomonas aeruginosa, Staphylococcus aureus, and Haemophilus influenza, while other species less frequently encountered are Stenotrophomonas maltophilia, Burkholderia cepacia, and non-tuberculosis mycobacteria. P. aeruginosa remains the most prevalent species among the CF population, accounting for approximately 46% of cases, according to the Cystic Fibrosis Foundation patient registry, 2016.

Respiratory bacterial infections can influence the severity of cystic fibrosis symptoms and patient outcome. Treatment strategies have focused heavily on antibiotic therapy, with particular emphasis on the antipseudomonal class of agents such as tobramycin, ceftazidime, meropenem, colistin, ciprofloxacin, and tazobactam/piperacillin; however, bacterial resistance plays a major role in patient prognosis.  P. aeruginosa has many adaptive mechanisms at the site of colonization enabling resistance, such as virulence factor production in particular biofilm formation. When biofilms begin to form, cells become encased within an exopolysaccharide matrix preventing the penetration and action of antimicrobial agents. This has prompted the exploration of novel strategies to improve prevention and inhibition of P. aeruginosa.

Our work explored the effect of antibiotic combination therapy against P. aeruginosa isolates recovered from chronically colonized CF patients within different hospitals. Five clonally common strains were identified from different patient groups within five of the chosen hospitals, known as transmissible strains, and were included in the study. Other strains recovered in no more than one patient were also included for comparative analysis. Patient details, such as age and gender, were not available for any isolate. All strains were tested for biofilm-forming ability at 24 hours to ensure accurate comparative analysis after treatment. Tobramycin and ciprofloxacin were tested for antibiofilm activity in combination with ceftazidime and meropenem, as these agents showed strong antipseudomonal activity as single agents.

A further number of antibiotic combinations were assessed against 72 hour biofilm-grown strains for a better analysis of more mature P. aeruginosa biofilm structures. The biofilm-growth assay does have a number of limitations with studies arguing that the structures are not well developed and not an accurate representation of conditions in the lungs of CF patients; however, it is frequently used for an initial screening of many strains and agents simultaneously.

We found that of the five strains identified as transmissible there were no common virulence traits and each strain had a unique response to the treatment, indicating that the reasons for transmissibility remain unclear. For this reason, the screening method does have implications when choosing a suitable treatment option. Of the antibiotic combinations tested, tobramycin-ceftazidime and tobramycin-meropenem displayed a synergistic effect against a limited number of P. aeruginosa strains. Additionally, tobramycin-ceftazidime did display synergy against mature (72-hour) biofilms but to a lesser degree. Such interactions were not influenced by virulence factor production of each strain. The mechanisms remain unclear but may be due to different components of the biofilm structure targeted by each agent. For the remaining isolates, indifferent interactions were prevalent with a small number of combinations displaying antagonism.

This analysis adds to the current knowledge of antibiotic combination therapy against cystic fibrosis pathogens with an emphasis on isolates that were transmissible versus those that were rare. No single antibiotic combination was effective against all strains from this patient cohort, results were strain-specific. The number of clinical isolates obtained remains small and it would be useful to examine a larger collection over a longer period of time. Tobramycin-ceftazidime may be an effective treatment for P. aeruginosa infection depending on the strain but further analysis would be needed to support this finding.

These findings are described in the article entitled Combination antibiotics against Pseudomonas aeruginosa, representing common and rare cystic fibrosis strains from different Irish clinics, recently published in the journal Heliyon. This work was conducted by Priya Kapoor and Philip Murphy from Adelaide and Meath Hospital, Incorporating the National Children’s Hospital, and Trinity College Dublin.

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