What is the clinical significance and management of Streptococcus MG (Streptococcus milleri group) infections?

Clinical Significance and Management of Streptococcus MG (Streptococcus milleri group) Infections

Streptococcus milleri group (SMG) organisms are clinically significant pathogens that have a strong propensity for abscess formation and should be treated aggressively when identified in clinical specimens, particularly blood cultures, as they often indicate underlying surgical infection requiring source control.

Classification and Characteristics

The Streptococcus milleri group (SMG), also known as the Anginosus group, consists of three distinct species:

• Streptococcus anginosus
• Streptococcus constellatus
• Streptococcus intermedius

These organisms are part of the normal flora of the human oropharynx, gastrointestinal tract, and genitourinary tract, but can cause serious infections when they invade sterile sites 1.

Clinical Presentations

SMG infections typically present as:

1. Abscess formation - A hallmark characteristic of SMG infections

   • Intra-abdominal abscesses (55% of bacteremic cases) 2
   • Thoracic empyema and lung abscesses 3
   • Brain abscesses (particularly S. intermedius)
   • Soft tissue abscesses

2. Respiratory tract infections 3

   • Pneumonia
   • Pulmonary abscess
   • Thoracic empyema
   • Acute bronchitis

3. Bacteremia - Relatively uncommon but significant

   • Annual incidence: approximately 0.93 cases per 100,000 population 2
   • Often indicates a deep-seated infection requiring surgical intervention 4

4. Other infections

   • Endocarditis
   • Septic arthritis
   • Genital and urinary tract infections 2

Risk Factors

Patients at increased risk for SMG infections include those with:

• Advanced age (most patients are elderly) 2
• Moderate to severe underlying diseases (86.7% of cases) 3
• History of esophageal or gastric surgery (26.7%) 3
• Alcohol consumption (60%) 3
• Hepatitis and pancreatitis (33.3%) 3
• Diabetes mellitus (13.3%) 3
• Malignancy 3, 2
• Immunosuppression 4
• Recent surgical procedures (58.3% of bacteremic cases) 4

Laboratory Diagnosis

1. Culture and identification

   • Growth on sheep blood agar plates with variable hemolysis patterns
   • Biochemical or enzymatic tests for species identification
   • Detection of streptococcal cell wall carbohydrate antigens (Lancefield classification) 1
   • Most SMG isolates are non-hemolytic or alpha-hemolytic 2
   • Lancefield grouping shows carriage of F antigen (41%), C antigen (14%), or no detected group (45%) 2

2. Species distribution in clinical infections

   • S. anginosus: 64% of bacteremic isolates 2, 40% of respiratory isolates 3
   • S. constellatus: 27% of bacteremic isolates 2, 53% of respiratory isolates 3
   • S. intermedius: 9% of bacteremic isolates 2, 7% of respiratory isolates 3

Management Approach

1. Diagnostic Workup

• Blood cultures (essential when SMG bacteremia is suspected)
• Imaging studies to identify potential abscess formation:
  • CT scan or ultrasound for intra-abdominal collections
  • Chest CT for thoracic empyema or lung abscess
  • MRI for suspected brain abscess
• Microbiological identification of the specific SMG species

2. Antimicrobial Therapy

• First-line treatment: Penicillin or ampicillin

  • Caution: Increasing resistance has been reported
  • 14-33% of isolates show intermediate susceptibility to penicillin G 3, 5
  • 53% show intermediate or complete resistance to ampicillin 3

• Alternative options:

  • Carbapenems (effective in 93.3% of respiratory cases) 3
  • Third-generation cephalosporins (cefotaxime more active than cefaclor or cefotiam) 5
  • Fluoroquinolones (99% susceptibility rate) 5
  • Clindamycin (5% resistance rate) 5

• Duration of therapy:

  • Median duration: 11.5 days (range 6-25 days) for bacteremia 4
  • Longer courses may be needed for deep-seated abscesses

3. Surgical Intervention

• Source control is critical for successful treatment 4
• Drainage procedures for abscesses and empyema
• Surgical debridement of infected tissues when indicated
• Consider surgical consultation for all SMG bacteremia cases, as 58.3% of patients with SMG bacteremia have underlying surgical infections 4

Prognosis

• Overall mortality rate is relatively low (10%) for bacteremic infections 2, 4
• Most deaths are related to underlying conditions rather than directly to SMG infection
• Successful outcomes are achieved with appropriate antibiotic therapy and surgical source control

Common Pitfalls and Caveats

1. Misidentification: SMG organisms may be misidentified as viridans streptococci or other streptococcal species in routine laboratory testing.

2. Overlooking source: Failure to search for and drain abscesses can lead to treatment failure and recurrence.

3. Underestimating pathogenicity: Despite being part of normal flora, SMG organisms should not be dismissed as contaminants when isolated from sterile sites.

4. Inadequate antimicrobial therapy: Increasing resistance to penicillin and other beta-lactams necessitates antimicrobial susceptibility testing.

5. Insufficient duration of therapy: Deep-seated infections may require prolonged antibiotic courses.

6. Polymicrobial infections: SMG often coexists with anaerobes and other bacteria, requiring broad-spectrum coverage initially.

By recognizing the clinical significance of SMG infections and implementing appropriate diagnostic and therapeutic strategies, clinicians can effectively manage these potentially serious infections and improve patient outcomes.