Robust estimation of bacterial cell count from optical density

Abstract

Optical density (OD) at 600 nm is widely used to estimate cell density in liquid cultures, but lacks inter-instrument comparability and is not directly proportional to cell count. This study conducted a large-scale interlaboratory experiment across 244 labs using three low-cost OD calibration methods: CFU assay, colloidal silica (LUDOX), and serial dilution of silica microspheres. The silica microsphere calibration produced highly precise results (95.5% of residuals <1.2‑fold), allowed quality control and assessment of instrument linear range, and could be coupled with fluorescence calibration (MEFL per cell) to align plate reader data with flow cytometry measurements—fluorescence per cell differed by only 1.07‑fold between methods.,