Eosin-methylene blue EMB agar was initially formulated in by Holt-Harris and Teague, to visibly differentiate between the lactose fermenting and non-fermenting microorganisms through the use of eosin and methylene blue dyes. The medium also included sucrose to differentiate between coliforms that were able to ferment sucrose more rapidly than lactose and those that were unable to ferment sucrose.
Later in , EMB agar was modified by Levine to enable the medium to differentiate between faecal and nonfecal types of the coliform bacteria group. However, this medium differs from the one formulated by Holt-Harris and Teague in that it includes a single peptone as a base and supplementing it with dipotassium phosphate as a buffer and increased concentration of lactose and absence of sucrose.
Salmonellae and other non-lactose- fermenters from the coliforms could also be differentiated. This medium contains lactose and sucrose as formulated by Holt-Harris and Teague and also contains peptone peptic digest of animal tissue and phosphate as modified by Levine. The two indicator dyes, eosin and methylene blue, are used in a ratio to impart minimum toxicity while providing best differentiation.
Eosin Methylene Blue agar EMB agar is both selective and differential culture medium ideally used for isolating faecal coliforms. It is a selective medium due to inhibition of gram-positive bacteria by methylene blue.
EMB agar's differential property comes from the eosin and methylene blue complex formation under acidic pH. People who are working on EMB media ask the following questions frequently. To adequately address these questions, we need to dive into two fundamental studies. Following is the summary of those articles in biteable pieces. Although the media was formulated in , how it shows its differential properties were mostly unknown.
In , Wynne et al. The hypothesis behind the EMB agar's differential staining ability is the degree of the bacteria to ferment lactose and produce acids. When the acids are released into surrounding media, pH drops results in acidic media. Although, many enteric forms can consume lactose and sucrose as carbon source and produce acids, the level at which they can drop the pH of the medium varies. Wynne et al. The results of the experiments mentioned below.
The pH of the media at the beginning of the experiment is 7. From the values mentioned above, one can infer that the level of pH change in the media is species dependent. In general, we may call the first three organisms to be fermenters, but the level of fermentation and type of acids they produce corresponds to decrease in pH varies.
Levine modification contains 10g of lactose twice as in EMB agar and contains no sucrose. Any growth on the media should be regarded as contamination and the whole lot should be discarded. Performance testing of prepared EMB agar plates can be done by inoculating known strains of bacteria into the medium and observing growth and colonial characteristics.
News Ticker. About Acharya Tankeshwar Articles. The colonies were positive for catalase. The bacteria grew on mannitol salt agar fermenting mannitol, as shown by the change to yellow of the medium.
The pH indicator in mannitol salt agar is phenol red, which turns to yellow when the medium is acidified by the products of fermentation. The toxin secreted by S. The organism was probably introduced into the salad during preparation by the food handler and multiplied while the salad was kept in the warm ambient temperature during the speeches. Blood agar contains many unspecified nutrients, supports the growth of a large number of bacteria, and allows differentiation of bacteria according to hemolysis breakdown of blood.
Rogosa agar contains yeast extract. The pH is adjusted to 5. Skip to main content. Microbial Growth.
Search for:. Media Used for Bacterial Growth Learning Objectives Define the term culture medium Give examples of the following types of media:: complex, chemically defined, selective, and differential.
Figure 1. The three types of hemolysis on a blood agar plate. Think about It Distinguish complex and chemically defined media. Distinguish selective and enrichment media. Compare the compositions of EZ medium and sheep blood agar. The End-of-Year Picnic The microbiology department is celebrating the end of the school year in May by holding its traditional picnic on the green.
Key Concepts and Summary Chemically defined media have a a known quantities of each chemical component Selective media favor the growth of some microorganisms while inhibiting others.
Colonial Characteristics Escherichia coli - isolated colonies, mm diameter, with little tendency to confluent growth, exhibiting a greenish metallic sheen by reflected light and dark purple centres by transmitted light. Enterobacter aerogenes - mm diameter, raised and mucoid colonies, tending to become confluent, metallic sheen usually absent, grey-brown centres by transmitted light.
Other Candida species produce smooth yeast-like colonies. Since a typical appearance is variable it is advisable to use a combined method such as that of Walker and Huppert Appearance Dehydrated medium: purple coloured, free-flowing powder Prepared medium: dark purple gel. Precautions Further tests are required to confirm the presumptive identity of organisms isolated on this medium. Some strains of Salmonella and Shigella species will not grow in the presence of eosin and methylene blue.
Store the medium away from light to prevent photo-oxidation. References 1.
0コメント