THE PREPARATION OP NUTRIENT MEDIA 



51 



far as possible similar to the natural soil on which they 

 first grew. In the following pages will be found an account 

 of the preparation of the culture media employed in general 

 use by bacteriologists. The preparation of these nutrient 

 media is not difficult, but it requires great care and attention 

 to the directions in order to insure success. 



Nutrient media are employed in test-tubes, small tri- 

 angular (Erlenmeyer's) flasks, plates, or flat Petri dishes. 

 All test-tubes, flasks, etc., employed in the preparation of 

 nutrient media are thoroughly cleansed with strong nitric 

 acid, after which treatment they are well rinsed with water 

 to remove all traces of acid. The tubes are then allowed to 

 draia until nearly dry, when they are finally rinsed out 

 with a little strong alcohol, drained, and allowed to dry. 



Reaction of Media. — Some bacteria grow readily in a 

 medium having an acid reaction, while the faintest trace of 

 acid will prevent the growth of others. As a rule, the patho- 

 genic species require a neutral or slightly alkaline medium. 



(1) Preparation of Beef-broth. — One pound of beef-steak, 

 freed from fat and connective tissue, is cut 

 up and passed through a small mincing- 

 machine. The finely-minced meat is then 

 digested with 1,000 c.c. of water. It is 

 then boiled, with constant stirring, for about 

 twenty to thirty minutes in a tinned or | 

 enamelled saucepan, which is kept well 

 covered. The broth is then strained j 

 through muslin, and then made up with i 

 distilled water to 1,000 c.c. to replace that 

 evaporated off during the boiling. To the 

 broth is then added 5 grammes of sodium 

 chloride and 10 grammes of peptone. The 

 latter is first rubbed up with a little of the broth in a glass 

 mortar, after which it is added to the bulk. 



4—2 



Fig. 11. 

 Meat Peess. 



