The problematic nature of fluorescently labeled bacteria (FLB) in Spumella feeding experiments an explanation by using video microscopy

Video microscopy was applied to observe the feeding by Spumella on fluorescently labeled bacteria (FLB) as well as on unstained bacteria. Ingestion rates were similar for unstained bacteria and for FLB. However, FLB were egested after a vacuole passage time of about three minutes while the vacuole passage time of unstained bacteria exceeded twenty minutes. Significant differences were found between the cumulative number of subsequent observed ingestions of FLB and food vacuole content. The assumption of a similar treatment of FLB and unstained living bacteria is therefore incorrect for Spumella. Thus, ingestion rates calculated from the number of FLB in food vacuoles underestimate real ingestion of this species.