20 CULTURAL STUDIES OF SPECIES OF PENICILLIUM. 



5 per cent cane sugar be added, an equally large majority of the same 

 species cause an acid reaction. Some few remain alkaline. These 

 species growing upon cane sugar produce acids in quantity both to 

 neutralize the alkaline products of the decomposition of gelatin and 

 to change the reaction of the mass. The litmus reaction has been 

 found generally reliable in a medium composed of 15 per cent gelatin 

 in distilled water, in lactose-peptone gelatin after Conn's 4 formula, 

 and in potato agar during four years of cultural work where many 

 successive lots of media have involved the use of materials from 

 different sources. In complex media the decomposition products of 

 organic nitrogenous constituents and those of carbohydrates tend 

 either to neutralize each other or to intensify the reactions, but add 

 greatlj' to the difficulties of analyzing the results. The litmus 

 reaction therefore may be used with the simplest organic media, but 

 is best applicable to cultures in synthetic media where analysis of the 

 results is feasible. 



Gelatin. — The liquefaction of gelatin media by an organism in cul- 

 ture shows its ability to produce a particular form of proteolysis. 

 This reaction is so conspicuous and so adaptable to cultural use that 

 it has been recorded in all cultural studies. Various investigators 

 have pointed out the limitations of this reaction. In cultivation 

 nearly all of the species of Penicillium have been found to grow well 

 upon a 15 per cent solution of gelatin alone in distilled water. Com- 

 parison of the results in this medium with liquefaction of several forms 

 of gelatin media experimented with showed essential agreement. 

 Since the advantages all lie in the simplification of formulae, the data 

 as to liquefaction by these species have been compiled from series of 

 parallel cultures of all the species upon 15 per cent gelatin in dis- 

 tilled water. 



The value of this reaction is measurably vitiated by the fact that 

 any species which can subsist upon gelatin alone must be capable of 

 more or less proteolytic action upon it. The results of observation 

 confirm this statement. The value of these observations therefore 

 depends upon the indication of the comparative rate of activity of 

 different species in inducing proteolysis, not upon the presence or 

 entire absence of this action. Numerous tests, involving a range from 

 15° to 25° C, show that slight changes of temperature affect this 

 reaction only to the extent to which they affect the rate of growth, 

 but do not disturb the comparative value of the data obtained. 

 Within this range of temperature the most active species produce 

 liquefaction in 5 to 8 days, other vigorous liquefiers require 8 to 15 

 days, while many species producing no liquid or but traces of soften- 

 ing in 15 days produce a gradual liquefaction in the succeeding 2 to 4 

 weeks. In studying species of Penicillium, liquefaction of gelatin has 

 been found to separate such species as produce this proteolysis within 



