Staphylococcus spp. - Comprehensive Overview

Staphylococcus spp. - The Cocci of Medical Importance

General Characteristics of the Staphylococci

  • Common Habitat: Normal inhabitants of skin and mucous membranes.
  • Classification:
    • Gram-positive and encapsulated.
    • Spherical cells found in irregular clusters or short chains.

Growth and Physiological Characteristics of Staphylococcus aureus

  • Colony Appearance: Large, round, opaque.
  • Aerobic Nature: Facultative anaerobe, capable of growth in the presence or absence of oxygen.
  • Environmental Tolerance:
    • High salt tolerance (7.5-10%).
    • pH range: 4.2 to 9.3.
    • Heat tolerance (up to 60°C for 60 minutes).

Enzymes of S. aureus

  • Coagulase: Causes clot formation.
  • Hyaluronidase: Breaks down hyaluronic acid in connective tissue.
  • Staphylokinase: Dissolves blood clots.
  • Nuclease (DNase): Degrades nucleic acids.
  • Lipase: Breaks down fats.
  • Penicillinase: Provides resistance to penicillin.

Toxins of S. aureus

  • Hemolysin: Lysing red blood cells.
  • Leukocidin: Kills leukocytes.
  • Enterotoxin: Causes food poisoning.
  • Exfoliative Toxin: Leads to skin peeling.
  • Toxic Shock Syndrome Toxin (TSST): Causes toxic shock syndrome.

Epidemiology and Pathogenesis of S. aureus

  • Environment: Widespread; found in human nares, skin, nasopharynx, and intestine.
  • Carriage: Common in healthy individuals.
  • Risk Factors for Infection:
    • Poor hygiene and nutrition.
    • Tissue injury and preexisting infections (e.g., diabetes, immunodeficiency).
  • Nosocomial Infections: Often acquired in medical facilities.

Clinical Staphylococcal Disease

Localized Cutaneous Infections
  • Folliculitis: Inflammation of hair follicles.
  • Hidradenitis: Inflammation of sweat glands.
  • Furuncle: Abscess from inflammation of a hair follicle or sebaceous gland.
  • Carbuncle: Larger lesions formed by merging multiple furuncles.
  • Impetigo: Bubble-like skin swellings.
Miscellaneous Systemic Infections
  • Osteomyelitis: Infection spreads from skin to bones (e.g., femur, tibia).
  • Pneumonia: Multiple lung abscesses; high mortality rate (50%).
  • Bacteremia: Can lead to endocarditis.
  • Deforming Arthritis: Infection affecting joint spaces.
  • Meningitis: Occurs in about 15% of cases.
Toxigenic Staphylococcal Disease
  • Food Intoxication: Caused by stable enterotoxin in high-salt foods.
  • Staphylococcal Scalded Skin Syndrome (SSSS): Severe skin desquamation.
  • Toxic Shock Syndrome: Severe systemic response to toxins.

Host Defenses Against S. aureus

  • Phagocytic Response: Action of neutrophils and macrophages.
  • Inflammation: Body's defensive response to infection.
  • Cell-mediated Immunity: Adaptive immune response.

Coagulase-negative Staphylococci (CNS)

  • Staphylococcus epidermidis: Associated with endocarditis and urinary tract infections.
  • Staphylococcus saprophyticus: Known for urinary tract infections.
  • Staphylococcus hominis: Found in skin areas with apocrine glands.
  • Staphylococcus capitis: Located in regions such as the scalp and face.

Identification of Staphylococcus in Clinical Samples

  • Specimen Sources: Pus, tissue exudates, sputum, urine, blood.
  • Culture Media: Blood agar and mannitol salt agar.
  • Diagnostic Tests:
    • Gram Stain: Identification of bacterial type.
    • Catalase Test: Distinguishes staphylococci from streptococci.
    • Coagulase Test: Differentiates S. aureus from other staphylococci.
    • Latex Bead Agglutination Test: Tests for surface protein (A).

Clinical Concerns in Staphylococcal Infections

  • Antibiotic Resistance:
    • High levels of penicillin resistance due to penicillinase.
    • Emergence of Methicillin-resistant Staphylococcus aureus (MRSA).
    • Types of MRSA: Hospital-acquired (HA-MRSA) and community-acquired (CA-MRSA).
  • Treatment: Effective agents include vancomycin, ceftaroline, linezolid, and daptomycin.
Treatment of Staph Infections
  • Procedures: Surgical intervention to drain abscesses.
  • Medications: Generally treated with cephalexin, sulfa drugs, tetracyclines, clindamycin. Intensive therapy required for severe cases (e.g., endocarditis, septicemia).
Prevention of Staph Infections
  • Hygiene Practices: Regular handwashing, proper wound care, and sanitation.
  • Surgical Precautions: Cleansing of wounds and incisions, isolation of infected persons, careful management of catheters and needles.