Dextrose Starch Agar

Dextrose Starch Agar is a microbiological growth medium formulated to support the cultivation and differentiation of various microorganisms, particularly bacteria and fungi. Its composition typically includes proteose peptone (nitrogen source), dextrose (glucose) as an energy and carbon source, soluble starch to neutralize toxic fatty acids, sodium chloride for osmotic balance, disodium hydrogen phosphate as a buffering agent, gelatin as a proteinaceous supplement, and agar as the solidifying agent.

Typical composition per liter:

Proteose peptone: 15 g

Dextrose: 2 g

Soluble starch: 10 g

Sodium chloride: 5 g

Disodium hydrogen phosphate: 3 g

Gelatin: 20 g

Agar: 10 g

pH is adjusted to about 7.3 ± 0.2 at 25°C.

The medium provides a nutritious environment favored by fastidious organisms such as Neisseria meningitidis and Streptococcus pyogenes. The starch in the agar also facilitates detection of amylolytic activity by the microorganisms through starch hydrolysis, which can be visualized by iodine staining.

Preparation of the medium involves dissolving 65 g of the powder in 1000 mL of distilled water, boiling to dissolve completely, dispensing into tubes or plates, sterilizing by autoclaving (121°C, 15 min), and cooling in a slanted position for solidification.

This medium finds use in microbiology labs for cultivation, differentiation, and maintenance of stock cultures of specific bacteria and fungi.