Streptococci

Ovoid Gram positive organisms that come in pairs (diplococci) or chains, and are subdivided into:

• α-Haemolytic
  Produce green rings around colonies on agar. Includes:
  • S. pneumoniae
  • S. bovis
  • S. mutans
• β-Hemolytic
  Cause lysis of cells, resulting in clear rings around colonies, and further divided by the Lancefield scheme. Includes:
  • Group A: S. pyogenes
  • Group B: S agalactiae
• γ-Haemolytic
  Generally no change around colonies, although may be also α-haemolytic. Includes:
  • S. bovis
  • S. mutans
• Enterococci

Enterococci were once considered Group D streptococci, but have broken out into their own genus based on DNA analysis.

They are included here for posterity and organisational convenience.

α-Haemolytic

S. pneumoniae is an obligate parasite and normal nasopharyngeal commensal. Infection:

• Also known as pneumococcal infection
• Requires some ↑ susceptibility:
  • Extremes of age
  • Recent viral pneumonia
    With streptococcal bacterial superinfection.
  • Immunosuppression
    • Smokers
    • Asplenic
      • Splenectomy
      • Sickle-cell disease
    • Malnutrition
    • Alcoholism
• Include:
  • Meningitis
    Most common cause in H. influenzae vaccinated individuals.
  • CAP
    Most common cause.
  • Otitis media
  • Sinusitis
  • Mastoiditis

Lancefield Scheme

Divides β-haemolytic streptococci into several groups based on cell wall composition.

Group A

Group A is synonymous with S. pyogenes:

• Reservoir in skin and mucous membranes of humans
  Spread by contact from person to person.
• Produces hyaluronidase which destroys connective tissue
• Cell walls contain an M protein which:
  • Is essential for infection
  • Has a high number of antigenic types
    Repeat infection is possible and common.
  • Antibodies to M protein are common precipitants of rheumatic fever
    Preventable if antimicrobials provided within 10 days.

Infections include:

• B
  • Pharyngitis
    Purulent inflammation of tonsils and oropharynx. Labeled scarlet fever if it occurs with endotoxin production.
• I
  • Minor skin infections
    • Erysipelas
    • Impetigo
  • Necrotising fasciitis

Complications include:

• Toxic shock syndrome
• Rheumatic fever
• Acute glomerulonephritis

Streptococcal toxic shock is covered under Toxic Shock Syndrome.

Group B

Group B is synonymous with S. Agalactiae:

• Carried in urogenital and GI tract
  • Transmitted sexually or at birth

Infections include:

• Neonatal
  Risk ↓, but not eliminated by, caesarian section. Intrapartum antibiotics given if high risk features.
  • Meningitis
  • Septicaemia
• Endometritis
• Opportunistic
  • Diabetic foot infections

Enterococci

Enterococci are:

• Normal gut flora
• May be α/β/γ-haemolytic
• Can survive in the presence of bile and high-salt solutions
  And other detergents, including cleaning chemicals.
• Are naturally resistant to β-lactams including carbapenems, and commonly resistant to other agents

Enterococci include:

• E. faecalis
  • Associated with immunocompromise:
    • Critical illness
    • Cancer
    • Neutropaenia
    • Steroids
    • Renal failure
  • Severe illness
    Bacteraemia ~50% mortality.
• E. faecium
  • More resistant
  • Less significant illness
    Bacteraemia ~10% mortality.
  • Generally more resistant than E. faecalis
    More likely to be VRE.

Vancomycin Resistant Enterococcus

Clinically significant enterococcus mutation that is:

• Resistant to a most antibiotics
  Due to the inherent resistance of Enterococci spp., in addition to the vancomycin resistance.
• Divided by van gene expression into:
  • VanA
    Teicoplanin resistant; extremely vancomycin resistant. Common in Australia.
  • VanB
    Teicoplanin sensitive; moderate vancomycin resistance.
• Able to transfer resistance:
  • Between enterococci
  • To S. Aureus

Risk factors:

• Patient
  • Age
• Disease
  • Critically unwell
• Treatment
  • Long hospital stay
  • TPN/EN feeding
  • CVC
  • Transplant
  • Haemodialysis
  • Antibiotic exposure
    Anything that kills other gut flora, allowing enterococci to expand:
    • Vancomycin
    • 3^rd generation cephalosporins
    • Anti-anaerobes
  • Long duration of antibiotic therapy
• Environment
  • Proximity to other VRE patients
    • Other VRE patients with diarrhoea
  • Nursing home
  • Extended hospitalisation

Other van genes exist, they are less clinically relevant.

Management

Specific therapy:

• Pharmacological
  • S. pneumoniae
    Commonly resistant to penicillins and other β-lactams:
    • Empiric therapy generally consists of:
      • Vancomycin
      • Ceftriaxone 1-2g IV Q12-24H or Cefotaxime 2g IV Q4-6H
  • Group A
    • Benzylpenicillin
      Resistance not developed.
  • Group B
    • Benzylpenicillin
    • Ampicillin
      Alternative to benzylpenicillin.
    • Cephazolin
      Alternative if penicillin allergy.
    • Clindamycin
      Alternative if resistance to β-lactams.
    • Vancomycin
      Alternative if penicillin allergy and clindamycin resistant.
  • VRE
    • Contact precautions
    • Antimicrobials
      • Linezolid
      • Daptomycin
• Procedural
  • Source control

Preventative:

• Pneumococcal vaccine

---

References

1. Harvey RA, Cornelissen CN, Fisher BD. Lippincott Illustrated Reviews: Microbiology (Lippincott Illustrated Reviews Series). 3rd Ed. LWW.