Phylogenetic analysis of picoplankton in Lake Biwa and application to legal medicine

Three strains of picoplankton designated as brown, green, and pink belongin g to the Synechococcus genus in cyanobacteria (approximately 1 mu m in size ) are found ubiquitously in Lake Biwa, Japan. However, they could not be mo rphologically discriminated from other bacteria such as Proteobacteria and Bacillus by microscopy. In this study, we attempted to use the polymerase c hain reaction (PCR) analysis of the 16S ribosomal DNA (rDNA) from picoplank ton for the diagnosis of death by drowning. A segment of 16S rDNA was seque nced in order to investigate their phylogenetic relationships and to design the specific primers. The PCR products from three picoplanktons were compa red with those from five other cyanobacteria, Melosira (diatom), Staurastru m (green alga), bacteria from Lake Baikal, and humans. The picogram order o f template DNA from picoplankton was specifically amplified by the primers. When the template of picoplankton was mixed with human tissue, at least 10 ng of template DNA was needed to obtain a PCR product. The efficiency of P CR was increased more than hundredfold by isolating the picoplankton from h uman lung tissue. The specific PCR products of the picoplankton were obtain ed from a formalin-fixed drowning body (lung and liver) that was found in a downstream river and Lake Biwa. The PCR analysis of the picoplanktion 16S rDNA is considered useful for the diagnosis of death by drowning.