THE PREPARATION OF CULTURE MEDIA 129 



13. Filter through absorbent cotton, passing the filtrate through the 

 same filter until clear. 



14. Titrate and record final reaction. 



Place agar in cotton-plugged sterile flasks or plugged sterile test 

 tv.bes and sterilize for thirty minutes on three successive days. 



Lactose-Litmus-Agar (Wurtz). — 1. Put 1,500 c.c. distilled water in 

 previously weighed agate-ware vessel. 



2. Add 15 gms. thread agar and boil over free flame for thirty to 

 forty-five minutes, watching and stirring constantly till the agar is 

 completely dissolved. 



3. Add 5 gms. Liebig's extract of meat, 5 gms. NaCl, 10 gms. Witte's 

 pepton, and dissolve completely. 



4. Restore loss by evaporation to 1,035 gms. 



5. Determine volume, titrate, and adjust reaction to one per cent 

 acid. 



6. Place in a flask and cool to 60° C. 



7. Add the whites of two eggs beaten up in 50 c.o. of water and mix 

 thoroughly. 



8. Heat for thirty minutes in Arnold sterilizer, shake thoroughly, 

 and heat again for fifteen minutes. 



9. Adjust weight. 



10. Filter through absorbent cotton to clear. 



11. Add two per cent pure lactose (milk sugar).' 



12. Add enough pure five per cent litmus solution^ to bring to 

 purple color when cold. 



13. Tube and sterilize. 



Welch's Modification of Guamieri's Medium.^ — This medium is made 

 on a meat-infusion basis, according to the directions given for the prep- 

 aration of meat-infusion agar. It contains 5 grams of agar, 80 grams of 

 gelatin, 5 grams of NaCl, and 10 grams of pepton to one liter. It should 



' Add lactose and litmus to 250 c.c. for 25 tubes; keep the remainder, with- 

 out lactose, stored in small sterile flasks for further use. 



'^ The litmus solutions used in the preparation of media are best made up as fol- 

 lows: Litmus in substance — Merck's purified, or Kaulbaum's — is dissolved in water 

 to the extent of 5 per cent. The solution is made by heating in an Arnold steriUzer 

 for about one to two hours, shaking occasionally. The solution is then filtered through 

 paper and sterilized. It should be kept sterile, as molds will grow in it otherwise. 



A standard litmus solution,, which is marketed for laboratory purposes, known 

 as "Kubel and Tiemann's" solution, may be used. 



' Welch, Bull. Johns Hopkins Hosp. 

 10 



