Fish Microbiology and Quality Assurance

11. Aminoacid decorboxylase test

The ability of the microorganisms to decorboxylate aminoacids such as lysine, ornithine and arginine is tested by inoculating test cultures to media containing individual aminoacids and observing increase in pH. Under anaerobic condition microorganism initially ferment glucose resulting in lowering of pH and subsequent decorboxylationof aminoacids results in ammonia production which causes increase in pH. The pH changes are indicated by the bromocresol purple incorporated to the medium.

Procedure:
To test tubes containing aminoacids are inoculated with test culture, overlayed with sterile liquid paraffin and incubated at 30⁰C for 24 to 48 hours. The change in colour of the medium from purple to yellow and back to purple is indicative of positive result.

Basal medium for aminoacid decorboxylase test

Soya peoptone : 5.0 g
Yeast extract : 3.0 g
Sodium chloride : 10. 0 g
Glucose : 1.0 g
Bromocresol purple : 0.016 g
Distilled water : 1 L
pH : 7.2 ± 0.2

The basal medium is divided in to four equal parts and aminoacids lysine, ornithine and arginine added at 0.5 %, 0.5 % and 4.0 % concentration respectively, transferred to test tubes in 3 ml
volumes and autoclaved at 110⁰c for 15 minutes. The aminoacid free medium is used as control.