It selectively promotes the growth of Gram-negative bacteria inhibits Gram-positive bacteria and aids in the differentiation of lactose fermenter and non-lactose fermenting colonies. EMB agar, first described by Holt-Harris and Teague, contained lactose and sucrose as source of carbohydrates. Levine further modified the medium by adding peptone and phosphate and removed sucrose from the formula and increased the lactose content. This aided in the differentiation of fecal and non-fecal types of the coliforms and also salmonellae and other non-lactose fermenters from the coliforms.
Another commonly used media for selective isolation of Gram-negative rods and differentiation of the member of Enter obacteriacea e as lactose fermenter and non-lactose fermenter is MacConkey Agar.
In summary, how a colony stains on EMB agar, is an interplay between the pH changes the bacteria brings in the medium, relative ratio of dyes and buffering capacity of the media. Although the reasons mentioned above for the colony colour are relatively straightforward, it can be more complex in reality. For example, strong fermenters can form EMB complex and takes up the dye.
Since the Eosin is in excess, and shown to be taken into cells under acidic conditions, plenty of chance to take up Eosin along with EMB complex. This situation becomes more complicated when pH changes are linked to change in the colour of these dyes. Alternatively, the commercially available EMB agar media powders can be used. Weigh the mixture of content as prescribed by the manufacturer. The test sample can be directly streaked on the medium plates and then the plates should be incubated, protected from light.
However, in order to obtain isolated colonies, standard procedures should be followed. A non-selective medium should be inoculated along with conjunction with EMB Agar. Identification of isolates should be done with further confirmatory tests. The pH of the media plays a critical role in the colony colour. Care to be taken to keep the pH to the recommended value. Save my name, email, and website in this browser for the next time I comment. Table of Contents. Add a header to begin generating the table of contents.
Introduction 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.
Principle Eosin Methylene Blue agar EMB agar is both selective and differential culture medium ideally used for isolating faecal coliforms. Neither Eosin Y red , methylene blue blue nor a mixture of the of them is green coloured. Where does the green sheen come from? Why some fermenting bacteria develop green sheen while others do not? Sometimes, I see blue colour colonies appear in the medium why? Figure 1. The medium is supplemented with the pH indicator neutral red, which turns to hot pink at low pH.
Selective and differential media can be combined and play an important role in the identification of bacteria by biochemical methods. Figure 2. On this MacConkey agar plate, the lactose-fermenter E.
Serratia marcescens, which does not ferment lactose, forms a cream-colored streak on the tan medium. Another commonly used medium that is both selective and differential is eosin-methylene blue EMB agar. EMB contains the dyes eosin and methylene blue that inhibit the growth of gram-positve bacteria. Therefore, EMB is selective for gram-negatives. In addition, the gram-negatives that grow can be differentiated based on their ability to ferment lactose.
When bacterial cells ferment lactose, acid is produced that precipitates the dyes in the medium and the colonies develop a green metallic sheen Figure 3. Figure 3 E. The gram-negative bacterium grows and ferments lactose, giving the colonies a green metallic sheen.
The microbiology department is celebrating the end of the school year in May by holding its traditional picnic on the green. The speeches drag on for a couple of hours, but finally all the faculty and students can dig into the food: chicken salad, tomatoes, onions, salad, and custard pie. By evening, the whole department, except for two vegetarian students who did not eat the chicken salad, is stricken with nausea, vomiting, retching, and abdominal cramping.
Several individuals complain of diarrhea. One patient shows signs of shock low blood pressure. Blood and stool samples are collected from patients, and an analysis of all foods served at the meal is conducted.
Bacteria can cause gastroenteritis inflammation of the stomach and intestinal tract either by colonizing and replicating in the host, which is considered an infection, or by secreting toxins, which is considered intoxication. The differential media make it easy to distinguish colonies of different bacteria by a change in the color of the colonies or the color of the medium. Color changes are the result of end products created by interaction of bacterial enzymes with differential substrates in the medium or, in the case of hemolytic reactions, the lysis of red blood cells in the medium.
In Figure 1, the differential fermentation of lactose can be observed on MacConkey agar. The lactose fermenters produce acid, which turns the medium and the colonies of strong fermenters hot pink. The medium is supplemented with the pH indicator neutral red, which turns to hot pink at low pH.
Selective and differential media can be combined and play an important role in the identification of bacteria by biochemical methods. The microbiology department is celebrating the end of the school year in May by holding its traditional picnic on the green. The speeches drag on for a couple of hours, but finally all the faculty and students can dig into the food: chicken salad, tomatoes, onions, salad, and custard pie.
By evening, the whole department, except for two vegetarian students who did not eat the chicken salad, is stricken with nausea, vomiting, retching, and abdominal cramping.
Several individuals complain of diarrhea. One patient shows signs of shock low blood pressure. Blood and stool samples are collected from patients, and an analysis of all foods served at the meal is conducted. Bacteria can cause gastroenteritis inflammation of the stomach and intestinal tract either by colonizing and replicating in the host, which is considered an infection, or by secreting toxins, which is considered intoxication. Signs and symptoms of infections are typically delayed, whereas intoxication manifests within hours, as happened after the picnic.
Blood samples from the patients showed no signs of bacterial infection, which further suggests that this was a case of intoxication. Since intoxication is due to secreted toxins, bacteria are not usually detected in blood or stool samples. MacConkey agar and sorbitol-MacConkey agar plates and xylose-lysine-deoxycholate XLD plates were inoculated with stool samples and did not reveal any unusually colored colonies, and no black colonies or white colonies were observed on XLD.
All lactose fermenters on MacConkey agar also ferment sorbitol. These results ruled out common agents of food-borne illnesses: E. Figure 2. Gram-positive cocci in clusters.
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