36 LIQUID MEDIA 



should sterilize his hands and then collect the milk as it leaves the udder in 

 sterile flasks. (For further details, see Chap. XII.) 



Each flask is about three-parts filled, sealed in the flame, [or plugged with 

 sterile wool and covered with an india-rubber cap,] and heated in a water 

 bath at 60-65 C. for eight days in the manner described on p. 12. 



When sterilization is completed, the milk can be tubed into sterile tubes, 

 as described in connexion with the preparation of serum (p. 45). 



(C) If the technique of the milking process can be relied upon, it will be 

 sufficient to fill as many tubes as are required, and to incubate them at 30 C. 

 for some days before using the milk as a culture medium. In spite of every 

 precaution some of the tubes will be contaminated, and any tube in which 

 the milk has clotted or which on microscopical examination shows the 

 presence of organisms must be rejected. 



Urine. 



Though urine was widely employed in the early days of bacteriology, it 

 has now almost ceased to be used as a culture medium. 



(a) 1. Boil some recently passed urine. 



2. If the reaction be markedly alkaline after boiling, add a little tartaric acid 

 solution, testing the reaction with litmus paper. 



3. Filter, tube and sterilize at 115 C. 



The composition of the urine is distinctly altered by this proceeding, the urea 

 in solution being decomposed at the temperature of boiling water. 



(6) It is better to sterilize by filtering through a Chamberland bougie (Chap. I.). 



(c) To collect urine in a sterile manner, and so avoid the necessity for steriliza- 

 tion with the attendant alteration in composition, proceed as in Chap. XII. ("Urine"). 



The urine which has been collected in a flask may be tubed by any of the methods 

 described for tubing serum (p. 45). Incubate the tubes at 37 C. for 48 hours, 

 and reject any which are then cloudy. 



Serum. 



Serum is obtained by allowing blood to clot spontaneously or from the 

 fluid of pleural effusions. It is used sometimes as a liquid but much more 

 commonly as a solid medium after being coagulated by heat. 



The technique for the collection of serum will be studied under the head 

 of solid media (pp. 45 et seq.}. 



Blood. 



Blood is frequently used as a culture medium. 



To use it as a liquid medium coagulation must be prevented, and this 

 may best be done by defibrinating the blood. The blood is collected asepti- 

 cally (pp. 45 and 48 and Chap. XII.) in a sterile flask containing glass beads 

 and shaken for about 10 minutes, then aspirated into a [Cobbett's bulb or] 

 Chamberland flask (pp. 45 and 47) and tubed. 



Among the many substances which it has been suggested might be added to 

 blood to prevent coagulation, neutral sodium citrate and extract of leeches' heads 

 may be mentioned. 



By the sodium citrate method the blood is collected as it leaves the vein in a 

 flask or tube containing a certain quantity of the following sterile solution : water, 

 1000 c.c. ; sodium chloride, 8 grams ; sodium citrate, 15 grams. 



Extract of leech heads is obtained by placing the heads in 75 per cent, alcohol 

 for 5 or 6 days. When hardened, the heads are dried and ground up in a mortar. 

 The powder is dissolved in distilled water (100 c.c. per head), boiled, filtered and 

 sterilized at 105 C. for 5 to 10 minutes. The extract is then introduced into 

 the tubes in which the blood is to be collected. 



These last two methods are not so good as defibrination. 



