Question

You complete a starch hydrolysis test and observe that only your colony is yellow, while all of the surrounding medium is darkly colored. Is this result positive or negative? Explain.

Answer 1

You complete a starch hydrolysis test and observe that only your colony is yellow, while all of the surrounding medium is darkly colored. Is this result positive or negative? Explain.

The rsult is POSITIVE FOR STARCH HYDROLYSIS. The bacteria is test positive for starch hydrolysis. While all of the surrounding medium is darkly colored, the only yellow color around the colony is called as the CLEAR ZONE. Clear zone occurs as a result of starch hydolysis by the organism produce monosaccharides which cannot bind the iodine molecule indicates yellow color. The starch hydrolyzing activity of the strain is directly proportional to the size of the clear zone.

This experiment is used to identify the bacterial strains that has enzyme called a-amylase and oligo-1,6-glucosidase and can hydrolyze starch into simple sugars to utilize starch as the carbon source. The broken smaller glucose subunits can directly proceed into the glycolytic pathway. Positive bacterial strain for starch hydrolysis is usually identified with the clear zone. Bacillus subtilis is an example of positive starch hydrolysis strain.

The iodine is yellow in color which is added to the starch medium. In presence of starch , iodine forms blue, purple or dark color as iodine is trapped in the helical structure of starch whereas the hydrolysed starch that is monosachharides cannot produce blue color and remains yellow.