Staphylococci

A microbiologist is identifying species of Staphylococcus based on basic lab characteristics.

• Novobiocin Sensitive

• S. epidermidis

• Novobiocin resistant

• S. aureus

• Produces golden yellow colonies on BA

• S. saprophyticus

A clinician is trying to identify causative agents based on clinical presentation.

• • S. saprophyticus

• Catheter-associated infections/Infected pacemaker

• Staphylococcus aureus

• UTI in young females

• S. epidermidis

• Scalded skin syndrome

Match the virulence factor with its function.

• Inactivates IgG Fc region

• Coagulase

• Lyses red blood cells

• TSST-1

• Forms fibrin clot

• Protein A

• Causes skin desquamation

• Exfoliative toxin

• Superantigen leading to toxic shock

• Hemolysins

Answer explanation

Virulence Factors of S. aureus – Answer Key with Explanations

Protein A→ 2. Inactivates IgG Fc regionProtein A binds to the Fc portion of IgG antibodies, preventing opsonization and phagocytosis, allowing the bacteria to evade immune response

.B. Coagulase→ 4. Forms fibrin clotCoagulase converts fibrinogen to fibrin, leading to clot formation around the bacteria, helping it to localize and avoid phagocytosis

.C. Hemolysins→ 5. Lyses red blood cells Hemolysins (especially alpha-toxin) create pores in host cell membranes, causing lysis of RBCs and tissue damage.

D. Exfoliative toxin→ 3. Causes skin desquamationThis toxin targets desmoglein-1 in the epidermis, leading to separation of skin layers and causing Staphylococcal Scalded Skin Syndrome (SSSS).

E. TSST-1 (Toxic Shock Syndrome Toxin-1)→ 1. Superantigen leading to toxic shockTSST-1 is a superantigen that causes massive, non-specific T-cell activation and cytokine release, leading to systemic toxicity and shock.

Match the test with the expected result for S. aureus.

• Positive – bubble formation

• Coagulase test

• Gram-positive cocci in clusters

• Mannitol salt agar

• Beta hemolysis

• Gram stain

• Fermentation with yellow colonies

• Blood agar

• Positive – clot formation

• Catalase test

Answer explanation

1. Protein A

Protein A is a surface protein found in the cell wall of S. aureus. It binds to the Fc region of immunoglobulin G (IgG), thereby preventing opsonization and subsequent phagocytosis by neutrophils. This immune evasion strategy allows the bacterium to survive and replicate within host tissues.

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2. Coagulase

Coagulase is an enzyme that converts fibrinogen to fibrin, resulting in the formation of a clot around the bacterial cells. This fibrin layer acts as a protective barrier, shielding the bacteria from immune cells and allowing it to localize and persist within host tissues, such as in abscesses.

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3. Hemolysins

Hemolysins, especially alpha-toxin, are cytolytic toxins that form pores in the membranes of host cells, including red blood cells. This leads to cell lysis, tissue destruction, and contributes to the spread of infection. Hemolysins also damage immune cells, aiding in immune evasion.

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4. Exfoliative Toxin

Exfoliative toxin targets and cleaves desmoglein-1, a protein crucial for maintaining intercellular adhesion in the superficial epidermis. As a result, the skin layers separate, leading to desquamation. This is the underlying mechanism in Staphylococcal Scalded Skin Syndrome (SSSS), commonly seen in infants and young children.

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5. Toxic Shock Syndrome Toxin-1 (TSST-1)

TSST-1 is a superantigen that bypasses normal antigen presentation by directly linking MHC class II molecules with T-cell receptors. This non-specific T-cell activation causes a massive release of pro-inflammatory cytokines ("cytokine storm"), leading to the systemic effects of toxic shock syndrome—high fever, hypotension, rash, and multiorgan failure.

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Match toxins produced by S. aureus with clinical manifestations

• Toxic shock syndrome

• Beta toxinAlpha-toxin

• uses necrosis of WBCs and tissue

• Enterotoxin

• Desquamation in scalded skin syndrome

• TSST-1

• Hemolysis and tissue damage

• Exfoliative toxin

• Rapid-onset vomiting and diarrhea

• Panton-Valentine leukocidin

Answer explanation

A. Enterotoxin → Rapid-onset vomiting and diarrhea

Staphylococcal enterotoxins (especially types A and B) are heat-stable exotoxins that act as superantigens in the gastrointestinal tract. They stimulate vagus nerve endings in the stomach lining and trigger the vomiting center in the brain. This leads to sudden onset of nausea, vomiting, abdominal cramps, and sometimes diarrhea within 1–6 hours after ingestion of contaminated food. Notably, symptoms occur without fever or inflammation, as the toxin acts directly and is preformed in food (not requiring bacterial growth in the body).

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B. Exfoliative Toxin → Desquamation in Scalded Skin Syndrome

Exfoliative toxin (also called epidermolytic toxin) specifically targets desmoglein-1, a protein in desmosomes responsible for skin cell adhesion. Its cleavage results in separation of the superficial epidermis, leading to widespread peeling of the skin without significant inflammation. This manifests clinically as Staphylococcal Scalded Skin Syndrome (SSSS), predominantly seen in neonates and young children, and characterized by flaccid bullae and a positive Nikolsky sign.

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C. TSST-1 (Toxic Shock Syndrome Toxin-1) → Toxic Shock Syndrome

TSST-1 is a potent superantigen that activates a massive number of T-cells by binding non-specifically to MHC class II and T-cell receptors. This results in an excessive release of cytokines (cytokine storm), leading to the classic symptoms of toxic shock syndrome, including high fever, hypotension, diffuse rash (often with skin peeling), vomiting, diarrhea, and multiorgan dysfunction. It is classically associated with prolonged tampon use, wound infections, or surgical sites colonized with S. aureus.

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D. Panton-Valentine Leukocidin (PVL) → Causes Necrosis of WBCs and Tissue

PVL is a pore-forming cytotoxin that targets neutrophils and monocytes, leading to their lysis and contributing to tissue necrosis. It is strongly associated with severe skin and soft tissue infections, particularly in community-acquired MRSA (CA-MRSA) strains. PVL is a key virulence factor in conditions like necrotizing pneumonia, furuncles, and abscesses, especially in otherwise healthy individuals.

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E. Alpha-Toxin → Hemolysis and Tissue Damage

Alpha-toxin (α-toxin), also known as alpha-hemolysin, is a pore-forming exotoxin that causes lysis of red blood cells and other host cells, including epithelial and endothelial cells. Its effects lead to tissue necrosis, vascular leakage, and inflammation, contributing to the severity of infections such as pneumonia, sepsis, and deep tissue abscesses.