Can bacteriophage endolysins be nebulised for inhalation delivery against Streptococcus pneumoniae?

Endolysins are bacteriophage-derived protein molecules highly effective for bacterial killing. Cpl-1 and ClyJ-3 are native and chimeric endolysins, respectively, having antimicrobial activity against Streptococcus pneumoniae which causes lung infections. We conducted the first feasibility study on nebulisation of Cpl-1 and ClyJ-3, with a focus on the antimicrobial activity, structural changes of the proteins and aerosol performance. Bacterial colony counts, live cell imaging and Fourier-transform infrared (FTIR) spectroscopy were used to evaluate the proteins before and after jet or vibrating mesh nebulisation. These nebulised aerosols were inhalable with a volume median size of 3.8–4.2 µm (span 1.1–2.3) measured by laser diffraction. How­ever, neb­u­li­sa­tion caused al­most com­plete loss in bioac­tiv­ity of ClyJ-3, which were corroborated with the live cell imaging observation and protein structural damage with a large intensity reduction in the amide absorption bands between 1300 and 1700 cm−1. In contrast, the bactericidal activity of Cpl-1 showed no significant difference (p ≥ 0.05) before and after mesh nebulisation with 4.9 and 4.6-log10 bacterial count reduction, respectively. However, jet nebulisation reduced the bioactivity of Cpl-1 and the effect was time-dependent showing 1.7, 1.0-log10 bacterial count reduction at 7 and 14 min with complete loss of antimicrobial activity at 21 min after nebulisation, respectively. The results were consistent with time-dependent changes in live cell images and FTIR amide band changes at 1655, 1640, 1632 and 1548 cm−1. In conclusion, it is feasible to nebulise endolysins for inhalation delivery but it depends on both the protein and the nebuliser, with the mesh nebuliser being the preferred choice.