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This is one of 10 Profiles on Animal-microbe interactions. It covers the pathogenic bacterium Proteus vulgaris and a method for rapid identification of bacteria from clinical specimens. Other
Profiles on this site that cover animal-microbe
interactions are: |
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| Proteus vulgaris The genus Proteus is classified in the enteric bacteria, together with Escherichia coli, Salmonella, Shigella, Enterobacter and Serratia. All these bacteria are small, Gram-negative rods and are facultative anaerobes: they ferment sugars in anaerobic conditions but can use a wide range of organic molecules in aerobic conditions. Some of the enterics are major pathogens of humans, but Proteus species are mainly soil inhabitants, particularly common in decomposing organic matter. Proteus and the related genus Providencia can quite frequently cause urinary tract infections. |
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Proteus has two interesting and notable features, shown in the images above. First, the cells are highly motile and often swarm across the surface of agar plates (Figure A). Swarming gives rise to a very thin film of bacteria on the agar surface, but swarming periods are interspersed with periods when the cells stop and undergo a cycle of growth and division so that the colony has a distinct zonation, clearly seen in Fig. A. The other notable feature of both Proteus and Providentia is the ability to degrade urea to ammonia, by production of the enzyme urease. This distinguishes them from the other enterics and is used in a simple diagnostic test (Figure B). Bacteria isolated from urine samples are inoculated onto a nutrient agar containing urea and the indicator phenol red. After overnight incubation, the ammonia produced by Proteus or Providentia raises the pH and changes the colour of the medium from yellow to red. |
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Diagnostic methods in microbiology Simple biochemical tests like the one above have always been an important aid to identification of bacteria, because the different bacterial groups and species have characteristic metabolic activities. A number of sophisticated tools are now available for clinical diagnosis. Some are based on monoclonal antibodies, and others on simple, rapid biochemical methods. The API 20E system shown below has become popular for rapid identification of members of the Enterobacteriaceae and other Gram-negative bacteria. The plastic strips consist of 20 small wells containing dehydrated media components (top row). The bacterium to be tested is suspended in sterile saline and added to each well, then the strip is incubated for 16-24 hours and the colour reactions are noted as either positive or negative. The test results can be entered into a computer programme to identify the bacterium. Four strips inoculated with four different bacteria are shown in the Figure. In each case the spectrum of results was different. |
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 Top row, Proteus vulgaris; second row, unidentified enteric bacterium; third row, Klebsiella pneumoniae; bottom row, Vibrio alginolyticus. |
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A few of the tests on these strips are outlined below. On the left-hand side:
On the right-hand side:
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