Triple Sugar Iron Agar



Explanation of TSI Reactions

Many of the enteric organisms will ferment glucose with the production of acids which will change the color of the medium in the butt and along the slant from red to yellow because of a reduction in the pH (within the first few hours). However, since the glucose is present in small amounts (0.1%), the supply is soon exhausted and the organisms growing on the surface of the slant in the presence of oxygen are forced to catabolize peptones and amino acids for their energy supply. Alkaline end-products are produced from these substances which revert the pH of the slant to an alkaline pH and thus change the color of the agar slant back to red (after 18-24 hours). Organisms such as Salmonella spp. or Shigella spp. and other organisms which attack glucose but do not ferment lactose or sucrose will produce an alkaline slant and acid butt in TSI slants in 18 to 24 hours. Since metabolism is progressing at a slower rate in the butt, this reversion does not usually take place in the butt until 48 hours or longer.

If the glucose is metabolized to CO2, the gas will be seen as bubbles or cracks in the agar butt. If hydrogen sulfide is formed during growth, a gray or black streak of iron sulfide is seen originating where the inoculating needle entered and throughout the agar butt.

Organisms which attack lactose and/or sucrose, such as Escherichia, will produce acid slants and acid butts usually with the formation of gas. In these cases, the acid slants do not revert to an alkaline status because lactose (1%) and sucrose (1%) are being fermented and are present in concentrations ten times that of glucose.

Some organisms (e.g., Pseudomonas, Acinetobacter) fail to ferment even glucose, and because they are strictly aerobic, they fail to grow in the butt of the tube. In these cases, the butt will be unchanged in color, and the slant either alkaline or unchanged.



Summary of Possible TSI Reactions

K = alkaline = Red;  A = acid = Yellow;  NC = No change;  G = gas produced;  H2S = hydrogen sulfide produced

Acid or alkaline results in the slant are reported first, followed by the butt results (e.g., K/A would be read as "K over A" or "alkaline over acid" and refers to an alkaline slant and acid butt).

  K/A è Only Glucose Fermented; Peptone Utilized  
  A/A è Glucose and Lactose/Sucrose Fermented  
  K/K è Peptone Utilized; No Carbohydrates Fermented  
  K/NC è Peptone Utilized Aerobically Only; No Sugars Fermented  
  NC/NC è No or Little Growth; Neither Sugars nor Peptone Catabolized  
  A/A, G è A/A + Gas Produced  
  A/A, H2S è A/A + H2S Produced  
  A/A, G, H2S è A/A + Gas + H2S Produced  
  K/A, G è K/A + Gas Produced  
  K/A, H2S è K/A + H2S Produced  
  K/A, G, H2S è K/A + Gas + H2S Produced  



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