171 



IIVI)K(M;KN IOX COXClOXTIiATloX AM) TITUATAliLK ACllHrV IN 

 RELATION TO BACTEUIOI.OGirAL MEDIA. 



I. T-. TiAi.itwix. 



It lias loiif; hcoii rccoiiiiizcd that tlic rcadioii of iiicdia is a very iiii- 

 portaiit oonsideration in the cultivation of liaclcria. 'Ilic liactcrioiofiist is 

 confrontocl with tho piolih-iii of dotorniiniiii; or iiicasuiiii.^ this reaction in 

 two instances. First, in tho adjustment of the orij^inal reaction of the nieilia 

 and .secondly in the measurement of acid production by bacteria. 



In the past two methods of measurement hjiv(> been iised. based on two 

 different chemical phenomena. The ohier method of the two is tlie titra- 

 tion of the media with a standard acid or basic solution, using phenolph- 

 thalien as an indicator. In usinj; this method media was almost iniiversally 

 made -f 1 to phenolphthalein. or 1% of normal acid was added after the neu- 

 trality point was reached. This method was based on a measurement of 

 the total acid or base in the solution. The newer method is a mea.sure 

 of the concentration of the free hydrogen ions in the solution. This may he 

 accomplished by either the electrolytic or the colorimetric method. The 

 electrolytic method is the more accurate of the two. but requires more 

 time and complicated apparatus, to which the bacteriologist rarely has 

 access. The colorimetric method, since the introduction by Clark and Lubs 

 of a series of phthalein indicators whose sensitive ranges have been ac- 

 curately determined, is accurate enough for bacteriological work, is applic- 

 able to the solutions with which the bacteriologist is working and is quick 

 and simple in operation. 



Since it is the concentration of free or disassociated hydrogen ions and 

 not the total amount of acid present that affects the bacterial growth, it is 

 readily seen that a method whicli measures hydrogen ion concentration is 

 preferable to one which measures titratable acidity. Also it should be 

 clearly understood that a determination of the titratable acidity gives 

 no indication of the hydrogen ion concentration. A single example using 

 two common acids will illustrate this point. Normal acetic acid has ten 

 times the titratable acidity of tenth normal hydrochloric acid, yet tenth 

 normal hydrochloric acid has about 22.4 times the hydrogen ion concen- 

 tration of normal acetic aciil. 



Another very serious soui'cc of error in the dctcrmiiiation of the reaction 

 of media by the old method of titrating with a standard solution lies in the 

 buffer effect of various ingredients of the media. By the buffer effect of a 

 substance we mean its ability to combine with an acid or base in the union- 

 ized condition. Peptone, mainly due to the proteoses and phosphates which 

 it contains, has a marked buffer effect. Thus the addition of an acid or a 

 base to a peptone solution may change the hydrogen ion concentration but 

 very slightly. Also the extent of the buffer effect of a peptone solution is 

 dei^endent ui)on the brand of the peptone and the technic followed in making 

 up the solution. As mentioned above, the buffer effect of pejitone is largely 

 due to its content of i»rotcoses and phosi»hates, according to Kligler"s work 



