靛基質（Indole）試驗：

　　某些細菌能分解蛋白胨中的色氨酸，生成吲哚。吲哚的存在可用顯色反應表現出來。吲哚與對二甲基氨基苯醛結合，形成玫瑰吲哚，為紅色化合物。

　　試驗方法：將待試純培養物小量接種于試驗培養基管，于36±1C培養24h時后，取約2ml培養液，加入Kovacs氏試劑2~3滴，輕搖試管，呈紅色為陽性，或先加少量乙醚或二甲苯，搖動試管以提取和濃縮靛基質，待其浮于培養液表面后，再沿試管壁徐緩加入Kovacs氏試劑數滴，在接觸面呈紅色，即為陽性。

　　實驗證明靛基質試劑可與17種不的靛基質化合物作用而產生陽性反應，若先用二甲苯或乙醚等進行提取，再加試劑，則只有靛基質或5-甲基靛基質在溶劑中呈現紅色，因而結果更為可靠。

Margaret (Peg) Johnson, Mesa Community College, Mesa, Ariz., USA

　　This image depicts the results of negative and positive indole tests. The indole test is frequently employed to distinguish Klebsiella or Enterobacter bacteria (indole negative) from Escherichia coli (indole positive). The presence of E. coli is used by public health officials as an indicator of fecal contamination of food and water supplies.

　　Prior to this test, Enterobacter aerogenes was used to inoculate one tryptone broth, while another tryptone broth was inoculated with Escherichia coli. The two broths were then incubated for 24 h and mixed with Kovas reagent (p-dimethylamino-benzaldehyde). Tryptone broth is rich in the amino acid tryptophan. Tryptophanase, an enzyme, is capable of cleaving tryptophan and producing indole, pyruvic acid, and ammonia. Indole can be detected by the development of a red color after adding Kovas reagent. Organisms that do not produce tryptophanase will be indole negative, as no indole will be present to react with the Kovas reagent.