The usual gelling agent for solid or semisolid medium is agar , a hydrocolloid derived from red algae. Agar is used because of its unique physical properties it melts at o C and remains liquid until cooled to 40 o C, the temperature at which it gels and because it cannot be metabolized by most bacteria.
Hence as a medium component it is relatively inert; it simply holds gels nutrients that are in aquaeous solution. Culture media may be classified into several categories depending on their composition or use. A chemically-defined synthetic medium Table 4a and 4b is one in which the exact chemical composition is known. A complex undefined medium Table 5a and 5b is one in which the exact chemical constitution of the medium is not known.
Defined media are usually composed of pure biochemicals off the shelf; complex media usually contain complex materials of biological origin such as blood or milk or yeast extract or beef extract, the exact chemical composition of which is obviously undetermined.
A defined medium is a minimal medium Table 4a if it provides only the exact nutrients including any growth factors needed by the organism for growth. The use of defined minimal media requires the investigator to know the exact nutritional requirements of the organisms in question. Chemically-defined media are of value in studying the minimal nutritional requirements of microorganisms, for enrichment cultures, and for a wide variety of physiological studies.
Complex media usually provide the full range of growth factors that may be required by an organism so they may be more handily used to cultivate unknown bacteria or bacteria whose nutritional requirement are complex i.
Complex media are usually used for cultivation of bacterial pathogens and other fastidious bacteria. Figure 2. Legionella pneumophila. Direct fluorescent antibody DFA stain of a patient respiratory tract specimen.
Delisle and Lewis Tomalty. Queens University, Kingston, Ontario, Canada. 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. 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. EMB agar is a medium used in the identification and isolation of pathogenic bacteria. It contains digested meat proteins as a source of organic nutrients.
Two indicator dyes, eosin and methylene blue, inhibit the growth of gram-positive bacteria and distinguish between lactose fermenting and nonlactose fermenting organisms. Lactose fermenters form metallic green or deep purple colonies, whereas the nonlactose fermenters form completely colorless colonies.
EMB agar is an example of which of the following? Haemophilus influenzae must be grown on chocolate agar, which is blood agar treated with heat to release growth factors in the medium. 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.
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