50 PRINCIPLES OF BACTERIOLOGY 



bacterial change has occurred. It is first put in the ice-chest for 

 twelve hours to allow the cream to rise. The milk is then siphoned off 

 from below the cream into a flask and its reaction tested. After correc- 

 tion it is put in tubes or flasks and sterilized. If acid to phenolphthalein, 

 normal sodium hydrate should be added to make it 1 per cent. 



LITMUS MEDIA. When it is desirable to determine whether bacteria 

 produce in their growth acid or alkali from one or more of the constit- 

 uents of the media litmus is frequently added. To prepare the litmus 

 solution take the lump litmus, powder finely, and boil with distilled 

 water so that a saturated solution is obtained. Filter and then boil for 

 thirty minutes on two successive days. The litmus solution is added 

 to the neutral media in sufficient quantity to give the desired depth of 

 color. The less heating that is done after mixing the better the results. 

 Merck's purified litmus in 1 per cent, aqueous solution should be used 

 in careful work. 



PETRUSKY'S LITMUS WHEY (as modified by Durham). Fresh milk 

 is slightly warmed and clotted by means of essence of rennet. The 

 whey is strained off and the clot is hung up to drain in a piece of muslin. 

 The whey, which is somewhat turbid, is then cautiously neutralized 

 with 4 per cent, citric acid solution, neutral litmus being used as an 

 indicator. When it gives a good neutral violet color with the litmus it 

 is heated at 100 C. for one hour; thereby nearly the whole proteid is 

 coagulated. It is thus filtered clear, and neutral litmus is added to a 

 convenient color for observation. 



NEUTRAL RED. This dye is added to the peptone and bouillon- 

 sugar media to the amount of 1 to 5 per cent, of a concentrated solution. 

 Its reduction by the growth of bacteria is a valuable point in differentia- 

 tion. 



NITRATE BOUILLON. Dissolve 10 grams of peptone in 1 litre of 

 spring or tap water and add 0.02 grams of potassium nitrate (which 

 is free of nitrites). This is placed in test-tubes and sterilized. 



POTATOES. Potatoes are used for some special purposes. The 

 potatoes may, after thorough scrubbing and removal of "eyes/ 7 be 

 soaked in bichloride of mercury (1 :1000) for twenty minutes, and then 

 sterilized on three consecutive days for one-half hour in the steam ster- 

 ilizer. To use they are cut in thick slices and put in deep Petri dishes. 

 For more careful work the potatoes are first cut into proper sizes for 

 tubes, and then soaked for twelve hours in running water to remove the 

 acidity. 



BLOOD AND BLOOD-SERUM MEDIA, (a) Fresh Blood Media. These 

 are made by streaking sterile defibrinated or fresh human, rabbit, or other 

 blood over nutrient agar contained in tubes or dishes. Sometimes fresh 

 blood is added to fluid nutrient agar at 40 C. or to bouillon and a 

 mixture thus obtained. Media made with fresh blood contains not only 

 the haemoglobin, but also intact red blood cells. Blood media are used 

 for the growth of the influenza bacillus, for pneumococci and other 

 bacteria, and for the observation of the reduction of the haemoglobin 

 by the growth of certain bacteria. 



