Simple Method for Demonstrating Myxobacterial Slime.

the time, distinct differences between encapsulated and unencapsulated bacteria (Fig. 3 and 4). Experience so far has shown that whatever is uncertain with skim milk is usually uncertain with nigrosin. Evidence that the skim-milk method is extremely sensitive was obtained with a bacterium with some of the characteristics of a cytophaga. The separated cells were connected by mucoid strands, so fine as to be invisible except in darkfield preparations, which extended beyond the limits of the halos. The presence of the strands was detected by obstruction to the flow path of the milk particles. These fibers were observed to contract somewhat and draw the cells closer together on contact with the milk, so that it would be reasonable to assume that the capsules observed by the skim-milk method may also be subject to some osmotic shrinkage. Figures 5 and 6 allow comparison of encapsulated and unenca)sulated Sphaerotilus from the same slime preparation. In most academic and diagnostic laboratories, sterile milk culture tubes are usually at hand and are ready for immediate use. The skim milk is especially useful in phase-contrast work because of minimal light and heat absorption, compared with nigrosin, and is more suitable than India ink for the delineation of the capsules because the milk dispersion is finer than that of the carbon particles. The skim-milk method is not advocated as a replacement for all other methods, but as an alternative method, since no single procedure is reliable for all microorganisms under all conditions. Nevertheless, it is useful, simple, convenient, and has some advantages over other methods, although subject to the limitations described.