Staphylococcal Disease

Diagnosis

Indications for Testing

  • Pustular disease; abscess, cellulitis
  • Bacteremia associated with deep-seated staphylococcal infection may present a puzzling picture since the focus of infection may not be readily apparent

Laboratory Testing

  • Culture
    • Superficial disease – culture of wound or affected area; recommend incision and drainage of all abscesses for wound culture
      • Further testing by PCR for mecA to rule out resistant staphylococci
    • Deep-seated infections – gram stain and culture of the organism from appropriate clinical specimens
  • Serum antibody testing
    • Prolonged exposure to staphylococci during deep-seated infection can result in elevated antibodies to staphylococcal ribitol teichoic acid (RTA)
      • High antibody levels (≥1:4) against staphylococcal RTA in patients with staphylococcal bacteremia may indicate staphylococcal endocarditis or complicated staphylococcal bacteremia

Differential Diagnosis

Clinical Background

Staphylococcus aureus bacteria are gram-positive cocci that cause diseases of varying severity, from skin and soft-tissue infections (eg, folliculitis, impetigo, cellulitis, abscesses) to deep-seated infections (eg, endocarditis, osteomyelitis).

Epidemiology

  • Incidence – 28-35/100,000
  • Transmission
    • Portal of entry for superficial infections is frequently unknown but may be from a surface abrasion 
    • Portal of entry for deep infections includes surgical sites or indwelling catheters

Organism

  • Gram-positive cocci; coagulase positive
  • May acquire drug resistance, including resistance to nearly all beta-lactam agents 
    • Known as methicillin-resistant Staphylococcus aureus (MRSA)

Risk Factors

  • Methicillin-sensitive Staphylococcus
  • MRSA
    • Antibiotics in past month (odds ratio 2.4)
    • Abscess (odds ratio 1.8)
    • Reported spider bite (odds ratio 2.8)
    • History of MRSA (odds ratio 3.3)
    • Close contact with MRSA patient (odds ratio 3.4)
    • Snorting or smoking illegal drugs (odds ratio 2.9)
    • Incarceration ≤12 months (odds ratio 2.8)

Clinical Presentation

  • Superficial skin infections
    • Folliculitis
    • Impetigo
    • Cellulitis (more commonly caused by beta-hemolytic Streptococcus)
      • Infection penetrates the subcutaneous tissue layer
        • Erythema and warmth; swelling and tenderness of affected area
    • Staphylococcal scalded skin syndrome
  • Soft-tissue infections
    • Abscess
      • >50% are caused by Staphylococcus
      • Includes furuncles and carbuncles
      • Area of fluctuance and erythema demarcate the abscess
    • Necrotizing fasciitis
      • Type 1 frequently caused by Staphylococcus
      • Rapid progression of local symptoms, including systemic symptoms (eg, fever)
        • Pain out of proportion to apparent involvement
        • Marked edema
  • Deep infections
    • Endocarditis – associated with native and prosthetic valves, indwelling catheters, or IVDU
    • Osteomyelitis – frequently associated with underlying immune deficiency
    • Deep tissue and muscle abscesses
    • Septic arthritis – associated with implanted joints, rheumatoid arthritis, immunocompromised state
  • Other infections

Treatment

  • Superficial – frequently cured with topical or systemic antibiotic therapy and abscess drainage
  • Deep – involves tissue debridement/abscess drainage and systemic antibiotic therapy

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Staphylococcus Surveillance Culture 0060124
Method: Culture/Identification

Identify staphylococcal colonization

 

Decolonization has variable success in preventing subsequent infections

Blood Culture 0060102
Method: Continuous Monitoring Blood Culture/Identification

Evaluate for organism as etiology of infected wound

Blood samples from two different anatomic sites required

Testing is limited to the University of Utah Health Sciences Center only

 
Wound Culture and Gram Stain 0060132
Method: Stain/Culture/Identification

Rule out systemic or deep-seated staphylococcal infection

   
Antimicrobial Susceptibility - Staphylococcus 0060707
Method: Automated Broth Microdilution

Identify antimicrobial resistance

The following agents are tested – clindamycin, daptomycin, erythromycin, gentamicin, levofloxacin, linezolid, nitrofurantoin, oxacillin, penicillin, quinupristin/dalfopristin, rifampin, tetracycline, trimethoprim/sulfamethoxazole, and vancomycin

For serious infections with coagulase-negative staphylococci, testing for the presence of mecA may be appropriate and will be performed in order to interpret the results for β-lactam agents

Selective reporting based on source

   
Antimicrobial Susceptibility - mecA Gene by PCR 0060211
Method: Qualitative Polymerase Chain Reaction

Gold standard for detecting oxacillin/nafcillin resistance in Staphylococcus aureus 

Determines presence of gene that confers resistance to nearly all beta-lactam antibiotics

Available as a stand-alone test, but typically included with ARUP's full susceptibility panel on staphylococci

Negative mecA PCR does not exclude oxacillin resistance

 
Teichoic Acid Antibodies 0050775
Method: Semi-Quantitative Immunodiffusion

Screen for antibodies to staphylococcal RTA

Negative result does not exclude staphylococcal disease

Positive antibody assay does not denote protection from subsequent infection