ABSTRACT
Sydney, a microbiology student, is trying to identify an unknown bacterial species as part of a laboratory assignment. Using a series of tests, he is able to narrow his choices down to two species: Citrobacter freundii and Proteus vulgaris. In his laboratory manual, Sydney notes that P. vulgaris can digest the amino acid phenylalanine, but C. freundii cannot. He therefore decides to inoculate his unknown species onto a phenylalanine slant. After allowing the bacteria to grow for 24 hours at 35°C, he adds ferric chloride to his tube and observes that a greenish color quickly forms (Figure 7.16). This allows Sydney to deduce that his unknown bacterial species can digest phenylalanine and therefore must be P. vulgaris. 1. What is a phenylalanine test and how does it work? 2. Why can only some species digest phenylalanine?
