![]() For example, an inoculum containing large amounts of bacteria might give a strong positive reaction, while one containing a very small number of bacteria might only produce a weakly positive, or perhaps a negative reaction. When using differential media, it is always important to remember that reactions in these media can vary based on the incubation time, the incubation temperature, the number of bacteria in the initial inoculum and how the medium is inoculated, among other variables. In addition to EMB and MS, there are many other types of differential media that can be used in the identification of bacteria. Each of these bacteria produced different colored colonies on this media, depending on their ability to ferment lactose. In other words, each bacterial species has a characteristic metabolism that can be used to distinguish them from other species, and therefore unknown bacteria are often identified based on both their appearance under the microscope as well as their metabolic properties.Įxamples of differences in metabolic properties were observed in Lab 2, when you inoculated E. Therefore, additional methods are required to identify individual species.Įven though different bacterial species may appear identical under the microscope, they all have a different genetic makeup (different DNA), and therefore produce different types of enzymes that allow them to carry out a characteristic set of biochemical reactions. aerogenes, and many other bacteria are all Gram negative rods and are indistinguishable by Gram staining. However, staining procedures are not sufficient to identify bacteria at the species level. Be able to relate the results of the tests performed to the different types of metabolic activities carried out by individual bacterial species.Īs you observed in Labs 3 and 4, staining procedures can be used to discriminate between bacteria based on morphology, arrangement, cell wall structure, and their possession of other structures associated with bacterial cells such as flagella, capsules and spores.Be able to perform the following biochemical tests and understand how they work: carbohydrate fermentation, gelatin hydrolysis, starch hydrolysis, casein hydrolysis, urea hydrolysis, citrate utilization, catalase activity, tryptophan hydrolysis (indole test).Learn how to use differential and selective media to identify bacterial species.
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