DIFFERENTIAL INCUBATION 255 



results of incubation. (If any of the subcultures fail, 

 further material can be obtained from the correspond- 

 ing emulsion ; or if it has dried, by moistening it with a 

 further drop of sterile distilled water.) 



e: Incubate all the subcultures and identify the 

 organisms picked out. 



4. Differential Media. 



(a) Selective. Some varieties of media are specially 

 suitable for certain species of bacteria and enable them 

 to overgrow and finally choke out other varieties; e. g., 

 wort is the most suitable medium-base for the growth 

 of torulae and yeasts and should be employed when 

 pouring plates for the isolation of these organisms. To 

 obtain a pure cultivation of yeast from a mixture con- 

 taining bacteria as well, it is often sufficient to inoculate 

 wort from the mixture and incubate at 37 C. for 

 twenty-four hours. Plant a fresh tube of wort from 

 the resulting growth and incubate. Repeat the proc- 

 ess once more, and from the growth in this third tube 

 plant a streak on wort gelatine, and incubate at 20 C. 

 The resulting growth will almost certainly be a pure 

 culture of the yeast. 



(b) Deterrent. The converse of the above also 

 obtains. Certain media possess the power of inhibiting 

 the growth of a greater or less number of species. 

 For instance, media containing carbolic acid to the 

 amount of i per cent, will inhibit the growth of prac- 

 tically everything but the Bacillus coli communis. 



5. Differential Incubation. In isolating certain bac- 

 teria, advantage is taken of the fact that different 

 species vary in their optimum temperature. A mix- 

 ture containing the Bacillus typhosus and the Bacillus 

 aquatilis sulcatus, for example, may be planted on 

 two slanted agar tubes, the one incubated at 40 C., 

 and the other at 12 C. After twenty-four hours' 

 incubation the first will show a pure cultivation of the 



