Development of a fast and precise potency test for BCG vaccine viability using flow cytometry compared to MTT and colony-forming unit assays

In a precarious world of rapidly growing pandemics, the field of vaccine production has witnessed considerable growth. Bacillus Calmette-Guérin (BCG) is a live-attenuated vaccine and a part of the immunization program in 157 countries. The quality control is based on a potency test through viable cell enumeration. The colony-forming unit (CFU) assay is the official method, however, it often yields fluctuating results, suffers from medium cracking, and requires lengthy analysis (~ 28 days). Flow cytometric analysis was proposed earlier, but it was coupled with a Coulter counter for measuring the entire bacterial population (live/dead). In the present study, thiazole orange/propidium iodide dyes supplemented with fluorogenic reference beads were employed for viable counting, eliminating the need for a Coulter counter. Both the flow cytometry and the colorimetric technique employing tetrazolium salt were validated and compared to the CFU assay. The colorimetric assay displayed high precision, accuracy, and a strong positive correlation with the CFU assay. The flow cytometry assay demonstrated high precision and a notable ability to distinguish different forms of BCG cells (live, injured, and dead). It also exhibited a perfect positive correlation with the CFU assay. Both methods reduced the analysis time by > 26 days and eliminated the need for human intervention by automating the test.