140 CULTURE MEDIA IN RELATION TO METABOLIC PRODUCTS 



If alcohol has been formed from the sugar it will be necessary to 

 obtain it by fractional distillation. The details of exact acid and 

 alcohol determinations require a more complicated apparatus and 

 extensive set of reagents, hence they cannot be given here. The 

 mere fact of acid formation can be easily demonstrated on lactose 

 litmus gelatin or litmus milk. If enough acid is produced the lavender 

 color of these media is changed to red. 



Gas Production. Gas production is controlled by inoculating 

 sugar bouillon contained in a fermentation tube. Various kinds of 

 sugars, such as saccharose, dextrose, maltose, lactose, mannite, are 

 used, as it is often more important to determine the 

 FIG. 71 varieties of sugar which are or are not split up by 



certain species of bacteria. The closed limb of the 

 fermentation tube should be graduated so that the 

 amount of gas formed can be estimated approxi- 

 mately from day to day by the readings. After the 

 formation of gas has ceased the proportion of 

 carbon dioxid and hydrogen present can be ap- 

 proximately estimated by the following method: 

 Fill the open bulb of the tube completely with a 

 2 per cent, caustic soda solution. Close the bulb 

 with the thumb or a rubber stopper and invert 

 the tube several times so that the gas is intimately 

 Fermentation mixed with the fluid, which now contains caustic 

 tube. soda. The latter will absorb the carbon dioxid. 



Return the remainder of the gas to the closed end 

 of the tube, remove the thumb or stopper and allow the fluid to cool 

 so that the residual gas is no longer expanded by heat. The loss 

 in gas represents the carbon dioxid and the balance is hydrogen. 

 The proportion of carbon dioxid to hydrogen is often characteristic 

 for certain bacteria. 



Sulphuretted Hydrogen. To ascertain the formation of sulphuretted 

 hydrogen a special culture medium known as iron peptone or lead 

 peptone is necessary. These culture media are prepared as follows: 



1. Take peptone, 30 grams, shake with water heated to 60 C. 



2. Wash emulsion into a liter flask with 80 c.c. of water. 



3. Add chloride of sodium, 5 grams, and phosphate of sodium, 3 

 grams, and make up to 1000 c.c. 



4. Heat for thirty minutes in a water bath or steam sterilizer to 

 dissolve completely, then filter clear. 



5. Fill into tubes 10 c.c. to each and add 0.1 c.c. of a 2 per cent, 

 neutral solution of tartrate of iron. This causes a yellowish-white 

 precipitate to form. 



6. Sterilize for twenty minutes on three consecutive days. 



The lead peptone is prepared in the same manner except that 0.1 

 c.c. of a 1 per cent, neutral solution of lead acetate replaces the tar- 

 trate of iron added to each tube. The tubes are inoculated with the 



