306 INFECTION AND IMMUNITY 



failure of union in the circulating blood of the living animal is due 

 to actions analogous to those of protective colloids, and represent 

 a protective mechanism which represents the sudden union of anti- 

 gen and antibody in the circulation. 



Precipitating antisera for tests should be clear. If turbid, the 

 sera should be filtered through small Berkefeld or porcelain candles. 



PREPARATION OF BACTERIAL FILTRATES AND PROTEIN SOLUTIONS FOB 

 PRECIPITIN TESTS. Bacteria may be grown in nutrient broth having 

 an initial reaction of neutrality or five-tenths per cent acidity to 

 phenolphthalein. The cultures are incubated for times varying 

 from a week to several months, and are then filtered through por- 

 celain or Berkefeld candles until perfectly clear. Bacterial extracts 

 may also be made by emulsifying agar cultures in salt solution, 

 placing at 37.5 C. in the incubator for a week or longer, and filter- 

 ing. More rapid extraction of bacteria may be accomplished by 

 repeated, rapid freezing and thawing of salt-solution emulsions, by 

 shaking in the shaking machine or by centrifugalizing, rubbing up 

 the sediment with dry salt, and the addition of distilled water to 

 isotonicity. 



Protein solutions to be tested should be made in salt solution. 

 When dealing with blood stains, as in doing the test for forensic 

 purposes, the stains should be dissolved in salt solution, an ap- 

 proximate dilution of one in five hundred being aimed at. This 

 solution if turbid should be filtered through a small porcelain filter. 

 It should be clear and colorless, show a faint cloud on boiling with 

 dilute acetic acid, and show distinct froth when shaken. 



When the reaction is to be done for determining the nature of 

 meat (detection of horse-meat substitution for beef, etc.), about 20 

 to 40 grams of the suspected meat are macerated in a flask, and 

 covered with 100 c.c. of salt solution. This mixture is allowed to 

 infuse at room temperature for three to four hours, and is then 

 placed in the refrigerator for twelve hours or more. At the end 

 of this time 2 c.c. are shaken into a test tube. If profuse frothing 7 " 

 appears, the extract is ready for use. It is then filtered clear, either 

 through paper, or through a Buchner or Nutsche filter. Berkefeld 

 filters may also be used. The solution is then diluted until the 

 addition of concentrated IIN0 3 produces only a slight even turbidity. 



7 P. Th. Milller, ' ' Technik. d. serodiagnos. Methoden. ' ' 



