Background and aims Pseudomonas aeruginosa (PsA) is associated with considerable morbidity and mortality in Non-Cystic Fibrosis bronchiectasis (NCFB) and Cystic Fibrosis (CF) patients. Ticarcillin, a carboxypenicillin, is occasionally used in NCFB and CF to treat pulmonary exacerbations. In CF, a subpopulation of PsA exists that is hypersusceptible to ticarcillin (Tichs) in vitro, (minimum inhibitory concerntration [MIC] <4 µg/ml). This phenotype, is associated with reduced MICs to β-lactams, fluoroquinolones, tetracyclines and a degree of resistance to aminoglycosides.
The aim of this study was to investigate whether this Tichs strain exists in NCFB patients and compare this to the prevalence rates from CF and control cohorts. We also assessed whether this strain correlated with enhanced susceptibility to temocillin and other anti-pseudomonal antibiotics.
Methods 18 isolates of PsA from NCFB patients, 23 PsA isolates from CF patients and 18 PsA isolates from controls with no chronic lung disease were analysed. MICs for each isolate were determined by agar dilution using ISO20776–1 for the antibiotics listed in Table 1 and interpreted using EUCAST breakpoints.
Results The NCFB isolates had the highest prevalence of the Tichs strain of the three cohorts we tested, with a prevalence of 76%, compared to a prevalence of 48% in the CF cohort and 0% in the controls. Resistant strains of PsA were more prevalent in the CF cohort compared to the NCFB and control cohorts, except for temocillin and ticarcillin where the CF and NCFB cohorts had lower MICs compared to the control cohort (Table 1).
The Tichs strain in NCFB and CF was associated with reduced MICs to all antibiotics apart from ciprofloxacin in comparison to the non- Tichs strain. In CF, the Tichs strain was also associated with increased MICs to gentamicin.
Conclusion Our data supports the existence of a Tichs strain of PsA in NCFB patients, which existed in greater prevalence compared to our CF cohort. This appears to correlate with reduced MICs to temocillin, to which PsA would normally be resistant. Therefore, temocillin may provide a useful alternative to the current anti-pseudomonal antibiotics in treating NCFB and CF patients.