Question

Dichotomous key for common skin bacteria:
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 lab experiment on skin bacteria by inoculating a phenol red glucose tube and a mannitol salt agar plate with her skin sample. After 24 hours of incubation at 37°C, she performs a Gram stain and phenol red 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 a similar experiment, but with slightly different results. Her initial results point her to a different direction, so instead of completing the PR trehalose test, she completes the coagulase slide test

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’s results. What is the name of the organism growing on her skin?

4. Is it possible that one or both students have MRSA on their skin? Explain.

Answer 1

Answer-

According to the given question-

Anna’s Skin Lab Results	Interpretations:	Interpretation
Phenol red glucose	Yellow after incubation	Which means that the bacteria is capable of fermenting the glucose and produces acidic by product in the medium which decreases the pH of medium and changes the color of medium from red to yellow.
MSA plate	Growth, yellow agar	In Mannitol Salt Agar plate the bacteria is capable of ferment mannitol and produces acidic by product in the medium which decreases the pH of medium and changes the color of medium from red to yellow. this medium promote growth of Staphylococcus species which can tolerate high salt concentration of medium.
Gram stain	Gram-positive, spherical cells arranged in clusters	Staphylococcus aureus are gram positive and are arranged as cluster.
Coagulase test (slide)	Cells clump	Staphylococcus aureus produces enzyme coagulase which have the capability to converts soluble form of fibrinogen present in plasma into insoluble form of fibrin and cause agglutination.

So from Anna’s Skin Lab Results we can observe that the organism is Staphylococcus aureus which is a gram positive bacteria, arranged as cluster , capable of fermenting glucose, mannitol and gives positive result for Coagulase test.

Approximately 15 % of ordinary S. aureus strains which are Methicillin-resistant Staphylococcus aureus or MRSA give coagulase negative results so they are checked by tube coagulase test . So Anna’s bacteria are belongs to Methicillin-resistant Staphylococcus aureus.