Bacteriological filters (candles) prepared from kieselguhr, a fossil diatomaceous earth. The pore size is relatively large, and three sizes are graded V, N and W (viel, normal, wenig)—coarse, normal and fine respectively—that is, in decending porosity. Serratia marcescens should be retained by even the V filter. The filters are moulded in the shape of a hollow candle with a metal collar at the open end. The direction of filtration may be from within outwards or vice versa.