(1) Bacteria of Skin & Throat

What are the normal microbiota of the skin?

Propionibacterium, Staphylococcus spp., Micrococcus, and Staphylococcus aureus (coagulase positive).

Which skin bacterium is coagulase positive?

Staphylococcus aureus.

What is the Gram reaction of Staphylococcus species?

Gram-positive.

What is the cell morphology of Staphylococcus species?

Cocci (spherical cells) that typically occur in clusters.

Which skin bacteria are catalase positive?

Staphylococcus spp. and Micrococcus.

Which bacterial genus is catalase negative?

Streptococcus.

What is the purpose of the catalase test?

To determine whether a bacterium produces the enzyme catalase by adding hydrogen peroxide.

What indicates a positive catalase test?

Immediate bubbling after hydrogen peroxide is added.

What does a negative catalase test look like?

No bubbles form.

What enzyme breaks down hydrogen peroxide into water and oxygen?

Catalase.

What is the purpose of Mannitol Salt Agar (MSA)?

It is selective for salt-tolerant bacteria and differential for mannitol fermentation.

Why is Mannitol Salt Agar selective?

It contains a high concentration of NaCl that inhibits most bacteria except salt-tolerant organisms such as Staphylococcus.

Why is Mannitol Salt Agar differential?

It contains mannitol and a pH indicator that differentiates bacteria that ferment mannitol from those that do not.

What does growth on Mannitol Salt Agar indicate?

The organism is salt tolerant.

What does a yellow Mannitol Salt Agar plate indicate?

The organism fermented mannitol and produced acid, causing the medium to turn yellow.

What does a red or pink Mannitol Salt Agar plate indicate?

The organism grew but did not ferment mannitol.

Which Staphylococcus species commonly ferments mannitol?

Staphylococcus aureus.

What specimen was collected during the skin microbiology lab?

A skin swab collected with a sterile saline-moistened cotton swab.

What medium was inoculated with the skin specimen?

Mannitol Salt Agar.

Why is the plate streaked for isolation?

To obtain isolated colonies for identification.

What colony is selected during the following lab?

A catalase-positive, Gram-positive coccus.

What test is performed after observing colonies on Mannitol Salt Agar?

A Gram stain.

Why is a Gram stain performed?

To determine the Gram reaction and morphology of the isolate.

After the Gram stain, what biochemical test is performed?

The catalase test.

What additional tube is inoculated after the catalase test?

A glucose fermentation tube.

Which test is performed during the next lab to identify Staphylococcus aureus?

The coagulase test.

What is the purpose of the coagulase test?

To determine whether the organism produces the enzyme coagulase.

What does a positive coagulase test indicate?

The organism is Staphylococcus aureus.

What does a negative coagulase test indicate?

The organism is likely not Staphylococcus aureus.

What are the normal microbiota of the throat?

Staphylococcus, Neisseria, Haemophilus, and Streptococcus.

Which throat organism shown in the slides is beta-hemolytic?

Streptococcus pyogenes.

What medium is used to observe hemolysis?

Blood agar.

What is alpha (α) hemolysis?

Partial destruction of red blood cells that produces a greenish discoloration around colonies.

What is beta (β) hemolysis?

Complete destruction of red blood cells, producing a clear zone around colonies.

What is gamma (γ) hemolysis?

No destruction of red blood cells and no change in the medium.

Which type of hemolysis is produced by Streptococcus pyogenes?

Beta (β) hemolysis.

What should be observed on the blood agar plate in the throat lab?

The type of hemolysis produced by bacterial growth.

What does complete clearing around a colony on blood agar indicate?

Beta hemolysis.

What does a green zone around a colony on blood agar indicate?

Alpha hemolysis.

What does no change around a colony on blood agar indicate?

Gamma hemolysis.