Real-time analysis of chemotactic motion of Euglena cells confined in a microchip toxicity sensor

We developed the real-time analysis of chemotactic motion of microbial cells (Euglena gracilis), for on-chip cytotoxicity sensing for environmental chemical substances. The Euglena cells were confined in a closed-type micro-aquarium in a PDMS microchip, and their movements were taken by a CMOS video camera. When 1.5%-H₂O₂ was introduced into a microchannel running aside of the micro-aquarium, the H₂O₂ molecules permeated into the micro-aquarium by diffusion through porous PDMS wall, and the cells fell into continuous rotation instead of single step turning and/or straightforward swimming. Such an abnormal swimming behavior is the result of metabolic disturbing effects evoked by radical oxygen species released from H₂O₂. In order to sensing the metabolic disturbing effects, we achieved real-time categorization of the swimming traces into straightforward swimming or continuous rotation; firstly the swimming traces in the video image were sectioned into squares, and then the aspect ratio and filling factor for each square were calculated. High aspect ratio or small filling factor corresponded to straightforward swimming, whereas low aspect ratio and high filling factor to continuous rotation. This motion analysis enables to measure the metabolic disturbing effects on swimming Euglena cells quantitatively, which is important to detect unidentified toxic substances in environments.