Gram-Positive and Gram-Negative Cocci of Medical Importance

Chapter 18: Gram-Positive and Gram-Negative Cocci of Medical Importance
Staphylococci: General Characteristics

  • Common Gram-positive cocci, forming irregular clusters, found on skin and mucous membranes.

  • Key species: Staphylococcus aureus, Staphylococcus epidermis, Staphylococcus saprophyticus.

Staphylococcus aureus

  • A normal flora inhabitant of the nose and skin (found in ≈1/3\approx 1/3 of people).

  • A resilient mesophile (37extoC37 ext{oC} optimum) and facultative anaerobe, tolerant to high salt, extreme pH, and high temperatures.

  • Produces numerous virulence factors.

Virulence Factors of S. aureus

Enzymes

  • Coagulase: Coagulates plasma/blood (diagnostic for ≈97%\approx 97\% of human isolates).

  • DNase: Digests DNA.

  • Penicillinase: Inactivates penicillin.

Toxins

  • Hemolysins (α, β, γ,δ\alpha,\ \beta,\ \gamma, \delta): Lyse red blood cells.

  • Leukocidin: Lyse white blood cells.

  • Enterotoxin: Causes food poisoning.

  • Toxic Shock Syndrome Toxin (TSST): A superantigen causing systemic symptoms and organ damage.

  • Exfoliatin Toxin: Causes skin blistering and peeling.

Blood Agar Plate (BAP) and Hemolytic Activity

  • BAP is a differential medium with 5%5\% sheep blood to observe bacterial interaction with red blood cells via hemolysins.

Types of Hemolytic Activity Observed on BAP

  • Alpha (α\alpha) Hemolytic: Incomplete hemolysis with a greenish tint.

  • Beta (β\beta) Hemolytic: Complete hemolysis with a clear zone (e.g., S. aureus, Streptococcus pyogenes).

  • Gamma (γ\gamma) Hemolytic: No hemolysis.

Pathogenicity of S. aureus: Toxigenic Diseases

  • Food Poisoning: Ingestion of heat-stable enterotoxins.

  • Staphylococcal Scalded Skin Syndrome (SSSS): Caused by exfoliatin toxin, leading to skin blistering and desquamation.

  • Toxic Shock Syndrome (TSS): Systemic toxemia from TSST release, causing shock and multi-organ failure (up to ≈50%\approx 50\% associated with tampon use).

Pathogenicity of Methicillin-Resistant Staphylococcus aureus (MRSA)

  • MRSA: Multi-drug resistant S. aureus strain, typically skin/wound infections. Carried by ≈2/100\approx 2/100 people.

    • Hospital-acquired (nosocomial) MRSA: More aggressive, can cause bacteremia leading to endocarditis, necrotizing pneumonia, and surgical site infections.

  • Vancomycin-resistant S. aureus (VRSA): Another significant multi-drug resistant strain.

Antibiotic Resistance: Influences and Mechanisms

  • Factors influencing resistance: Poor patient compliance, over-prescribing, improper disposal.

  • Molecular mechanisms:

    • Conjugation: Gene transfer via plasmids.

    • Transformation: Uptake of free DNA from the environment.