Recent advances in optical methods of biomass monitoring

Optical density (OD) has long been used as a tool for assessing biomass in liquid microbial cultures due to its simplicity and the common availability of laboratory spectrophotometers.  As a relative measure of the growth of a unicellular organism on a given spectrophotometer, OD can be highly reproducible.  Less well appreciated is that the measurement is not universal between instruments and typically no longer varies linearly with biomass once stationary growth has been reached.  Angled scattering measurements provide differing sensitivity to cell size and differing ranges of biomass sensitivity.  In recent years, instruments measuring back-scattering at multiple angles and source-detector separations have been developed that provide a wide enough range of sensitivity that it is possible to monitor biomass continuously (“on-line”) across many orders of magnitude while maintaining linear response.  Techniques for minimizing sensitivity to window thickness have also been developed, allowing measurements to be made non-invasively, through the wall of a vessel.  Sources of potential interference such as bubbles and expressed proteins will also be discussed, along with recently developed methods for mitigating their effects.