122 METHODS OF PURE CULTURE. 



The nutrient solutions hitherto described play a great part in 

 earlier mycological literature, on which account their constitution 

 is now given, though at present they are but seldom used. 



On the other hand, a second nutrient solution given by Pasteur,, 

 viz., yeast-water, is still frequently used. To prepare this solution,, 

 about 100 grms. of thick brewer's barm (or 75 grms. of pressed 

 yeast) are placed in a tin can with one litre of water over the fire, 

 and boiled for a quarter of an hour, and are then passed through a, 

 folded filter. If the liquid passing through is turbid it is returned 

 to the filter, and in this way a clear, pale yellow filtrate is obtained, 

 which is made up to one litre by the addition of distilled water, 

 and is then sterilised (either in bulk or in portions) by exposure- 

 to 100 C. in a steamer on three consecutive days, or by a single 

 operation of twenty minutes at 120 C. under pressure. By a 

 preliminary addition of 5 to 10 per cent, of sugar a very useful 

 nutrient medium for yeast is obtained. When acidified with 

 acetic acid and qualified with alcohol, yeast-water rendered good 

 service in Pasteur's studies in acetic fermentation. 



For the cultivation of beer-yeasts the most suitable medium is- 

 hopped beer-wort, sterilised in the Pasteur flask as already de- 

 scribed. The hop-resin in this liquid exerts a toxic action on 

 many organisms, and among them the lactic acid bacteria, which 

 play an important part in distillery work ; so that hopped wort- 

 must not be employed to cultivate these organisms in the 

 laboratory, unhopped wort being advisable for this and sundry 

 similar purposes. Unhopped wort is an advantageous medium for 

 numerous fermentative organisms, and therefore requires special 

 care in sterilising. 



Wine-must serves for the artificial multiplication of wine- 

 yeasts and fruit-yeasts, and a concentrated form of it is kept in 

 stock in the laboratory. On this point fuller particulars will be- 

 found in Chapter xx. 



Saprogenic and most pathogenic bacteria thrive particularly 

 well in meat-juice. This is used in the form of so-called bouillon, 

 and, following the lines indicated by the researches of PETIII and 

 MAASSBN (I.), is prepared as follows : Half a kilogram (i.i Ib.) of 

 finely minced beef, free from fat, is placed in a tin pan or earthen 

 crock along with one litre (if pints) of well- water, and, after 

 standing for an hour at the ordinary temperature, is heated to about 

 60 C. during three hours, with frequent stirrings. At the con- 

 clusion of this period of extraction the mixture is boiled for half 

 an hour and then filtered through a folded filter. The pale yellow 

 effluent liquid is made up to one litre when cold, and exhibits an 

 amphoteric reaction. Its primary salts of orthophosphoric acid 

 (e.g. KH 2 P0 4 ) redden blue litmus paper, whilst on the other hand 

 the secondary phosphates (e.g. K 2 HP0 4 ) also present behave in 

 the contrary manner. In presence of phenolphthalein, however, 

 only the tertiary phosphate (K 3 P0 4 ) acts as a base, and conse- 



