abbreviations: A/A: glucose and lactose/sucrose fermented A/A,G: A/A + gas produ

Question

abbreviations:

A/A: glucose and lactose/sucrose fermented

A/A,G: A/A + gas produced

A/A,G,H2S: A/A +gas + H2S produced

A/A,H2S: A/A + H2S produced

K/A: only glucose fermented peptone utilized

K/NC: peptone utilized aerobically only, no sugars ferminated

NC/NC: no or litle growth, neither sugars nor peptone catabolized

K/K: peptone utilized; no carbohydrates fermented

K/A,G: K/A + Gas produced

K/A, H2S: K/A + H2S produced

K/A,G,H2S: K/A + Gas + H2S produced

Explanation / Answer

The test is the triple sugar iron test for Enterobacteria. Glucose is present in low concentration of 0.1% Lactose and sucrose is present in 1% concentration. Phenol red is the indicator, which turns yellow in acidic conditions. It is red in basic condition (pH 8.2). Coliform bacteria ferment sucrose much faster than lactose. If none of the sugars is fermented, the slant will be red due to deamination of amino acid that creates alkaline conditions. If glucose is fermented (but no lactose or sucrose), slant will appear red as the small amount of acid produced is oxidized to carbon dioxide and water by the organism. When lactose is fermented with gas, there will be cracks in the yellow agar due to gas production. Hydrogen sulphite reacts with iron in the medium to form black color.

A and D: Species such as Salmonella (A) and Proteus (B) ferment the glucose at the aerobic surface of the slant as a carbon source. They cannot utilize the lactose and sucrose present in the medium. They also produce hydrogen sulphite, which turns the media black. As the B species produce large amounts of hydrogen sulphite. Hence, the whole medium turns black. These organisms use the protein (peptone) at the aerobic surface of the slant as a carbon source, the pH increases and the top of the slant becomes red. The black is due to hydrogen sulfide production. Substantial hydrogen sulfide production by D makes it difficult to determine color in the butt of the slant.

C: The organism ferments glucose similar to A and D but does not produce hydrogen sulphite. Hence, the media turns yellow. There is also gas production

The organism present in "C" ferments only the glucose similar to Salmonella and Proteus (less acid) but does not produce hydrogen sulfide (note gas production).

B: This organism can ferment all three sugars (glucose, sucrose and lactose). Hence, a large amount of acid is produced, lowering the pH. The medium turns yellow. There is a little bit of gas production. Escherichia coli ferment all three sugars, the pH reduces, and the slant is slightly yellow.

A: K/A, H2S: K/A + H2S produced

B: A/A,G: A/A + gas produced

C: K/A,G: K/A + Gas produced

D: K/A, H2S: K/A + H2S produced

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