Dichotomous key for common skinbacteria:
I. Cells arranged in tetrads, glucose not fermented(Micrococcus)
A. Colonies have yellow pigment M. luteus
B. Colonies have red or pink pigment M. roseus
II. Cells arranged in clusters, glucose is fermented(Staphylococcus)
A. Acid produced from mannitol
1. Coagulase-positive S. aureus
2. Coagulase-negative
a. Acid produced from trehalose S. saprophyticus
b. No acid from trehalose S. capitis
B. Acid not produced from mannitol
1. Acid produced from trehalose S. saprophyticus
2. No acid from trehalose S. epidermidis
Elsa completes the in-class labexperiment on skin bacteria by inoculating a phenol red glucosetube and a mannitol salt agar plate with her skin sample. After 24hours of incubation at 37°C, she performs a Gram stain and phenolred trehalose test on a colony of bacteria from her MSA plate.(Note: the PR trehalose test is similar to the PR glucose test,only the carbohydrate is trehalose instead of glucose).
Elsa’s lab partner, Anna, completes asimilar experiment, but with slightly different results. Herinitial results point her to a different direction, so instead ofcompleting the PR trehalose test, she completes the coagulase slidetest
Anna’s Skin Lab Results | Interpretations: |
Phenol red glucose | Yellow after incubation |
MSA plate | Growth, yellow agar |
Gram stain | Gram-positive, spherical cells arranged in clusters |
Coagulase test (slide) | Cells clump |
- Use the dichotomous key on page 41 again to interpret Anna’sresults. What is the name of the organism growing on her skin?
- Is it possible that one or both students have MRSA on theirskin? Explain.
