The bacteria quickly metabolized the glucose, initially producing an acid slant and an acid butt (acid over acid; A/A) in a few hours. The Emben-Meyerhof-Parnas pathway is used both aerobically (on the slant) and anerobically (in the butt) to produce ATP and pyruvate. On the slant, the pyruvate is further metabolized to CO2, H2O, and energy. After further incubation (18 hours) the glucose was consumed, and because the bacteria could not use lactose or sucrose, the peptones (amino acids) were utilized as an energy source aerobically, on the slant. Utilization of peptones causes the release of ammonia (NH3) resulting in the pH indicator, phenol red, turning from yellow to red. In the anerobic butt, the bacteria use the Embden-Meyerhof-Parnas pathway to metabolized the glucose producing ATP and pyruvate, which is converted into stable acid endproducts, thus the butt remains acidic.
The black precipitate indicates that the bacteria were able to produce hydrogen sulfide (H2S) from sodium thiosulfate. Because H2S is colorless, ferric ammonium citrate is used as an indicator resulting in the formation of insoluble ferrous sulfide. Formation of H2S requires an acidic environment; even though a yellow butt cannot be seen because of the black precipitate, the butt is acidic. The results would be recorded as alkaline over acid (K/A), H2S positive.
Bacteria producing a K/A with H2S include: Citrobacter freundii*, Edwardsiella tarda, Proteus mirabilis*, and Salmonella spp*.
Bacteria commonly producing an A/A with H2S include: Citrobacter freundii*, Proteus mirabilis*, and P. vulgaris*.
* = variable reactions
Reference: MacFaddin, J.F. (2000) Biochemical Tests for Identificaion of Medical Bacteria, 3rd. ed. Lippincott Williams & Wilkins, Philadelphia. |
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