Oxidizer Fermenter Nonutilizer
of interest (i.e., lysine, ornithine, or arginine), and a pH indicator.
Organisms are inoculated into the tube medium that is then overlaid with mineral oil to ensure anaerobic conditions. (See Chapter 13) Early during incubation, bacteria utilize the glucose and produce acid, resulting in a yellow coloration of the pH indicator. Organisms that can decarboxylate the amino acid then begin to attack that substrate and produce the amine product, which increases the pH and changes the indicator back from yellow to purple (if bromcresol purple is the pH indicator used; red if phenol red is the indicator). Therefore, after overnight incubation, a positive test is indicated by a puiple color and a negative test (i.e„ lack of decarboxylase activity) is indicated by a yellow color. With each amino add tested, a control tube of the glucose-containing broth base without amino acid is inoculated. This standard's color is compared with that of the tube containing the amino acid following incubation.
Because it is a two-step process, the breakdown of arginine is more complicated than that of lysine or ornithine. Arginine is first dehydrolyzed to dtrulline, which is subsequently converted to ornithine. Ornithine is then decarboxylated to putresdne, which results in the same pH indicator changes as just outlined for the other amino adds.
Unlike decarboxylation, deamination of the amino add phenylalanine occurs in air. The presence of the end product (phenylpyruvic add) is detected by the addition of 10% ferric chloride, which results in the development of a green color. Agar slant medium is commerdally available for this test.
Single Substrate Utilization. Whether an organism can grow in the presence of a single nutrient or carbon source provides useful identification information. Such tests entail inoculation of organisms to a medium that contains a single source of nutrition (e.g., dtrate, mal-onate, or acetate) and, after incubation, observing the medium for growth. Growth is determined by observing the presence of bacterial colonies or by using a pH indicator to detect end products of metabolic activity.
Establishing Inhibitor Profiles. The ability of a bacterial isolate to grow in the presence of one or more inhibitory substances can provide valuable identification information. Examples regarding the use of inhibitory substances are presented earlier in this chapter.
In addition to the information gained from using inhibitory media and/or antimicrobial susceptibility testing, other more specific tests may be incorporated into bacterial identification' schemes. Because most of these tests are used to identify a particular group of bacteria, their protocols and prindples are discussed in the appropriate chapters in Part HI. A few examples of such tests indude:
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