Microbiology Lab Exam 1: Staphylococcus aureus Overview

Microbiology Lab Exam 1: Staphylococcus aureus (S. aureus)

Basic Description Features

  • Shape: Cocci (round bacteria)
  • Arrangement: Clusters resembling grapes
  • Gram Stain: Gram-positive, which indicates a purple stain.
  • Oxygen Use: Facultative anaerobe, able to live with or without oxygen.
  • Motility: Non-motile, indicating no flagella.
  • Spores: Does not form spores.
  • Catalase Test: Positive; produces bubbles when hydrogen peroxide is applied.
  • Coagulase Test: Positive, distinguishing it from other staphylococci.
    • Mnemonic: “Staphy loves clusters and coagulase.”

Appearance in Culture

  • Colony Characteristics on Agar Plates:
    • Golden-yellow in color ("aureus" translates to "golden" in Latin).
    • Colonies appear smooth, round, and shiny, with golden pigment aiding in surviving oxidative stress.

Natural Habitat (Normal Flora)

  • Common Locations:
    • Skin
    • Nasal passages
    • Throat
    • Perineum
  • Pathogenicity: Typically harmless on the surface but can become pathogenic if it enters tissue or the bloodstream through injuries like cuts, catheter insertion, or surgical incisions.

Transmission Methods

  • Direct Contact: Contaminated hands, wounds, personal items (e.g., towels).
  • Indirect Contact: Contaminated surfaces or instruments.
  • Autoinfection: From one’s own nasal flora entering a wound.
  • Risk Areas: Hospitals are significant sites for “nosocomial infections.”

Diseases Caused by S. aureus

A. Skin & Soft Tissue Infections
  • Abscesses/Boils: Known as furuncles.
  • Carbuncles: Multiple interconnected abscesses.
  • Cellulitis: Infection of skin and subcutaneous tissue.
  • Impetigo: Characterized by honey-colored crusted lesions, especially common in children.
  • Wound Infections: Infections following surgical or traumatic injuries.
  • Folliculitis: Infection of hair follicles.
  • Key Fact: S. aureus releases various enzymes and toxins that damage tissue leading to pus formation (a pyogenic infection).
B. Toxin-Mediated Diseases
  • Food Poisoning:
    • Toxin Involved: Enterotoxin (heat-stable).
    • Symptoms: Rapid onset vomiting, nausea, diarrhea occurring within 2–6 hours after consumption of contaminated food (e.g., custard, mayonnaise, potato salad).
  • Toxic Shock Syndrome (TSS):
    • Toxin Involved: TSST-1 toxin.
    • Symptoms: Sudden fever, hypotension, rash resembling sunburn, multiorgan failure, commonly linked to tampon use or wound infections.
  • Scalded Skin Syndrome (Ritter’s disease):
    • Toxin Involved: Exfoliative toxin.
C. Serious Deep or Systemic Infections
  • Osteomyelitis: Infection of bone tissue.
  • Endocarditis: Infection of heart valves, particularly in IV drug users.
  • Pneumonia: Often occurs following influenza.
  • Bacteremia/Sepsis: The presence of bacteria in the blood leading to severe systemic infections.
  • Meningitis: Inflammation of the protective membranes covering the brain.
  • Post-surgical wound infections: Complications arising from surgical procedures.
  • Empyema: Presence of pus in the pleural cavity.

Laboratory Identification

  • Gram Stain: Shows purple cocci in grape-like clusters.
  • Tests for Confirmation:
    1. Catalase Test: Positive; bubbles indicate Staphylococcus, not Streptococcus.
    2. Coagulase Test: Positive for S. aureus only.
    3. Mannitol Salt Agar (MSA): S. aureus can grow in high salt concentrations and ferments mannitol, turning the agar yellow through acid production; other staphylococci do not ferment mannitol, thus the agar remains pink.

Antibiotic Resistance

  • Methicillin-sensitive S. aureus (MSSA):
    • Treatment options include oxacillin, nafcillin, and cephalexin.
  • Methicillin-resistant S. aureus (MRSA):
    • Resistant to all β-lactam antibiotics (e.g., penicillin, methicillin).
    • Treated with alternative antibiotics such as vancomycin, linezolid, or daptomycin.
    • Notable for hospital and community outbreaks, especially occurring among healthcare workers, athletes, and post-surgical patients.

Nursing & Infection Control

  • Precautions:
    • Use of gowns and gloves.
    • Rigorous hand hygiene practices — utilizing both alcohol-based sanitizers and soap.
    • Thoroughly clean medical equipment between patient use.
    • Screen carriers using nasal swabs.
    • Mupirocin ointment may be used for nasal decolonization.

Treatment Summary

  • Type of Infection & Drug of Choice:
    • MSSA: Nafcillin, oxacillin, cephalexin.
    • MRSA: Vancomycin, linezolid, TMP-SMX (Bactrim).
    • Skin Infections: Incision and drainage combined with antibiotics.
    • Toxin Diseases: Supportive care and removal of source (e.g., tampons, wound packing).

Microscopy / Culture Appearance

  • Gram Stain Feature: Purple cocci in clusters resembling grapes.
  • Colony Color: Golden-yellow.
  • Agar Test: MSA yields a yellow zone upon fermentation.
  • Hemolysis: Beta-hemolytic, producing a clear zone on blood agar due to hemolysin toxin.

Important Toxins & Enzymes

  • Coagulase:
    • Function: Clots plasma; aids in evading the immune response.
  • Hemolysins:
    • Function: Destroys red blood cells.
  • Leukocidin:
    • Function: Destroys white blood cells.
  • Exfoliative Toxin:
    • Cause: Induces skin peeling (affects scalded skin syndrome).
  • Enterotoxin:
    • Cause: Responsible for food poisoning symptoms.
  • TSST-1:
    • Triggers toxic shock syndrome.

NCLEX-Style Quick Summary

  • Identifying Features:
    • Gram-positive cocci in clusters indicating S. aureus.
    • Catalase positive, coagulase positive, and beta hemolysis signifies the diagnostic triad.
  • Common Infections Causation:
    • Known for causing wound infections and abscesses.
    • MRSA necessitates contact isolation due to risk of infection spread.
  • Associated Diseases:
    • Food poisoning, toxic shock syndrome, scalded skin syndrome.
  • Characteristic Pigment:
    • Produces a golden pigment, and exhibits mannitol fermentation yielding a yellow agar color.
  • Mnemonic for Memorization:
    • “S. aureus = S. for Skin, S. for Shock, S. for Scalded, S. for Stomach.” (Refers to skin infections, toxic shock, scalded skin, and food poisoning).