Bailey and Scott’s diagnostic microbiology. Colonies cannot be subcultured from the medium after the addition of Gram’s iodine due to the oxidative nature of the reagent and the resulting cell death.It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification.It aids in the differentiation of species of genera Corynebacterium, Clostridium, Bacillus, Bacteroides, Fusobacterium, and members of Enterococcus spp.Negative test:A blue, purple, or black coloration of the medium (depending on the concentration of iodine).Positive test:A clear zone around the line of growth after addition of iodine solution indicates that the organism has hydrolyzed starch.Examine for the clear zone around the line of bacterial growth.Following incubation, flood the surface of the plates with iodine solution with a dropper for 30 seconds.Incubate the bacterial inoculated plates for 48 hours at 37☌.Using a sterile technique, make a single streak inoculation of organism to be tested into the centre of labeled plate. ![]() Peptic digest of animal tissue 5.000, Sodium chloride 5.000, Yeast extract 1.500, Beef extract 1.500 Starch, soluble 2.000 Agar 15.000 Final pH ( at 25☌) 7.4☐.2 Method Agar is the solidifying agent and starch is the carbohydrate. Beef extract and pancreatic digest of gelatin provide nitrogen, vitamins, carbon and amino acids. Starch agar is a simple nutritive medium with starch added. Since no color change occurs in the medium when organisms hydrolyze starch, iodine solution is added as an indicator to the plate after incubation. While the non-hydrolysed starch forms dark blue color with iodine, its hydrolyzed end products do not acquire such dark blue color with iodine.Ĭonsequently, transparent clear zones are formed around the colonies that hydrolyze starch while the rest of the plate show a dark blue coloration as iodine forms the colored complex with starch. ![]() If the bacteria have the ability to hydrolyze starch, it does so in the medium, particularly in the areas surrounding their growth while the rest of the area of the plate still contain non-hydrolysed starch. In the starch hydrolysis test, the test bacteria are grown on agar plates containing starch. In this manner, nutrient sources, such as starch, that are too large to be absorbed through the cell membrane can be broken down into smaller molecules and transported into the cell via diffusion. Many bacteria produce extracellular enzymes used to catalyze chemical reactions outside of the cell. ![]()
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