CULTIVATION OF BACTERIA. 487 



3. A Few Words Regarding the Technic of Ordinary 

 Cultures. 



The platinum needle must be heated red-hot each time before it is 

 used and before putting it down. 



(a) Fluid culture media are inoculated with a loopful of pure 

 culture. 



(6) Gelatin and agar stab cultures are made with a straight 

 needle, only a single stab being made in each tube, but it should ex- 

 tend almost to the bottom of the tube. 



(e) Agar and gelatin streak cultures and potato cultures 

 are inoculated by a gentle superficial stroke over the surface with the 

 platinum loop. It is sometimes necessary to rub the culture into the 

 potato. 



(d) Gelatin plate cultures : 



1. To isolate certain bacteria in pure culture : The gelatin in three 

 tubes is melted, and after it is cooled dow r n to 30, a loopful of a fluid 

 or a trace of a solid pure culture is introduced into one of them and 

 well mixed. From this first tube one or two loopfuls of gelatin are 

 carried to a second tube, and from this, after mixing, two or three 

 loopfuls are again transferred to the third tube. After anything which 

 may be upon the edge of the tubes has been burned off, the contents 

 of each tube are poured into separate sterile plates, the cover being 

 quickly raised for this purpose, and the plate inclined gently to and fro 

 in order to distribute the gelatin as uniformly as possible. During 

 the transferring from one tube to another it is recommended that they 

 be held inclined, to prevent the falling into them of foreign germs. 

 The plates thus prepared are then placed in a culture chamber with 



*a constant temperature of 22 (or room temperature is used), and after 

 two or three days the individual colonies which have developed are 

 studied macroscopically and with slight (fifty times) magnification. 

 Usually, of the three plates, only two are useful ; at least one has been 

 sown too thick or too thin. 



2. If one wishes to ascertain the number of bacteria, for example, 

 in w r ater, 1 c.c., 0.5, and 0.1 c.c. of the water is placed in three tubes 

 of liquefied gelatin, well mixed, and poured into plates. To ascertain 

 the number of germs, if they are very numerous, the Wolfhiigel count- 

 ing plate is used ; if only a few colonies appear, then the plate is in- 

 verted, the bottom divided into sextents with ink, and each visible 

 colony marked with a dot. Plates made to determine the number of 

 bacteria in drinking-water must be counted several times (on the 

 second, third, and fifth days). In the case of fluids with very many 

 germs (sour milk, canal-water, etc.), 1 c.c. is first placed in 100 c.c. 

 of sterilized water, and this then treated as above. Solid bodies are 

 first rubbed up in water. In the examination of air a definite quan- 

 tity is drawn through a tube filled with sterilized sand, the sand then 

 being washed in sterilized water and plates prepared from it. 



(e) Agar plate=cultures are prepared in the same way. The agar 

 must not be too cool when poured into the dish or it will solidify at 

 once, forming an uneven surface. On the contrary, if it is too hot, the 

 bacteria" are killed by the temperature. Recently it has been much 



