94 LABORATORY EXERCISES IN BACTERIOLOGY. 



(c) Lactose-litmus Agar. This modification is used in the study of acid production 

 by bacteria and as a means of differentiation between acid-producing bacteria and 

 organisms not possessed of the power of thus converting the sugar. It is prepared by 

 adding to agar of slightly alkaline reaction ( 0.5) two per cent, of lactose, and after 

 sterilization tinting the liquid medium before it congeals to a pale blue color with a 

 sterile litmus solution. Acid-producing bacteria growing upon this medium cause 

 a pinkish discoloration of the mass in and about the colonies, contrasting with the 

 blue color of the rest of the medium and the colonies of bacteria not possessing the 

 acid-producing power. 



4. Peptone Solution. This is often and with advantage used in place of bouillon. 

 It is made by dissolving ten grams of dried peptone and five grams of table salt in one 

 liter of distilled water; after which it is to be filtered, distributed, and sterilized. It 

 is not essential to adjust the reaction, as it is almost constantly neutral or nearly so. 

 The medium is especially favorable for use in the study of indol and phenol production. 



Rosolic Acid-peptone Solution. This substance serves as a means of determining 

 reaction changes produced by bacteria. It consists of the above peptone solution, to 

 which has been added two per cent, of a 0.5 per cent, solution of rosolic acid in eighty 

 per cent, alcohol. After diffusion of the dye through the peptone solution, the medium 

 is distributed to tubes and sterilized by fractional steaming. It is of a pale pink 

 color, which is intensified by alkaline changes and discharged by acid alterations 

 produced in the culture. 



5. Blood-serum. This substance, while an important nutrient for a large number 

 of bacteria, is of especial value for the cultivation of various pathogenic germs which 

 grow but feebly or not at all upon the ordinary media. The serum is generally ob- 

 tained from the blood of beeves slaughtered in the abattoir, or of the horses used in 

 the places of antitoxin manufacture ; occasionally the blood of smaller animals, as 

 calves, sheep, and the laboratory experiment animals, is used. Exceptionally the 

 serum is taken from human blood, as that from the placenta. There are differences 

 in the values of the serum derived from these various sources, but for the ordinary 

 scope of bacteriologic work they are not of importance. As already suggested, how- 

 ever, this feature is one to which future study is likely to add considerable interest. 



The serum may be used either in its liquid state or after solidification by heat. 

 After coagulation by heat it cannot again be liquefied, and is therefore a suitable medium 

 for cultivation at incubator temperature. The same fact, however, indicates the 

 impossibility of this substance as a medium for plate cultures, although mixtures of 

 serum with agar or gelatine (from these, however, the coagulable albumins are lost 

 in the manufacture) are sometimes employed for the purpose. Solidified blood-serum 

 is liquefied by the proteolytic action of bacteria, but to a less degree than gelatine. 



(a) The collection and preparation of solidified serum, especially by the older method, 

 requires no little care and time ; and the busy practitioner may without disadvantage 

 for the cultivation of any of the organisms requiring attention in clinical work, as 

 the diphtheritic germ, substitute for the serum the whole blood. Before it has time 

 to clot the blood is distributed to a number of tubes, coagulated by exposure to a 

 temperature of 90 C. in the oven, and then sterilized in the common fractional manner 

 in the steam-bath. The opaque black surface of the medium thus made contrasts 

 sharply with the pale or white colonies of most of the organisms and is not disad- 

 vantageous ; nor is the cultural value of the whole blood appreciably less for the majority 

 of bacteria than that of the solidified serum. 



(6) When the serum is to be used as a liquid medium (instead of bouillon), it 



