18 MASS. EXPERIMENT STATION BULLETIN 414 



most part, were from the same stock cultures as those employed in the dissolved- 

 iron study. Readings were made after 48 hours' incubation. Results are shown 

 in table 5. 



These results closely resembled those obtained with metallic iron. By separate 

 experiment with acetic acid as the acidifying agent in the medium, it had been 

 demonstrated that at 37° C. erythrosin began to be precipitated at pH 5.1 and 

 that precipitation was practically complete at pH 4.9. 



T.\BLE 5. — Precipitation of Erythrosin. 



Cultures Positive Negative 



1 



17 



113 



15 



G. Indol Tolerance of Coliform Bacteria 



This study (15) was based upon reports of other investigators that, although 

 indol has been shown to inhibit bacterial growth, E. coli is quite resistant to its 

 effects. It has even been suggested that indol production may be the mechanism 

 by which E. coli retains its dominant position among intestinal flora (literature 

 cited in published paper). 



Tubes of Standard-Methods lactose broth were prepared and quantities of 

 indol (solution in 70 percent alcohol) were added to give the concentrations shown 

 in table 6. The bacterial cultures inoculated into the medium consisted of 173 

 strains of E. colt, 155 of A. aerogenes, and 83 of coliform intermediates. These 

 cultures were classified by means of Voges-Proskauer, methyl-red, sodium citrate, 

 and uric acid tests. The final results, after 48 hours' incubation, are shown in 

 table 6. 



T.\BLE 6. — Indol Toler.a.nce of Coliform Bacteria. 



Dilution of indol and number of strains inhibited Total 



cultures 

 Organism 1:1500 1:1800 1:2000 1:2300 1:2500 1:3000 tested 



Results indicate that E.- coli had the greatest resistance to indol, A. aerogenes 

 the least, with the intermediates in between. The difference, however, was not 

 great enough to have any apparent significance or use in indicating the fecal 

 origin of E. coli or the non-fecal origin of coliform bacteria. 



H. Bile Salts and Aerobacter aerogenes 



Norwood and Webster (16) have suggested that in the human intestine E. 

 coli of the colon may be a derivative of A. aerogenes existing in the small intestine, 

 and their report suggested the next study (17). 



