SEPTIC TANK STUDIES 



li 



Indol is produced in tryptophane broth by Escherichia coli and related strains 

 of the group and is considered to be a characteristic of coliform bacteria of sewage 

 origin. The same is true of a positive methyl-red test which means that the 

 coliform bacteria of sewage origin will produce sufficient acid in a medium con- 

 taining dextix)se to give a positive acid reaction with the methyl-red indicator. 

 The Voges-Proskauer reaction depends on the production of a substance known 

 as acetyl-methyl-carbinol from dextrose, and a positive test is characteristic of 

 Aerobacter aerogenes and other coliform bacteria of non-sewage origin. The 

 same is true of a positive sodium-citrate test which means the capacity of the 

 bacteria to grow in a medium containing sodiujii citrate as the sole source of 

 necessar\' carbon. The Citrobacter (5), closely related to E. coli, violates this 

 rule because it gets its name from its ability to utilize sodium citrate. Of these 

 tests, the methyl-red test is regarded by many authorities as the most dependable 

 single test for sewage-type coliform bacteria. A positive methyl-red test may 

 occur with any combination of reactions to the other three tests. 



Table 4. — Classification of Coliform Bacteria From Septic Tank 



Effluents Expressed as Percentage^ of Total Cultures 



Isolated From the Respective T.a.nks. 



Results of the differential tests, as seen in table 4, are based on the following 

 group reactions: 



Indol 



Methyl- 

 red 



Voges- 

 Proskauer 



Sodium 

 Citrate 



Escherichia coli 

 Citrobacter 

 Aerobacter aerogenes 



