A test run meant to allow athletes to familiarise with the marathon swimming course in the Seine River was cancelled Tuesday over concerns about the Paris waterway quality. Fluctuating bacteria levels have been a constant concern throughout the Games due to the swimming portion of the triathlon and the marathon swimming events both planned in the river. The cancellation of Tuesday’s marathon swimming test event comes a day after the triathlon mixed relay event was held in the river that runs through the centre of the French capital. How can Seine bacteria be harmful?Most strains of E. coli and…