4.7: Triple Sugar Iron Medium
Triple Sugar Iron is a slant medium with two growth environments: aerobic (on the slant) and anaerobic (in the “butt”). The medium contains three sugars in varying concentrations and a pH indicator that turns yellow at pH measurements below 6.8, and a deeper red at pH measurements above 8.2. Bacteria that ferment typically produce one or more types of acid as a byproduct, therefore, fermentation (both aerobic on the slant and anaerobic in the butt) is noted as a change in the color of the media. The medium also identifies strict aerobes that only grow on the slant surface, and also bacteria that produce H 2 S, either as a way to produce ATP anaerobically using sulfur or sulfate as a final electron acceptor, or as a result of the breakdown of proteins that contain high numbers of sulfur-containing amino acids (cysteine or methionine).
The results of this test are reported as appearance of the slant/appearance of the butt, using A to indicate acid reaction (yellow color), K to indicate an alkaline reaction, and NC to indicate no change in the medium. H 2 S (detected as a blackening in the media) and the production of gas (CO 2 ) as a byproduct of fermentation are also reported if observed (see Figure 4).
As an example, and for practice, the interpretation and outcomes for the 4 TSI tests shown are provided in the table below. Note that many other possible reactions may also occur so proper interpretation of this test is important.
| Table 1. TSI reactions shown in the cultures in Figure 4, from left to right. | |
| Outcome | Interpretation |
| Uninoculated control | For color comparison with inoculated samples |
| K/NC | Aerobic respiration (dark red on the slant) only. Bacteria are strict aerobes. |
| A/A; gas | Fermentation of all three sugars with CO 2 produced. Bacteria are facultative anaerobes. |
| K/A; H 2 S | Aerobic respiration (dark red on slant), fermentation of glucose (acid only in butt), anaerobic respiration (black in butt). Bacteria are facultative anaerobes. |
| K/A | Aerobic respiration (dark red on the slant); fermentation of glucose (acid only in butt). Bacteria are facultative anaerobes. |
How would you interpret the outcome of the TSI slant, the appearance of which is described below?
| Appearance | Outcome and Interpretation |
| Slant is a dark red color, butt is yellow with noticeable cracking and bubbling. |