Which medium is selective

A culture medium is simply water and nutrients that support microbial growth. Primary culture media for clinical microbiology are divided into 3 primary categories — nutritive, differential and selective media. Media that support the growth of many different microorganisms without distinguishing genera or species are nutritive. In contrast, differential media allow several different types of bacteria to grow, but also contain compounds that allow microbial genera or even species to be visually differentiated.

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. Analysis of the chicken salad revealed an abnormal number of gram-positive cocci arranged in clusters Figure 2. A culture of the gram-positive cocci releases bubbles when mixed with hydrogen peroxide. The culture turned mannitol salt agar yellow after a hour incubation.

All the tests point to Staphylococcus aureus as the organism that secreted the toxin. Samples from the salad showed the presence of gram-positive cocci bacteria in clusters.

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. Gancyclovir is an exception to the rule as it is used to specifically kill cells that carry its respective marker, the Herpes simplex virus thymidine kinase HSV TK.

Some examples of selective media include:. Differential media or indicator media distinguish one microorganism type from another growing on the same media. This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators such as neutral red, phenol red, eosin y, or methylene blue added to the medium to visibly indicate the defining characteristics of a microorganism.

This type of media is used for the detection of microorganisms and by molecular biologists to detect recombinant strains of bacteria. Current and past strategies for bacterial culture in clinical microbiology. Clin Microb Rev. Article Google Scholar. Madigan M, Martinko J. Brock biology of microorganisms. Hoboken: Prentice Hall; Google Scholar. Prokaryotes: the unseen majority.

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