nACTERIOLOGICAL TECHNIC 85 



species of microbe may undergo considerable change in extended cultur- 

 ing, as indicated in the changed culture characters. In fact, some of the 

 changes are so extreme as to confuse even the most expert bacteriologists. 

 B. Isolating Bacteria by the Plate Method. — In order to separate 

 or isolate the several species and varieties of bacteria in any con- 

 taminated substance, it is only necessary to dilute the inoculating 

 material sufficiently. For this purpose there is necessary, sterilized 

 Petri dishes containing heat-sterilized gelatin or other solid media through 

 which the bacteria from the contaminated substance are disseminated 

 in numbers so small that the colonies from each and every microbe 

 present may be visible to the naked eye (or aided by the microscope). 

 This is done as follows : 



Pig. 35. — 'Appearance of colonies on gelatin in a Petri dish. Differences in size of 

 colonies may indicate different species. Differences in color also indicating different 

 species, cannot be shown in the figure. {Williams.) 



To obtain isolation cultures of air bacteria it is only necessary to expose 

 the Petri dish (with a layer of gelatin or agar-gelatin medium, sterilized) 

 for about two minutes, immediately closing the dish and setting it aside 

 to await developments. Making isolation cultures from contaminated 

 solids or liquids is not quite so simple. Proceed as follows: Liquefy the 

 gelatin in four or five test-tubes and keep them at a temperature of not 

 more than 30° C, just high enough to keep the contents liquid; set them in 

 a beaker fiUed with warm water (30° C:) until needed. Number the tubes 

 from I to 5. 



Dip a platinum loop (bend the end of a straight needle into a small 

 loop) into the infected liquid, as bouillon, milk, water, tea, syrup, tincture, 

 fluidextract, etc., etc., and pass one loopful.into tube No. i (sterilize 



