Some bacteria are able to hydrolyze starch from the medium.
Medium composition: nutritive agar supplemented with soluble starch 2%.
Inoculate a starch agar plate with fresh culture and incubate 24 hours at 37 ˚C, 24-48
hours. After incubation, flood the plate with Gram’s iodine / Lugol solution (iodine 20g
in 100 ml 50˚ethanol).
When iodine is added to starch, it turns dark blue. If starch has been hydrolyzed, then
it is not available to react with the iodine and the area around the bacterial growth is
Positive result: clear zone around colonies.
Negative result: dark blue to purple medium.
1. H. Raducanescu, V.Bica-Popii,1986. Bacteriologie veterinara, Ed. Ceres, Bucuresti.
2. Murray, P.R., Baron, E. J., Jorgensen, J.J., Pfaller, M.A., and Yolken, R.H. Manual of Clinical Microbiology, 8th ed. ASM Press:
Washington, DC, 2003.
(c) Costin Stoica