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Biochemical Test Media for Lab Unknown Identification—Part 3 Send Print

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Created: Thursday, 29 August 2002
Last update: Monday, 16 August 2010
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Spirit blue lipase test (Enlarged view)
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Spirit blue lipase test (Labeled view)
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Starch agar (Enlarged view)
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Unified-Oxidation-Fermentation (Enlarged view)
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Most bacteria of medical importance can be grown on artificial culture media. Culture media can be nonselective or selective. Nonselective media allow a wide variety of bacteria to grow (e.g., nutrient agar or blood agar). Selective media allow only certain organisms to grow because they have specific inhibitors added to the media (e.g., the bile salts in MacConkey agar). Additionally, culture media may also be differential, which allows groups of biochemically related bacteria to be distinguished from other groups of bacteria.

This series of images illustrates some common biochemical media reactions for identifying bacteria. Although some bacteria can be identified by visual observation using microscopy, definitive identification usually requires further tests, many of them biochemical. Diagnostic laboratories use various biochemical media to isolate and identify bacteria from clinical specimens.

These images can be used for practice questions on quizzes, lab practical reviews, or as guides for students as they are reading their own tests in lab.

Figure 11. Spirit blue lipase test. This differential test determines if an organism produces the secreted enzyme lipase. The bacterial sample is streaked onto an agar plate containing tributyn, a triglyceride hydrolyzed by the enzyme lipase. If the bacteria secretes lipase, there will be a zone of clearing surrounding the sample. If the bacteria does not produce and secrete lipase, the agar will remain opaque. Positive: Serratia marcescens. Negative: Escherichia coli.

Figure 12. Starch agar. This differential test determines an organism's ability to produce amylase. The bacterial sample is incubated on an agar plate containing starch and iodine (as an indicator). If the organism produces amylase, a zone of clearing will surround the inoculation. Positive: Bacillus subtilis. Negative: Escherichia coli.

Figure 13. Unified-Oxidation-Fermentation (Uni-OF) glucose test (a variation of the Oxidation-Fermentation Hugh-Leifson Base). This single tube test determines if an organism is oxidative or fermentative. Carbohydrates may be metabolized by one of two processes: oxidation (aerobic) or fermentation (anaerobic). Uni-OF glucose media is supplemented with glucose as the carbohydrate source and a pH indicator. The lower half of the tube allows for anaerobic conditions, while the upper half contains aerobic conditions. If an organism is able to metabolize the glucose in the condition present, acid is produced and the media turns yellow. A color of blue or green indicates that metabolism did not occur. An organism with oxidative metabolism will display yellow in the upper half of the tube and green in the lower half. An organism with fermentative metabolism will display yellow in both halves of the tube. Blue or green in both halves indicates that the organism cannot metabolize glucose. Fermentative metabolism (yellow in both halves): Vibrio cholerae. Oxidative metabolism: (yellow in upper half, green in lower half): Bacillus subtilis. Negative (green in both halves): Pseudomonas.

See also:
Biochemical Test Media for Lab Unknown Identification—Part 1
Biochemical Test Media for Lab Unknown Identification—Part 2
Biochemical Test Media for Lab Unknown Identification—Part 4

Legend written by:
Kristen Catlin
American Society for Microbiology
Washington D.C. 20036

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