Staphylococcus Aureus

Summary

This video provides a comprehensive overview of *Staphylococcus aureus*, highlighting its defining characteristics, such as its gram-positive, catalase-positive, coagulase-positive nature, and its clustering behavior. It explores key virulence factors, including Protein A, penicillin-binding proteins (notably in MRSA), TSST-1 (toxic shock syndrome toxin), enterotoxin B (cause of food poisoning), and exfoliative toxin (responsible for scalded skin syndrome). The video also covers diagnostic tips for distinguishing *S. aureus* from other staphylococci and discusses treatment options, with emphasis on managing MRSA infections. A helpful mnemonic and clinical correlations for exam prep are included throughout.

Takeaways

• 😀 Staphylococcus aureus is a Gram-positive, catalase-positive, coagulase-positive, and beta-hemolytic bacterium that occurs in clusters.
• 😀 The term 'coccus' refers to the round shape of the bacterium, and 'clusters' means that Staphylococcus aureus forms groups of these cocci when observed under the microscope.
• 😀 A useful mnemonic to remember the characteristics of Staphylococcus aureus is associating it with a 'positive aura' since everything about it is positive (Gram-positive, catalase-positive, coagulase-positive).
• 😀 The key difference between Staphylococcus aureus and Staphylococcus epidermidis is that Staph aureus is coagulase-positive, while Staph epidermidis is coagulase-negative.
• 😀 Virulence factors of Staphylococcus aureus include: protein A (prevents complement activation and phagocytosis), altered penicillin-binding proteins (which confer methicillin resistance), super antigen TSST-1 (causes toxic shock syndrome), enterotoxin B (causes food poisoning), and exfoliative toxin (causes scalded skin syndrome).
• 😀 Protein A binds to the Fc portion of immunoglobulin G, preventing the immune system from recognizing and destroying the bacteria.
• 😀 Methicillin-resistant Staphylococcus aureus (MRSA) has altered penicillin-binding proteins, making it resistant to beta-lactam antibiotics such as methicillin.
• 😀 Toxic shock syndrome (TSS) is caused by the super antigen TSST-1, which over-stimulates T cells and induces massive cytokine release, leading to fever, shock, and organ failure.
• 😀 Enterotoxin B from Staphylococcus aureus is heat-stable and causes food poisoning within 2-6 hours of ingestion, regardless of cooking temperature.
• 😀 Exfoliative toxin from Staphylococcus aureus causes scalded skin syndrome by cleaving the inter-epidermal cell connections, leading to skin sloughing, particularly in children.

Q & A

What is the gram stain result for Staphylococcus aureus?

Staphylococcus aureus is gram-positive.

What are the defining biochemical characteristics of Staphylococcus aureus?

Staphylococcus aureus is catalase positive, coagulase positive, beta-hemolytic, and occurs in clusters.

What is the mnemonic used to remember the defining characteristics of Staphylococcus aureus?

The mnemonic is 'aura'—Staphylococcus aureus equals aura. The 'positive aura' helps to remember that it is gram-positive, catalase positive, coagulase positive, and beta-hemolytic.

How does Staphylococcus aureus differ from Staphylococcus epidermidis?

The major difference is that Staphylococcus aureus is coagulase positive, while Staphylococcus epidermidis is coagulase negative.

What is the role of Protein A in the virulence of Staphylococcus aureus?

Protein A binds to the Fc portion of immunoglobulin G, preventing complement activation and phagocytosis, allowing Staphylococcus aureus to evade immune clearance.

What does the alteration of penicillin-binding proteins in MRSA lead to?

In MRSA (Methicillin-resistant Staphylococcus aureus), the alteration of penicillin-binding proteins makes it resistant to methicillin, allowing the bacteria to maintain its structural integrity despite antibiotic treatment.

What is toxic shock syndrome toxin-1 (TSST-1), and how does it cause toxic shock syndrome?

TSST-1 is a superantigen that binds to MHC class II molecules and T-cell receptors, causing an uncontrolled release of cytokines like interleukin-2 and TNF-alpha, leading to severe inflammation, high fever, and multi-organ failure.

Which clinical conditions are most associated with toxic shock syndrome caused by TSST-1?

Toxic shock syndrome is most commonly associated with tampon use and nasal packing, both of which can provide a conducive environment for the growth of Staphylococcus aureus and production of TSST-1.

What does enterotoxin B do, and how is it related to food poisoning?

Enterotoxin B is a heat-stable toxin produced by Staphylococcus aureus that causes food poisoning. It remains active even if the food is cooked, and symptoms typically appear within 2 to 6 hours after ingestion.

What is exfoliative toxin, and what disease does it cause?

Exfoliative toxin is a protease that cleaves inter-epidermal cell junctions, leading to skin sloughing and causing Staphylococcal Scalded Skin Syndrome (SSSS), typically affecting young children.