Triple Sugar Iron (TSI) Calculators

0 calculators tagged with “Triple Sugar Iron (TSI)”

Triple Sugar Iron (TSI) agar is a differential bacteriological medium used to distinguish enteric gram-negative bacilli based on carbohydrate fermentation and hydrogen sulfide (H₂S) production. TSI contains glucose, lactose, and sucrose as fermentable sugars plus ferrous sulfate as an H₂S indicator and phenol red as a pH indicator. Inoculated by stab (butt) and streak (slant), TSI reactions over 18–24 hours reveal key metabolic characteristics that help identify Enterobacteriaceae and distinguish pathogens from commensals in clinical and food microbiology.

All Calculators

No calculators found for this topic.

Components of TSI Agar

  • Glucose: Present at 0.1% (low concentration) — fermented by virtually all Enterobacteriaceae
  • Lactose and sucrose: Present at 1% each — fermented by coliforms and non-pathogens; non-fermented by many pathogens like Salmonella and Shigella
  • Phenol red: pH indicator — yellow in acid conditions (fermentation), pink/red in alkaline conditions
  • Ferrous sulfate + sodium thiosulfate: H₂S source and indicator — H₂S produces black precipitate (FeS)

Interpreting TSI Results

Slant (surface): Reflects lactose/sucrose fermentation. Acid/yellow slant = lactose or sucrose fermented. Alkaline/red-pink slant = no lactose/sucrose fermentation (only glucose was used, and resulting acid was oxidized away).

Butt (deep): Reflects glucose fermentation and gas production. Acid/yellow butt = glucose fermented. Cracking/displacement of agar = gas (CO₂) produced. Separation of agar from tube = heavy gas.

H₂S: Black precipitate in the butt indicates H₂S production.

Common Pathogen Patterns

  • E. coli: A/A (acid/acid), gas, no H₂S
  • Salmonella: K/A (alkaline/acid), gas, H₂S positive
  • Shigella: K/A, no gas, no H₂S
  • Klebsiella: A/A, heavy gas, no H₂S

Glossary

Triple Sugar Iron (TSI) Agar
A differential bacteriological medium containing glucose, lactose, sucrose, phenol red (pH indicator), and ferrous sulfate (H₂S indicator) for identifying enteric gram-negative bacteria.
K/A Reaction
A TSI result where the slant is alkaline (K) and the butt is acid (A); indicates glucose fermentation only (no lactose/sucrose); typical of Salmonella and Shigella.
A/A Reaction
A TSI result where both slant and butt are acid; indicates fermentation of lactose or sucrose in addition to glucose; typical of E. coli and other coliforms.

Frequently Asked Questions

Read slant (top) and butt (bottom) separately. Slant reaction: yellow (acid) = lactose or sucrose fermented; pink-red (alkaline) = only glucose was fermented initially, then acid was oxidized, leaving alkaline surface. Butt reaction: yellow (acid) = glucose fermented; gas production = cracked or displaced agar. Black precipitate anywhere = H₂S produced. Combine these three observations (slant/butt/H₂S/gas) to characterize the organism.

When only glucose is fermented (lactose and sucrose are not), the small amount of acid produced from glucose is quickly oxidized away at the aerobic slant surface, leaving alkaline conditions from peptone degradation products (NH₃). The anaerobic butt retains acidity longer because oxygen-dependent oxidation cannot occur. This produces a K/A (alkaline slant / acid butt) pattern typical of non-lactose-fermenting enteric pathogens like Salmonella and Shigella.

A black precipitate (iron sulfide, FeS) in the butt or along the stab line indicates H₂S production. The organism reduces thiosulfate to sulfide, which reacts with ferrous ions in the medium. H₂S positivity is characteristic of Salmonella spp. and Proteus spp. among common enteric pathogens, and is negative for most E. coli, Shigella, and Klebsiella. H₂S production in the context of K/A pattern strongly suggests Salmonella in clinical microbiology.

KIA (Kligler Iron Agar) contains only glucose and lactose (no sucrose), while TSI contains glucose, lactose, and sucrose. In most clinical laboratory contexts, the two media give identical results because sucrose fermentation patterns largely overlap with lactose. TSI is now more widely used because it provides slightly more information. Both use the same reaction principle (phenol red pH indicator, ferrous sulfate for H₂S detection) and are interpreted identically.