understand how the lung environment affects colistin treatment efficacy and to develop new therapeutic strategies to improve patient outcomes

Cystic fibrosis (CF) affects >10,000 people in the UK. Early mortality characterises the disease, largely driven by chronic lung infection and inflammatory airway wall damage. By adulthood >60% patients are chronically infected with the Gram-negative organism, Pseudomonas aeruginosa (Pa). Current

therapy is limited, and once infection is chronic, the best that can be achieved is 'suppression' of bacterial load, usually with inhaled antimicrobials. The Strategic Research Centre for Pa infection was established by Prof Jane C Davies, with a focus on improving understanding of pathogenic mechanisms and therapies. Its partnership with the Royal Brompton Hospital's CF clinic, one of the

largest in Europe, through senior clinical academics, underpins the strong translational focus of the programme. The narrow pipeline of new antibiotics under development means that work to improve efficacy of existing agents is urgently needed. Colistin is the polymyxin antibiotic used most commonly to control chronic P. aeruginosa infection in

CF. Unfortunately, whilst colistin is usually effective in suppressing infection, it is almost never able to clear P. aeruginosa from the lungs once chronic infection is established. Intravenous colistin is also used as a 'last-resort' agent in severe disease, and thus, the emergence of resistance to

polymyxin antibiotics is a growing concern. Efforts to improve colistin efficacy have been hampered by a poor understanding of the antibiotic's mode of action and the lack of knowledge around the impact that the host environment has on bacterial susceptibility. Recent work from the Edwards lab, based in the MRC Centre for Molecular Bacteriology and Infection, has revealed that colistin targets

LPS in both the outer and cytoplasmic membranes, leading to bacterial lysis and killing (Sabnis et al., 2019). We have also shown that colistin resistance due to the mobile colistin resistance (MCR) family of LPS modifying enzymes (Liu et al., 2016) is due to modification of LPS at the cytoplasmic membrane (Sabnis et al., 2019).

We exploited this information to develop a combination therapeutic approach to enhance colistin activity. We found that the experimental antibiotic murepavadin caused the accumulation of LPS in

the cytoplasmic membrane of P. aeruginosa, which sensitised the bacterium >1000-fold to colistin mediated killing. Given the geographical heterogeneity in airway deposition of inhaled agents related to airway narrowing and mucus plugging, and the resulting variability in drug concentrations, successful approaches to enhance efficacy of lower drug concentrations could have direct clinical

impact. The crucial next step in this work is to determine how the host environment influences LPS processing and transport, which we hypothesise will have significant effects on colistin susceptibility and therefore treatment outcomes. For example, during this work, we found that LPS released by bacteria exposed to colistin can sequester the antibiotic, rendering it ineffective. We also found that

the presence of human serum renders P. aeruginosa tolerant of colistin. These findings indicate that colistin efficacy is affected by the in vivo environment, but this requires further investigation.