Cystic fibrosis (CF) is an autosomal, recessive genetic disease caused by mutations in the CF transmembrane conductance regulator. The resulting chloride and bicarbonate imbalance produces a thick static lung mucus. This mucus is not easily expelled from the lungs, and so can be colonised by bacteria leading to biofilm formation. Biofilms are intrinsically resistant to antibiotics and persister cells lead to chronic infection with multidrug-resistant strains. CF lung infection with Burkholderia cepacia complex (BCC), especially the subspecies B. cenocepacia, results in higher morbidity and mortality. Patients infected with BCC can rapidly progress to “cepacia syndrome”, a fatal necrotising pneumonia.
We aimed to identify whether a combination therapy that first disrupts B. cenocepacia biofilms with antioxidants and then kills the exposed, sessile bacteria with antibiotics, would be more efficacious. Using controlled in vitro spectrophotometry, synergy testing by fractional inhibitory concentration index (FICI) analysis, colony forming unit and microscopy assays: three antioxidants: ((N-Acetylcysteine (NAC), glutathione and vitamin C)) and three antibiotics: (ciprofloxacin, ceftazidime and tobramycin) were screened and assessed for their ability to disrupt the early and mature biofilms of six B. cenocepacia CF isolates.
A combination of NAC and ciprofloxacin produced a statistically significant biofilm disruption in all strains tested, with growth inhibition (>5-8 log10 compared to 108 CFU/mL for positive control) observed when exposed to 4890- or 8150 μg/mL NAC combined with 32- or 64 μg/mL ciprofloxacin. This level of disruption occurred despite a lack of displayed synergy between NAC and ciprofloxacin, as evidenced by a FICI of 1 - 1.5, suggesting a slight additive but wholly indifferent effect. Thus, NAC-mediated biofilm disruption may be aided by the acidic pH of NAC at higher concentrations.
Our study demonstrated that NAC is an effective disruptor that reduces the necessity for high concentrations of antibiotic. Further research will focus on the host toxicity, the effect of neutralised NAC and efficacy in ex vivo CF models.