We present a spectrophotometer (optical density meter) combined with electromagnets dedicated to the analysis of suspensions of magnetotactic bacteria. The instrument can also be applied to suspensions of other magnetic cells and magnetic particles. We have ensured that our system, called MagOD, can be easily reproduced by providing the source of the 3D prints for the housing, electronic designs, circuit board layouts, and microcontroller software. We compare the performance of our system to existing adapted commercial spectrophotometers. In addition, we demonstrate its use by analyzing the absorbance of magnetotactic bacteria as a function of their orientation with respect to the light path and their speed of reorientation after the field has been rotated by 90°. We continuously monitored the development of a culture of magnetotactic bacteria over a period of 5 days and measured the development of their velocity distribution over a period of one hour. Even though this dedicated spectrophotometer is relatively simple to construct and cost-effective, a range of magnetic field-dependent parameters can be extracted from suspensions of magnetotactic bacteria. Therefore, this instrument will help the magnetotactic research community to understand and apply this intriguing micro-organism.

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Analogous to the Beer–Lambert law. However, it should be noted that the relation between OD and cell concentration is only approximate.30 

54.

In this measurement, the absorbance is high (transmission of light is low) when the field is aligned along the light path. This measurement was performed with an older, single-stage photodiode amplifier, unlike the measurement shown in Fig. 5 taken with the new amplifier, which has an inverted response.

55.

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56.

Note that the most likely arrival time is exp(μσ2). Therefore, one cannot simply divide the distance traveled by the most likely arrival time to obtain the most likely velocity.

57.
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