Bacteria in thin soil sections stained with the fluorescent brightener calcofluor white M2R

Bacteria in thin soil sections were stained with the fluorescent brightener calcofluor white M2R. The fluorochrome was applied to the soil sample after fixation and before embedding in a polyester resin. Thin soil sections were prepared from the hardened blocks. Acridine orange, applied to the polished thin soil sections, was useful to counterstain the soil matrix. Best results were obtained with rhizobial cells grown in a culture medium and introduced into the test soils. After their introduction into the soil, most rhizobial cells were detected in combination with clay particles and organic matter surrounding the quartz particles. Surfaces of large pores, especially in "Beekeerd" loamy sand, were covered with the introduced cells. Indigenous soil bacteria were also stained, but most at a lower intensity. Comparison to observations on stained soil smears suggested that some of smaller coccoids, starving cells and bacterial spores remained unstained. Calcofluor white proved to be an excellent fluorochrome to study fungal hyphae and plant roots in thin soil sections. However, these tissues stained to such an extent that bacterial cells on these tissues were more difficult to detect.