An Improved Method for photographing Superficial Microbial Colonies

Abstract

THE lack of contrast in transparent superficial microbial colonies causes some difficulties during photography. The following method promises good results provided some precautions are taken. Suppose it is intended to photograph transparent, colourless colonies on nutrient agar plates. The surface of the medium is flooded with indian ink diluted in distilled water (1 : 100). The ink contains a pigment, carbon black, having a particle size of 200 Å. (Pelikan Tusche No. 541, Günther and Wagner, Hanover). When the upper layer becomes coloured (about 2–3 min.) the surplus solution is sucked off. As a result, in the weakly coloured medium the colonies remain colourless and in a photograph the distinction is clearly visible (Fig. 1). The principle of this procedure is founded on the slower penetration of coarse-grained colloid into the colonies and may be considered as the macro-adaptation of Burri's indian ink ‘negative staining’ method¹. One must be careful to workquickly before the colonies become coloured and the dish should not be stirred because the colonies easily emulsify. The use of crossed ‘Polaroid’ filters or ‘Polaroid J’ film (Fisher Scientific) under and over the dish gives more contrasted pictures. As light source, an X-ray illuminator is suitable.