Recreating chronic respiratory infections in vitro using physiologically relevant models

Abstract

Despite the need for effective treatments against chronic respiratory infections (often caused by pathogenic biofilms), only a few new antimicrobials have been introduced to the market in recent decades. Although different factors impede the successful advancement of antimicrobial candidates from the bench to the clinic, a major driver is the use of poorly predictive model systems in preclinical research. To bridge this translational gap, significant efforts have been made to develop physiologically relevant models capable of recapitulating the key aspects of the airway microenvironment that are known to influence infection dynamics and antimicrobial activity in vivo In this review, we provide an overview of state-of-the-art cell culture platforms and ex vivo models that have been used to model chronic (biofilm-associated) airway infections, including air-liquid interfaces, three-dimensional cultures obtained with rotating-wall vessel bioreactors, lung-on-a-chips and ex vivo pig lungs. Our focus is on highlighting the advantages of these infection models over standard (abiotic) biofilm methods by describing studies that have benefited from these platforms to investigate chronic bacterial infections and explore novel antibiofilm strategies. Furthermore, we discuss the challenges that still need to be overcome to ensure the widespread application of in vivo-like infection models in antimicrobial drug development, suggesting possible directions for future research. Bearing in mind that no single model is able to faithfully capture the full complexity of the (infected) airways, we emphasise the importance of informed model selection in order to generate clinically relevant experimental data.

FIGURE 1

Schematic representation of in vitro and ex vivo models used for biofilm infection studies. Microscopy images are reported for each model to show biofilm formation by Pseudomonas (P.) aeruginosa (used as representative example) in association with the airway epithelium or ex vivo lung tissue. a) Scanning electron microscopy (SEM) image of air–liquid interfaces of CFBE41o- cells with human tracheobronchial mucus (black arrow) and pre-formed biofilms of P. aeruginosa PAO1 (white arrows) after 1 h of co-incubation (scale bar: 3 μm) (reproduced from [53] under the terms of the CC BY licence). b) SEM images of P. aeruginosa PAO1 biofilms (white dotted frame) formed in association with three-dimensional A549 cells after 6 h of incubation (magnification: ×500; scale bar: 50 μm) (reproduced with modifications from [60] under the terms of the CC BY licence). c) Confocal laser scanning microscopy (CLSM) image of biofilm-like aggregates of green fluorescent protein labelled P. aeruginosa (ATCC 10145GFP) (green) on the epithelium of the cystic fibrosis (CF) lung chip at 24 h post-infection (scale bar: 10 μm) (reproduced with permission from [64]). d) CLSM image of P. aeruginosa PAO1 clusters (orange) formed in association with primary bronchial epithelial cells (pink) and cell-secreted mucus (green) at 13 h post-infection (scale bar: 200 μm) (reproduced from [65] under the terms of the CC BY licence). e) Visual observation of a pig lung explant after 4 days of incubation with a CF isolate of P. aeruginosa (reproduced with permission from [67]). SEM image of P. aeruginosa PA14 biofilms in the ex vivo pig lung model at 48 h post-infection. Image was processed with GIMP1.2 for false colouring (red: lung tissue; green: bacteria and biofilm matrix) (scale bar: 2 μm) (reproduced from [68] under the terms of the CC BY licence). Figure created with Biorender.com. PNEC: pulmonary neuroendocrine cell; RWV: rotating-wall vessel; SCFM: synthetic CF medium.