Many instruction manuals for the culture of microorganisms in liquid media state the culture is to be harvested at an OD600 value. Such a value, however, is only meaningful in the context of the specific bacterial strain that is cultured under defined growth conditions and measured using a specific instrument.
The reason for such requirements is that, in contrast with absorbance measurements, the light scatter that occurs during turbidity measurements is strongly dependent on the size and shape of the cells as well as the optical design of the photometer [1, 2]. Different strains thus scatter light in different ways, and different growth conditions may also influence the size and the shape of a cell. The influence of the photometer becomes evident once the sample is measured using different instrument types; discrepancies as high as 100% of the measured OD600 values are not unheard of . With respect to the intensity and the angle of incidence of the stray light that reaches the detector of the photometer, the distance between the aperture and the detector, as well as the size of the aperture behind the cuvette shaft, are of critical importance (figure 2) .