(2) Bacteria of Skin & Throat

What is microbial antagonism?

The ability of normal microbiota to inhibit the growth of pathogens by competing for nutrients and space and producing inhibitory substances.

Why are normal microbiota important?

They help protect the body from colonization by harmful microorganisms.

Which organism is Gram-positive, catalase-positive, and coagulase-positive?

Staphylococcus aureus.

Which organisms are Gram-positive cocci found on the skin?

Staphylococcus spp., Micrococcus, and Staphylococcus aureus.

What is the purpose of using sterile saline on the swab?

To moisten the swab and improve collection of microorganisms from the skin.

Why is only half of the Mannitol Salt Agar plate inoculated?

To allow proper streaking for isolation of individual colonies.

What does "streak for isolation" mean?

To spread bacteria across the agar surface so individual colonies can develop.

Why are isolated colonies important?

They allow accurate identification and biochemical testing of a single organism.

What is observed after incubation of the Mannitol Salt Agar plate?

Colony growth and whether mannitol fermentation occurred.

What should you look for when observing colonies on Mannitol Salt Agar?

Large opaque colonies and evidence of mannitol fermentation.

Why is glucose fermentation tested after isolation?

To aid in identification of the bacterial isolate.

What characteristic of Staphylococcus aureus is confirmed with the coagulase test?

Its ability to clot plasma by producing coagulase.

How quickly should the coagulase test be interpreted?

Within 1–2 minutes.

What indicates a positive coagulase test?

Visible coagulation or clot formation.

What is the purpose of Blood Agar?

To differentiate bacteria based on their ability to lyse red blood cells.

Which type of hemolysis completely destroys red blood cells?

Beta (β) hemolysis.

Which type of hemolysis partially destroys red blood cells?

Alpha (α) hemolysis.

Which type of hemolysis causes no destruction of red blood cells?

Gamma (γ) hemolysis.

What appearance does beta hemolysis produce on Blood Agar?

A clear, transparent zone surrounding the colony.

What appearance does alpha hemolysis produce on Blood Agar?

A green or brown discoloration around the colony.

What appearance does gamma hemolysis produce on Blood Agar?

No change in the agar surrounding the colony.

Which organism shown in the slides is associated with beta hemolysis?

Streptococcus pyogenes.

What are the normal microbiota of the throat?

Staphylococcus, Neisseria, Haemophilus, and Streptococcus.

What should be determined when observing the Blood Agar plate?

The type of hemolysis present.

Why is hemolysis important in bacterial identification?

Different bacteria produce characteristic patterns of red blood cell destruction.

Which enzyme does catalase break down?

Hydrogen peroxide (H₂O₂).

What are the products of the catalase reaction?

Water and oxygen gas.

What causes bubbles in a catalase test?

Release of oxygen gas.

Which bacterial genus is commonly differentiated from Staphylococcus using the catalase test?

Streptococcus.

What are the key identification tests for skin bacteria in this lab?

Gram stain, catalase test, glucose fermentation, and coagulase test.

What media are used in the Skin & Throat lab?

Mannitol Salt Agar and Blood Agar.

Which medium is selective and differential?

Mannitol Salt Agar.

Which medium is differential for hemolysis?

Blood Agar.

Which organism is specifically identified using the coagulase test?

Staphylococcus aureus.

What is the first step in identifying bacteria grown from the skin?

Observe colony morphology on Mannitol Salt Agar.

What is the second step in identifying bacteria from the skin?

Perform a Gram stain.

What is the third step in identifying bacteria from the skin?

Perform a catalase test.

What is the fourth step in identifying bacteria from the skin?

Inoculate a glucose fermentation tube and perform a coagulase test during the next lab.