Large and/or complex molecules/substances/nutrients (e.g. polysaccharides, lipids, proteins) need to be broken down into simpler parts before they are able to pass through into the cell membrane. Extracellular enzymes help hydrolyze and break down these macromolecules.
Starch hydrolysis. Starch is polymer of glucose units linked via glycosidic bonds. Amylase helps break starch down into dextrins (smaller polysaccharide units), and maltose (disaccharide). Maltase helps break down the maltose into glucose. Starch agar is a nutrient medium with starch added. When drops of iodine are added, remaining starch will turn blue-black. If exoenzymes have broken down the starch (i.e. if the organism is capable of starch hydrolysis), a clear zone should be evident around the growth. Presence of a clear zone indicates positive for starch hydrolysis.
Lipid hydrolysis. Lipases (esterases) help breakdown ester bonds of lipids into glycerol and fatty acids. Agar is supplemented with tributyrin (triglyceride) which will appear cloudy/milky. Presence of a clear zone (positive result) around the specimen growth indicate that the specimen is able to break down lipids.
Casein hydrolysis. Casein is a protein in milk. It’s made of amino acids units held together by peptide bonds (CO-NH). Proteases help to break down casein via a process called peptonization or proteolysis: peptones, polypeptides, dipeptides, and finally into amino acids. Agar is supplemented with milk (containing casein). A positive result is indicated by a clear zone around the growth.
Reference
Cappuccino, J. G., & Welsh, C. (2018). Microbiology: A laboratory manual.
