48 MEDICAL BACTERIOLOGY 



For preparation of synthetic media, see "Preliminary Report on Synthetic 

 Media," C. J. T. Borland, Journal of Bacteriology, 1916, vol. i, No. 2, p. 135. 



During titration the contents of the dish should be constantly stirred. When 

 NaOH is being added, as each drop falls from the burette it causes a bright rose 

 red to appear, which varnishes on stirring, until the neutral point is reached. 



When the contents of the dish show a faint, but distinct pink color, that does 

 not disappear on stirring and heating, the neutral point has been reached. 



Having neutralized a measured sample of culture medium with a measured 

 amount of acid or alkali, it is easy to compute just what quantity of one or the 

 other need be added to the bulk of medium to produce any desired degree of 

 acidity or alkalinity e.g., if we are using a NaOH solution and find that the 

 neutral point has been reached when 2 cc. of it have been dropped into the dish 

 containing 5 cc. of the culture medium, then to neutralize 100 cc. of the culture 

 medium, twenty times as much - NaOH would be required, or 40 cc. of I0 

 NaOH. As i cc. of - x NaOH is equivalent to 10 cc. of 10 NaOH, then 4 cc. 

 Y NaOH would neutralize 100 c.c of the culture medium. 



In establishing a standard for this method of titration the plus sign is taken 

 to indicate acidity, the minus sign to indicate alkalinity 



Culture media having a reaction that would require the addition of i cc. 

 of normal sodium hydrate to each 100 cc. of media to neutralize it is said to have 

 a reaction of +i. In other words, +i indicates that the media is so acid that 

 i per cent, of normal sodium hydrate would be required to neutralize it. 



For general purposes and when the number of bacteria per cubic centimeter 

 is to be determined in milk, foodstuffs, water, sewage, etc., +1.5 is the standard 

 reaction. 



As a matter of fact when standard peptone, sodium chloride, meat extract 

 or meat infusion are used in making culture media, when completed, the reac- 

 tion nearly always falls between +0.5 and +1.5. If the reaction is found 

 to lie between these limits no attempt should be made to adjust it. If it falls 

 outside these limits it should be adjusted to +1.5. 



The growth of bacteria is largely influenced by variations in the titratable 

 acidity of culture media. Some organisms are much more sensitive to varia- 

 tions in this than others, and bacteriologists since the days of Pasteur have given 

 careful attention to this fact. Recent investigations have shown that in some 

 instances, probably in many, the hydrogen ion concentration is equally as im- 

 portant as the titratable acidity of media. At the present time no method of 

 measuring or adjusting this is in general use. Undoubtedly, in the near future 

 this will be done. The student is recommended to read Clark and Lubs mono- 

 graph on this subject, which is the clearest and most informative presentation 

 of the subject. "The Colorimetric Determination of Hydrogen Ion Concen- 

 tration and Its Applications in Bacteriology," Journal of Bacteriology, vol. ii, 

 No. I, January, 1917. 



FILTRATION OF CULTURE MEDIA 



Culture media must be clear. This is usually accomplished by filtration. 



