152 PUBLIC HEALTH BACTERIOLOGY 



with water, adding i c.c. of phenolphthalein, and titrating 

 with N/io NaOH. Boil one minute before titration. 

 Calculate the amount of N/i NaOH required for the litre 

 of broth made. The convention at present in use is to 

 leave the broth acid to phenolphthalein to the extent that 

 i c.c. of N/i NaOH is required to neutralize 100 c.c. of 

 broth, or 10 c.c. per litre, and the medium is said to be 

 " acid + 10," or -f i per cent. Therefore add the calcu- 

 lated amount of N/i NaOH less 10. (Some add the full 

 amount of soda and then make acid to the desired extent 

 with N/i HC1.) Heat to boiling and test again ; if it needs 

 a further correction, boil again. Allow to cool to bring 

 down the precipitate of MgAm phosphate caused by the 

 change of reaction. Filter, place in flasks or tubes (about 

 5 c.c.), and sterilize in autoclave at 120 C. for 15 minutes, 

 or at 130 C. for i minute, or for 15 to 60 minutes on 

 three successive days in a steam sterilizer at 100 C. 



Broth can also be made from Liebig's Extract of Meat, 

 using 0*5 per cent of it instead of mince-meat. The other 

 procedure is similar. Neutralization can also be effected 

 by adding saturated solution of NaOH until red litmus is 

 just turned blue. 



Meat Extract or Fleischwasser can be used as a basis for 

 broth and the media derived from it. It is made by 

 warming 500 grm. of minced beef or horseflesh with 

 i litre of water at 50 C. for half an hour, and then boiling 

 for half to three-quarters of an hour. Filter, strain, make 

 up to i litre, and then pour into a flask ; if not to be used 

 at once, sterilize. 



Broth contains some muscle sugar or inosite, and to 

 get rid of this is at times inoculated with a young culture 

 of B. coli and incubated at 37 C. for 18 hours, and then 

 boiled to kill the organisms. 



Glucose and Lactose are added to sugar-free broth (usually 

 i per cent). Such media are not sterilized in the autoclave 

 but in a steamer, because the sugars are not stable at 

 high temperatures. 



Nutrient Gelatin is made from broth by adding 10 per 

 cent in winter and 15 per cent in summer of " gold label" 

 gelatin. Heat on water-rbath (as little as possible) to 

 dissolve ; readjust reaction (gelatin makes the medium 



