126 LABORATORY EXERCISES IN BACTERIOLOGY. 



Should there be difficulty, however, a glass rod is quickly flamed and introduced under 

 the bell jar and used to spread the medium before it sets. One of the sterilized plat- 

 forms is now placed in the culture dish and the culture plate adjusted upon it as soon 

 as the film of medium has become solid, the cover of the dish being immediately 

 reapplied. The same process follows in turn for the second and third plates, upon 

 which are spread the contents of the second and third diffusion tubes, the plates being, 

 arranged with their platforms in a set, one over the other in the culture dish. In 

 work of this sort gelatine is usually used as the medium, being more liquid at tem- 

 peratures which are harmless to the implanted microbes than agar, which at the same 

 temperature is less easily spread and apt to quickly solidify in an uneven layer where it is 

 deposited on the plate. Gelatine-agar may, however, be employed, and is preferable 

 to plain gelatine in that the dish and plates may be introduced into the incubator 

 without danger of the medium being liquefied and running off the plates. 



Plates prepared in the same way with uninoculated medium are sometimes used 

 for smear or stroke inoculation when it is wished to have a large surface for distribution 

 of the infectious material. For this purpose well-liquefied agar may be employed as 

 well as gelatine. 



FIG. 39. LEVELLING TRIPOD BEARING A DISH OF CRACKED ICE, COVERED BY A GLASS 

 PLATE. ON THE PLATE A LEVEL AND BELL JAR ; BENEATH THE LATTER A CULTURE 

 PLATE. 



As a result of spreading the diffusely inoculated medium over the extensive surface 

 of the culture plate, the colonies arising from the multiplication of the individual 

 microbes are usually scattered and isolated from each other, thus affording facility 

 for counting their number and for examination of their peculiarities of form and color, 

 as well as favoring any attempt at mechanical removal of one or other colony to a 

 fresh medium in order to obtain a pure culture. With a magnifying glass or the low r 

 power of the microscope one can frequently appreciate peculiarities of importance 

 in the identification of the organisms, which might not have been possible if the colony 

 were in the ordinary culture tube. These advantages have made the principle of plate 

 cultures of the utmost value to bacteriologists. "To plate a tube" is an expression 

 often used in the laboratory, by which is meant the spreading of the contents of a 

 tube over a broad surface, whether on plates, as above described, or in Petri dishes, 

 or over the interior surface of the tube, as in Esmarch tubes. 



Petri Dish Cultures. The objectionable features of the ordinary plate culture 

 are the danger of infection in preparation (common to all cultures in large culture 

 dishes), the rather tedious technique of spreading and solidifying the medium over a 

 cold surface, the possibility of the liquefaction of the medium during culture and 

 dripping from the plate, and the difficulty of examination of the colonies without 



