Real-time Monitoring by Interferometric Light Microscopy of Phage Suspensions for Personalised Phage Therapy

Abstract Phage therapy uses viruses (phages) against antibiotic resistance. Tailoring treatments to specific patient strains requires stocks of various highly concentrated purified phages. It therefore faces challenges: titration duration and specificity to a phage/bacteria couple; purification affecting stability; and highly concentrated suspensions tending to aggregate. To address these challenges, interferometric light microscopy (ILM), characterising particles (size, concentration, and visual homogeneity) within minutes, was applied herein to myovirus phage suspensions. Particle concentration was linearly correlated with phage titre (R²>0.97, slope: 3 particles/plaque forming units (PFU)) at various degrees of purification, allowing to estimate phage titre for suspensions ≥3.108 PFU/mL, thereby encompassing most therapeutic doses. Purification narrowed and homogenised particle distribution while maintaining therapeutic concentrations. When compared to dynamic light scattering, electrophoretic mobility, and UV/Visible-spectroscopy, ILM best detected aggregates according to our homemade scoring. The present proof-of-concept positions ILM as a valuable quality control tool for phage suspensions, paving the way for further investigations.