BACTERIA OCCURRING IN SEWAGE 69 



Microscopical Examination and Staining. — Colonies are 

 examined with a low power — about i inch — the Petri dish 

 being inverted under the microscope, or, if necessary, the cover 

 is removed. A minute portion of the growth is then mixed with 

 a drop of pure water on a cover-glass, dried by a very gentle 

 heat, and it is fixed by rapidly passing twice or thrice through 

 a flame with the residue upwards. A drop of the stain is then 

 spread over the preparation, or it may be floated face down- 

 wards on the staining solution, which sometimes requires 

 warming, and after a few minutes the specimen is carefully 

 rinsed with water, dried, and examined under the microscope 

 with a xV-inch immersion lens. For such rapid work methyl 

 blue is a most useful stain ; fuchsine, gentian violet, and other 

 dyes are also used, sometimes with a mordant for demon- 

 strating flagella, spores, etc. Many bacteria do not stain 

 readily, and the manner in which an organism takes up a stain 

 often helps in its identification. In London sewage and in the 

 bacterial beds there are some bacteria which, after being stained 

 with hot carbol fuchsin, are " acid fast," and thus resemble the 

 tubercle bacillus. An impression preparation is taken by gently 

 pressing the cover-glass on the colony, which must be on the 

 surface, and not too advanced in growth. The cover-glass is 

 then removed with the aid of the forceps, and after being 

 allowed to dry, the preparation is fixed, stained, and mounted. 

 When examined in this manner, the bacteria often show their 

 natural grouping, which is not defined in an ordinary prepara- 

 tion from the colony. (See Figs. 5, 8 and 12.) 



To study the growth of an organism, and to decide whether 

 it is motile, a " hanging-drop " examination should be made. 

 A drop from a fluid culture is transferred by a platinum loop 

 to the centre of a thin cover-glass held by forceps, and this is 

 inverted over the well of a hollow-ground slide, round which a 

 ring of vaseline has been painted so as to lute down the cover- 

 slip. The edge of the drop is at first focussed with a low power, 

 and then with the immersion lens. 



Many devices are employed for the rough separation of the 

 motile bacteria from non-motile forms, such as a capillary 

 communication between the mixed culture and a sterilized 

 medium, or placing the impure culture in a small parch- 

 ment bag, surrounded with sterile water, the actively motile 

 organisms being found to rapidly penetrate into the fresh 

 culture material. 



