These two test tubes represented two different growth environments. The left tube contained a growth medium containing the amino acid phenylalanine (+), while the tube on the right contained no phenylalanine (-). This was a test to determine the presence, or absence of Gram-positive members of the family of bacteria, Enterobacteriaceae, which could, or could not enzymatically break down, or deaminate phenylalanine. Note that the left tube contents exhibited a greenish coloration, indicating the enzymatic deamination of the phenylalanine contained within, converting it to phenylpyruvic acid. The tube on the right contained a bacterium, though of the same family, could not deaminate, or break down the phenylalanine, and therefore, produced a yellow color, or a negative result.