Media Principles – Ritu's Biology Hub

Medium	Functional Type	Principles
Blood agar	Enriched and Differential	Contains whole blood so the organisms are distinguished based on haemolysis pattern. It is an enriched because the presence of blood, a whole nutrient, allows growth of fastidious bacteria. It is a differential medium as it enables differentiation between various bacteria. Organisms produce an enzyme called haemolysis that utilizes haemoglobin from RBCs. · β – haemolysis – Clear zone around the colony as haemoglobin is completely degraded. Eg., S.pyogenes, S.aureus · α – haemolysis – Greenish or turbid zone around the colony as haemoglobin is partially degraded to methemoglobin. Eg., S.viridans · ϒ – haemolysis – No haemolysis. Eg., S.faecalis, S.epidermidis
Chocolate agar	Enriched	It is same as blood agar with the same composition. The difference lies in overheating of the blood and lysing RBCs that give it a chocolate brown colour. The heating process releases certain growth factors like NAD and factor X making them more readily available. Used to cultivate H.influenzae, N.gonnorhaea
Eosin methylene blue agar	Selective and Differential	Two dyes, eosin and methylene blue inhibit growth of gram positive bacteria. Hence, it is a selective medium. It contains lactose as the fermentable sugar. Lactose fermenters ferment lactose and generate acidic products. The dyes react with these products and get deposited in the colonies giving dark red to maroon colonies. Typical coliforms such as E.coli also exhibit a green metallic sheen. This medium is used for detecting indicator organisms in a water potability test.
Hektoen enteric agar	Selective and Differential	Hektoen Enteric Agar is used for the isolation of Salmonella sp. Bile salts, bromothymol blue and acid fuchsin inhibit the growth of most Gram positive organisms. Lactose and sucrose are the fermentable carbohydrates. Lactose and sucrose fermenters produce yellow colonies on fermenting the sugars and producing acidic products which turn the pH indicator, bromothymol blue, yellow while the non-fermenters show blue-green colonies. Sodium thiosulfate is metabolized to H₂S gas which reacts with ferric ions of ferric ammonium citrate producing a black insoluble precipitate of FeS.
MacConkey agar	Selective and Differential	It is selective because it contains bile salts that inhibit growth of gram positive and non-enteric gram negative bacteria. It is a differential medium because it allows us to differentiate between lactose fermenters and lactose non-fermenters. It contains a pH indicator, neutral red which turns pink under acidic and pale to orange-ish under alkaline conditions. Lactose fermenters ferment lactose present in the medium producing a lot of acids that lower the pH of the medium thereby giving rise to pink colonies. Lactose non-fermenters are not able to do so giving pale colonies.
Mannitol salt agar	Selective and Differential	MSA is a selective medium since the 7.5% NaCl concentration inhibits growth of most bacteria other than Staphylococci. It contains mannitol as the C source that makes it a differential medium allowing us to differentiate between pathogenic and non-pathogenic Staphylococci. It contains phenol red as the pH indicator that turns yellow under acidic conditions. Pathogenic strain eg., S.aureus utilize mannitol producing acidic metabolic products that reduce the medium pH turning it yellow in colour.
Sabouraud’s agar	General Purpose	It is widely used for the cultivation of fungi, specially for those associated with skin infections. It is also used to determine fungal contaminants in food, cosmetics and pharmaceutical products. Peptone is a N source and dextrose serves as the C source. High dextrose concentration and low pH favours fungal growth inhibiting bacteria.
Simmon’s citrate agar	Differential	It is a medium used to differentiate between Enterobacteriaceae and members of aerogenes group. Sodium citrate and ammonium dihydrogen phosphate act as C and N source respectively. Microbes also utilize other ammonium salts in the medium liberating alkaline products increasing the pH of the medium. This change in pH is sensed by the pH indicator, bromothymol blue, thereby making the medium turn green to blue.
Sulfur indole motility test	Differential	It is a differential medium used to differentiate various gram negative bacilli based on sulphide production, indole formation and motility. Bacteria utilize sodium thiosulfate forming H₂S gas that reacts with ferrous ammonium sulphate giving a black ppt. of FeS. Pancreatic digest of casein is a rich source of tryptophan. Organisms secreting tryptophanase converts it to indole. Indole reacts with p-dimethylaminobenzaldehyde in Kovac’s reagent giving a red band. A small amount of agar allows the detection of motility in the semi-solid medium. The organism is stabbed through the medium. Upon incubation, if the medium turns turbid it confirms motility.
Thioglycollate medium	General Purpose	Dextrose, pancreatic digest of casein, yeast extract and L-cystine provide the growth factors necessary for bacterial growth. L-cystine and sodium thioglycollate allows Clostridium to grow in this medium even under aerobic conditions. Sodium thioglycollate act as a reducing agent and neutralizes the toxic effects of mercurial preservatives and peroxides formed in the medium, thereby promoting anaerobiosis and making the medium suitable to test materials containing heavy metals. Any increase in the oxygen content is indicated by a colour change of the redox indicator, resazurin to red. The small amount of agar helps in maintaining low redox potential for stabilizing the medium.

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By RituparnaJune 20, 2016Basic MicrobiologyLeave a Comment

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