64 A MANUAL OF BACTERIOLOGY 



autoclave (p. 52) by heating to 115-125 C. for half an 

 hour on one occasion. Culture media may also be kept in 

 bulk in flasks, bottles or milk-bottles ; these need some- 

 what longer sterilisation than tubes. Some forms of 

 culture media can also be purchased ready for use. 

 Certain media can be obtained in powder form (Chop- 

 ping's) from Messrs. Baird and Tatlock, and in tabloid 

 form (Thompson's) from Messrs. Burroughs and Wellcome. 

 These are convenient when small quantities are required 

 for occasional use. 



Neutralisation. The reaction of the culture medium 

 influences the growth of micro-organisms upon it, and 

 some organisms will develop only when the reaction lies 

 within narrow limits. To obtain the proper reaction, 

 acid or alkali is added to the prepared media to the 

 required amount, the process being commonly termed 

 " neutralisation." For the pathogenic micro-organisms 

 a slightly alkaline medium is generally to be preferred, 

 and as most of the media are somewhat acid when pre- 

 pared, the addition of a certain amount of alkali is 

 required. A rough and ready method of obtaining the 

 proper degree of alkalinity is by the addition of a 10-20 

 per cent, solution of caustic soda (preferably) or of 

 sodium carbonate, glazed litmus paper being used as an 

 indicator. A few drops of the alkaline solution are added 

 at a time and well mixed with the medium, and between 

 each addition a drop of the medium is taken with a glass 

 rod and smeared on to both red and blue litmus papers. 

 At first the blue paper will be reddened, while the red 

 paper will be unchanged. Subsequently, both papers 

 will frequently be changed, the " amphoteric reaction." 

 Neutralisation should be continued beyond this stage by 

 the addition of more alkali, until the red paper is well 

 blued and the blue paper is unaltered ; the reaction will 

 now be about right. Should a medium as prepared be 



