MEASUREMENT OF H-ION CONCENTRATION 461 



Arnold sterilizer. Methyl-red was the only indicator that de- 

 colorized on sterilization. This was true of the glucose as well 

 as the lactose bouillon. These findings would justify the con- 

 clusion that the indicators enumerated, with the exception of 

 methyl-red can be introduced in the sugar media before sterili- 

 zation, if sterilization is done by the fractional method. It may 

 be of interest to note here that the reaction of the glucose bouil- 

 lon with brom-thymol-blue just before sterilization was pH 7, 

 and after sterilization it was found to be pH 6.8; while the lactose 

 bouillon was pH 7 before and the same after sterilization. This 

 would tend to indicate that the glucose bouillon becomes slightly 

 more acid on sterilization. 



Action of bacteria upon the indicators 



To each of the test tubes of the second set of glucose and lac- 

 tose bouillon respectively, was added after sterilization and with 

 sterile pipettes 0.8 cc. of each of the eight indicators enumerated. 

 The third set was left as a control. All three sets were then 

 placed in an incubator for twenty-four hours to test sterility. 

 When found sterile, the next day they were inoculated with a 

 pure culture of Bad. coli (freshly isolated from feces) and placed 

 in an incubator at 37°C. According to the findings as given in 

 table 11 none of the indicators except methyl red were attacked 

 by the bacteria, (Bad. coli) used in this test. Methyl red was 

 the only one of the whole group that was decolorized. It would 

 seem fair, therefore, to assume that the seven indicators used may 

 be added before sterilization and inoculation when dealing with 

 Bact. coli. When dealing with another organism, it would be 

 wise to test out its effect upon the indicators, or run control 

 cultures to check up findings. 3 



3 Incidentally it may be noted that the acid production of Bact. coli in glucose 

 was found to be the difference between pH 6.8 and 4.8 or pH 2 in twenty-four 

 hours, and that it did not increase in the next twenty-four hours The same or- 

 ganism in lactose produced much less acid in twenty-four hours (a difference of 

 pH 7 to 5.5 or pH 1.5), while in the next twenty-four hours it reached the same 

 acidity as in glucose (the difference of between pH 7 to 5 or pH 2). The third 

 set used as a control, checked up the findings in set no. 1 and no. 2. Methyl-red 

 if it be used, must be added fresh immediately before the reading is made. The 

 ability of sulphonthalein indicators to withstand the action of bacteria, has been 

 recognized by observers and referred to by the committee on descriptive chart. 

 Com. Soc. Am. Bact. (1919). 



JOURNAL OP BACTERIOLOGY, VOL V, NO. 5 



