Differential Tests

Differential Tests

1. Triple Sugar Iron (TSI) Agar

   • Sugar Fermentation
     • Glucose
       • Anaerobic fermentation seen in the butt; pH is acidic and media turns yellow from acidic byproducts
     • Lactose & Sucrose
       • Aerobic; seen in the slant of the tube. pH becomes acidic; media is yellow
     • Ferrous Sulfate
       • Detect H2S gas from sulfur reduction; black color
   • Peptonization
     • Amino acids metabolized to make ammonia; pH is alkaline; media is red

2. Simmon's Citrate Agar

   • Citric Acid Utilization
   • Sodium Citrate (sole carbon source)
     • Citrase uses Sodium Citrate as food, converts ammonium to ammonia
       • Ammonia produces makes media alkaline, blue media
   • Ammonium phosphate (single nitrogen source)
   • Bromthymol blue pH indicator

3. Sulfide-Indole-Motility (SIM)

   • Sulfur Reduction
     • Bacteria reduce sulfur directly or remove sulfur from cysteine, create H2S gas
       • Ferrous ammonium sulfate + H2S = ferric sulfide
         • Black precipitate if H2S is produced
     • Ferrous ammonium sulfate
   • Indole Production
     • Tryptophanase hydrolyzes tryptophan
       • Indole
         • Combines with DMABA in Kovac's Reagent to form Rosindole
           • Cherry Red if Indole is produced
       • Pyruvate
     • Kovac's Reagent
   • Motility
     • Ability of bacteria to move via Flagella
       • Cloudy growth outside of stab line
         • Bacteria are motile
       • Growth only along stab line
         • Bacteria are not motile