Question

9 starch agar plates have been inoculated with different bacteria in order to see if each bacterial species has amylase to hydrolyze the starch. After incubation, Gram's iodine must be added to see if the results were positive or negative. Before applying the Gram's iodine, the plates are incubated for 48 hours.

What should you expect to happen if you applied the Gram's iodine after 2 hours of incubation instead of the 48 hours and why?

Answer 1

agar containing starch is a differential media that helps in testing the ability of any organism to produce amylase enzyme that hydrolyzes starch. Starch is large biomolecules and cannot enter the bacterial cells. Therefore, certain bacteria secrete these exoenzymes that degrade starch and then utilize the small subunits.

Starch agar is a nutritive media containing starch. Since there is no change in color of media when organisms hydrolyze starch iodine is added to the plate after 48 hrs of incubation. Iodine turns blue-black in the presence of starch and a zone of clearance around the bacteria is indicative of enzyme produced by bacteria and released in media to hydrolyze starch.

Adding the Gram's iodine after 2hrs of incubation instead of 48 hrs incubation would not give bacteria enough time for growth and produce enzymes. Hence, the entire plate would appear blue-black without any zone of clearance leading to faulty results.