174 Methods of Observing Micro-organisms 



Care should be exercised to use a rather small drop, 

 especially for the detection of motility, as a large one vi- 

 brates and masks the motility of the sluggish forms. 



When the bacteria to be observed are in solid or semi- 

 solid culture, a small quantity of the culture should be mixed 

 in a drop of sterile bouillon or other fluid.' 



For observing the growth of bacteria where it is desirable 

 to prevent movement, Hill * has invented an ingenious 

 device which he calls the " hanging block.'' His directions 

 for preparing it are as follows: + 



"Pour melted nutrient agar into a Petri dish to the depth of about 

 one-eighth or one-quarter inch. Cool this agar, and cut from it a 

 block about one-quarter inch to one-third inch square and of the thick- 

 ness of the agar layer in the dish. This block has a smooth upper and 

 under surface. Place it, under side down, on a slide and protect it 

 from dust. Prepare an emulsion, in sterile water, of the organism 

 to be examined if it has been grown on a solid medium, or use a broth 

 culture; spread the emulsion or broth upon the upper surface of the 

 block as if making an ordinary cover-slip preparation. Place the 

 slide and block in a 37 C. incub'ator for five to ten minutes to dry 

 slightly. Then lay a clean sterile cover-slip on the inoculated surface 

 of the block in close contact with it, usually avoiding air-bubbles. 

 Remove the slide from the lower surface of the block and invert the 

 cover-slip so that the agar block is uppermost. With a platinum 

 loop run a drop or two of melted agar along each side of the agar 

 block, to fill the angles between the sides of the block and the cover- 

 slip. This seal hardens at once, preventing slipping of the block. 

 Place the preparation in the incubator again for five or ten minutes 

 to dry the agar-agar seal. Invert this preparation over a moist cham- 

 ber and seal the cover-slip in place with white wax or paraffin. Vaselin 

 softens too readily at 37 C., allowing shifting of the cover-slip. The 

 preparation may then be examined at leisure." 



With this means of examining the growing cultures, Hill 

 has acquired interesting knowledge of the fission and budding 

 of Bacillus diphtherias 



If the specimens to be examined must be kept for some 

 time at an elevated temperature, some such apparatus as 

 that of Nuttall will be found useful though expensive. 



II. STAINING BACTERIA. 



In the early days of bacteriology efforts were made to facili- 

 tate the observation of bacteria by the use of nuclear dyes. 

 Both carmin and hematoxylin tinge the nuclei of the bac- 

 teria a little, but so unsatisfactorily that since Weigert in- 

 troduced the anilin dyes for the purpose, all other stains 



* "Journal of Medical Research," vol. vn, No. 2; new series, vol. n, 

 March, 1902. 



