Formation of formic acid by gas negative E. coli strains in EC test

Some strains of E. coli type I which produce no gas in EC medium at 44.5°C were detected from the human and animal feces. The average rate of detection frequency of these bacteria was about 10% among the total E. coli type I isolates. The biochemical pattern of carbohydrate metabolism of these bacteria was investigated using nutrient broth containing formic acid, pyruvic acid, acetic acid or α-ketoglutalic acid, each of which is suposed to be intermediate compound in dissimilation pathway. Gas formation at 44.5°C was observed only in the broth containing formic acid. In the lactose broth, formic acid was not produced by these bacteria at 44.5°C, whereas it was produced by the typical E. coli I strain. From these results it is conseivable that the formic acid formation from pyruvic acid by the said atypical E. coli I strain may be inhibited at 44.5°C. On the other hand E. coli type II and III bacteria almost failed to produce formic acid from pyruvic acid even at 37°C.