Zebrafish larvae swim bladder serves as a model to study Mycobacterium abscessus infection kinetics, host-pathogen interactions and drug efficacy.

BACKGROUND: Mycobacterium abscessus, a nontuberculous mycobacterium (NTM), is a cause of chronic lung infection primarily affecting people with underlying conditions such as cystic fibrosis and other causes of bronchiectasis. Mostly caudal vein infection of zebrafish embryos has been used for modelling NTM infection which differs from the pathogenesis of human respiratory infection. Given the histological and physiological similarity of the zebrafish swim bladder to human lung, we developed a localized swim bladder infection model of M. abscessus, representing the three-way interactions between bacteria, epithelial cells, and immune cells seen in the human lung. METHODS: Wild-type zebrafish larvae were injected with M. abscessus:mCherry into the posterior area of the swim bladder at 5 days post-fertilization and growth of M. abscessus in the swim bladder region was verified by imaging and enumerating the CFU counts. Similar injections were performed in swim bladders of transgenic Tg(mpx:GFP) and Tg(mpeg:GFP) zebrafish larvae to observe host neutrophil- and macrophage-pathogen interactions respectively. The expression of the proinflammatory cytokine, TNF-α was also measured to quantify host immune response. To study drug efficacy in this model, zebrafish larvae were infected into the swim bladder with M. abscessus:eGFP, treated with clarithromycin or omadacycline by immersion and CFU counts were determined at 2- and 4-day post-treatment. RESULTS: M. abscessus formed a biofilm-like, localized temporal mycobacterial growth in the swim bladder, with minimal spread into the systemic circulation. This localized growth of M. abscessus induced recruitment of neutrophils and macrophages, forming granuloma-like structures in the swim bladder along with a significant increase in TNF-α. Both clarithromycin and omadacycline treatment resulted in a significant reduction of bacterial biofilm-like growth in the swim bladder. CONCLUSION: Overall, this study established the first zebrafish: M. abscessus swim bladder infection model to study localized NTM infection, host-pathogen interactions, and drug efficacy studies.