A diverse collection of microorganisms co-exist with life on Earth; most of them as inhabitants among plants or oceans, many as normal flora in humans, and some in remote locations in the most extreme habitats. Despite being ubiquitous in nature and typically harmless, bacteria get particular attention for causing disease in humans.
Correct identification of a microorganism allows for proper investigation of a particular species, and prevention or treatment of a disease if necessary. During lab, students were instructed to choose a test tube inoculated with an unknown organism and then prompted to initiate a series of appropriate lab tests to correctly identify the organism.
Unknown 3 was received in Trypticase Soy Broth. A Gram test was immediately performed to reveal that the bacterium was a Gram positive cocci. Once the morphology and Gram stain were determined and the presence of growth on a Trypticase Soy Agar slant was apparent, two tests were used to narrow down the list of possible organisms; those tests were the Methyl Red test and Urea hydrolysis.
The Methyl Red test is used to distinguish an organism’s ability to ferment mixed acid, which is verified by the change in color of the Methyl Red broth after a reagent is added. There was no change in color of the broth which indicated a negative result. Unknown 3 was also inoculated in a Urease broth, which tested for the presence of Urease, an enzyme responsible for hydrolyzing Urea into ammonia and carbon dioxide. If Urea had metabolized, indicating a positive test, the broth would have turned pink. The test was negative. The figures below provide a summary and supplement the analysis; Figure 1 describes the purpose of the test, the results and its implications, while Figure 2 outlines the steps taken to correctly identify the organism once known as Unknown 3.
Test Purpose Reagents/Media Observation Results Interpretation
Gram Stain Cell wall composition, morphology Crystal violet, Gram’s iodine, Gram’s Decolorizer, Safranin Counterstain Purple circles, clustered in tetrads + Gram positive cocci
Methyl Red Detect mixed acid fermentation MR-VP broth, Methyl Red reagent No change in color - Non-fermenter
Urea hydrolysis Detects enzyme urease Urea broth Yellow color - Does not metabolize urea
Unknown Organism #3
Gram (+) Rod Gram (-) Rod Gram (+) Cocci
S. aureus S. epididermidis
M. luteus E. faecalis