In clinical applications of API 20 E Identification System for Enterobacteriaceae and Other gram-negative rods, reagents are added only if the glucose (oxidation/fermentation) test results is yellow or at least three other tests are positive. If these are not met, a MacConkey Agar plate is streaked and additional tests are performed confirming glucose metabolism, nitrate reduction, and motility. Why do you think this is so? Be specific.