124 MICRO-ORGANISMS AND DISEASE [chap. 



alkaline (see a former chapter), is then inoculated with the 

 microbe and incubated. During the growth, on inspection 

 the gelatine next to the growth will be found to have become 

 violet and then red if the microbe produces acid, and the 

 more rapidly and conspicuously so, the more rapidly and 

 more acid it produces. If the gelatine remains bluish, then 

 no acid has been produced. In this case a neutral nutritive 

 gelatine is prepared and mixed with neutral litmus and then 

 inoculated with the microbe. On incubation, as the growth 

 appears, if the violet colour of the gelatine has turned blue 

 next to the growth then the microbe is an alkali-producer, if 

 the gelatine remains neutral then the microbe does not 

 produce either acid or alkali. As mentioned above, it is 

 common to find that the microbe produces acid, some rapidly 

 and distinctly {e.g. bacillus coli and typhoid), others only 

 slowly and in small amount {e.g. some varieties of the 

 vibrio of cholera). An interesting phenomenon is that many 

 microbes — even highly specialised microbes like the glanders 

 bacillus — grow well on potato (steamed), although the reac- 

 tion of this is acid (mallicacid) — in some potatoes very pro- 

 nounced, in others only very slight. Now the curious thing 

 about it is that some of the bacteria that show rapid and good 

 growth on potato show only very feeble or no growth if 

 planted on an acid medium, e.g. acid broth or acid 

 gelatine. 



3. Some microbes have the power to liquefy and pep- 

 tonise such resisting substances like solid agar and solid 

 blood-serum, though this power is possessed only by few 

 species. Most of the species that are capable of liquefying 

 and peptonising gelatine leave the agar and blood-serum 

 unaltered. The bacillus of Koch's malignant oedema, the 

 vibrio of Finkler, the vibrio of cholera (Koch), rapidly liquefy 

 blood-serum, but do not alter solid agar. 



