Lab 15 - Isolation and Identification of Staphylococcus

What are the basic morphological characteristics of Staphylococcus?

Staphylococcus are Gram-positive cocci that appear in irregular, grape-like clusters and are about 0.5–1.0 µm in diameter.

What is the catalase result for Staphylococcus?

All Staphylococcus species are catalase-positive and produce bubbles when exposed to hydrogen peroxide.

How does Staphylococcus respond to high salt?

They tolerate high NaCl concentrations (7.5–10%), allowing growth on selective media.

Where are Staphylococcus species commonly found?

They are found on the skin and in the nasal cavity and other mucous membranes.

What is the role of the capsule in Staphylococcus?

The capsule helps bacteria adhere to surfaces and colonize the host.

Name 5 clinically important species.

S. aureus, S. epidermidis, S. saprophyticus, S. haemolyticus, S. hominis.

Which Staphylococcus species is coagulase-positive?

Staphylococcus aureus is the only coagulase-positive species.

What does coagulase do?

It converts fibrinogen into fibrin, causing plasma to clot.

What is a positive coagulase test result?

Plasma gels or coagulates.

What is a negative coagulase test result?

Plasma remains liquid.

Where is S. aureus commonly found?

In the nasal passages, throat, axillae, and groin.

Why is S. aureus important?

It is the most pathogenic Staphylococcus species.

What infections does S. aureus cause?

Skin infections, abscesses, osteomyelitis, endocarditis, food poisoning, and toxic shock syndrome.

Where are coagulase-negative species found?

As normal microbiota of the skin.

What is S. saprophyticus known for?

Causing urinary tract infections, especially in young women.

What is S. epidermidis associated with?

Infections involving medical devices like prosthetics and catheters.

What colony morphology does Staphylococcus show on blood agar?

Round, raised, opaque colonies (1–2 mm).

What types of hemolysis can occur?

Some species show beta hemolysis; others show none.

What does a zone of inhibition mean with antibiotics?

It indicates the organism is sensitive.

What type of medium is MSA?

It is both selective and differential.

What makes MSA selective?

High salt concentration (NaCl).

What makes MSA differential?

Mannitol fermentation and phenol red indicator.

What color is uninoculated MSA?

Pink/red.

What does a yellow color on MSA indicate?

Mannitol fermentation producing acid.

Which species ferment mannitol?

S. aureus and S. saprophyticus.

Which species does NOT ferment mannitol?

S. epidermidis.

What does a zone of inhibition indicate in a novobiocin test?

The organism is sensitive.

What does no zone indicate?

The organism is resistant.

What does the Staphyloslide latex test detect?

Cell-bound coagulase.

What indicates a positive result in the latex test?

Clumping (agglutination) of latex beads.

What indicates a negative result in the latex test?

No clumping.

How is the catalase test performed?

Add hydrogen peroxide to bacteria on a slide.

What indicates a positive catalase test?

Bubble formation (oxygen release).

What is the substrate for catalase?

Hydrogen peroxide (H₂O₂).