microbiology

• Gram-positive cocci


  Two medically important genera: Staphylococcus and Streptococcus
• Staphylococci


  • Appear in grapelike clusters microscopically
  • Produce catalase (degrade hydrogen peroxide)
• Streptococci


  • Appear in chains microscopically
  • Do not produce catalase
• Catalase test

  1. Differentiates Staphylococcus sp. from Streptococcus sp.

  2. Catalase splits hydrogen peroxide into water and oxygen (tiny bubbles)
• Medically important Staphylococcus species

  • Staphylococcus aureus
  • Staphylococcus epidermidis
  • Staphylococcus saprophyticus
• Coagulase test


  Differentiates S. aureus (coagulase positive) from other Staphylococcus species
• Staphylococcus aureus


  • Gram-positive cocci in clusters
  • Non-spore forming, non-motile
  • Reservoir: humans, nose is main site of colonization
  • Grows aerobically as yellow/gold colonies on blood agar
  • Catalase-positive, coagulase-positive
• Staphylococcus aureus virulence factors

  • Protein A
  • Enterotoxin
  • Toxic shock syndrome toxin (TSST)
  • Exfoliatin
  • Alpha toxin
  • PV Leukocidin
• Staphylococcus aureus pyogenic diseases

  • Skin and soft tissue infections
  • Septicemia (sepsis)
  • Endocarditis
  • Osteomyelitis and septic arthritis
  • Post-surgical wound infections
  • Staphylococcal pneumonia
  • Conjunctivitis
  • Organ abscesses
• Staphylococcus aureus toxin-mediated diseases

  • Food poisoning (gastroenteritis)
  • Toxic shock syndrome
  • Scalded Skin Syndrome
• Coagulase-negative Staphylococci


  • Normal microbiota of skin
  • Gram-positive cocci, catalase-negative, coagulase-negative
  • White colonies on blood agar
  • Infections of intravenous catheters and prosthetic implants
  • Urinary tract infections in sexually-active women
• Streptococci


  • Spherical gram-positive cocci arranged in chains or pairs
  • Catalase negative
  • Can be classified by hemolytic characteristics on blood agar: α-hemolytic, β-hemolytic, γ-hemolytic
• Lancefield grouping


  Depends on the C carbohydrate in the cell wall, helpful in serologic identification of Streptococci species
• β-hemolytic Streptococci


  • Complete lysis of RBCs, pale/clear area surrounding colony on blood agar
  • Includes Streptococcus pyogenes (group A strep) and Streptococcus agalactiae (group B strep)
  • Two important antigens: C carbohydrate (cell wall, determines Lancefield grouping) and M protein
• Streptococcus


  Spherical gram-positive cocci arranged in chains or pairs
• Streptococcus


  • Catalase negative
  • Can be classified depending on their hemolytic characteristics in blood agar
• Hemolytic characteristics in blood agar

  • α-Hemolytic: form a green zone around their colonies as a result of incomplete lysis of red blood cells in the agar
  • β-Hemolytic: form a clear zone around their colonies because complete lysis of the red cells occurs
  • γ-hemolysis: no hemolysis
• Lancefield grouping


  Depends on the C carbohydrate in the cell wall; it is helpful in the serologic identification of the Streptococci species
• β-hemolytic Streptococci


  • Complete lysis of RBCs → pale/clear area surrounding colony on blood agar
  • Streptococcus pyogenes (group A strep), Streptococcus agalactiae (group B strep)
  • Two important antigens: C carbohydrate (located in the cell wall; determines the Lancefield grouping), M protein (most important virulence factor of S. pyogenes)
• Streptococcus pyogenes (Group A)

  • Habitat: Human upper respiratory tract and skin
  • Transmission: Droplet, Contact
  • Colony: β-Hemolytic colonies on blood agar
  • Gram stain: Gram-positive cocci in chains
  • Bacitracin sensitive
  • Hyaluronic acid capsule and M protein inhibit phagocytosis
  • Exotoxin and enzymes: Streptokinase, Hyaluronidase, DNAases, Haemolysins (Streptolysins, Leukocidins), Pyrogenic exotoxins (Erythrogenic toxin)
• Pathogenesis of Streptococcus pyogenes (Group A)

  1. Pyogenic inflammation: induced locally at the site of the organisms in tissue

  2. Exotoxin production: can cause widespread systemic symptoms in areas of the body where there are no organisms

  3. Immunologic: occurs when antibody against a component of the organism cross-reacts with normal tissue or forms immune complexes that damage normal tissue
• Streptococcus pyogenes (Group A) infections

  • Cellulitis
  • Impetigo
• Streptococcus pyogenes (Group A) Pharyngitis


  • Most common bacterial cause
  • Characterized by throat pain and fever
  • On PE, inflamed throat and tonsils, often with a yellowish exudate, are found, accompanied by tender cervical lymph nodes
  • Spontaneous recovery often occurs in 10 days, but rheumatic fever may occur
  • If untreated, may extend to the middle ear, sinuses, mastoids, meninges; may also develop into peritonsillar or retropharyngeal abscess
  • Treatment: Antibiotics (Penicillins), Supportive
• Rheumatic Fever


  • A consequence of pharyngeal infection with group A β-hemolytic streptococci
  • Immune mediated (type II hypersensitivity)
  • Antibodies to M protein cross-react with self antigens, often myosin (molecular mimicry)
  • Late sequelae: Rheumatic Heart Disease
  • Diagnosis: Evidence of Streptococcal infection + 2 major or 1 major + 2 minor
• Post-Streptococcal Glomerulonephritis


  • Immunologic: Initiated by antigen–antibody complexes on the glomerular basement membrane triggering immune response
  • Clinical features: hypertension, edema of the face and ankles, and "smoky" urine (due to red cells in the urine)
• Streptococcus agalactiae (Group B)


  • Habitat: Human vagina
  • Transmission: Droplet, Contact
  • Colony: β-Hemolytic colonies on blood agar
  • Gram-positive cocci in chains
  • Bacitracin resistant
  • May cause neonatal sepsis and meningitis
• α-hemolytic Streptococci


  • Partial oxidation of hemoglobin → greenish or brownish color without clearing around growth on blood agar
  • Streptococcus pneumoniae, Viridans streptococci
• Streptococcus pneumoniae


  • Habitat: human upper respiratory tract
  • Transmission: Droplet
  • Gram ⊕, α-hemolytic, lancet-shaped diplococci, facultative anaerobe
  • Optochin sensitive, bile soluble
  • Encapsulated, IgA protease
  • Most commonly causes MOPS: Meningitis, Otitis media (in children), Pneumonia, Sinusitis
• Streptococcus pneumoniae Pneumonia


  • Infection of the lung parenchyma
  • Most common bacterial cause is S. pneumoniae
  • Clinical features: Cough (with or without sputum production), fever, dyspnea, pleuritic chest pain
  • On PE: tachypnea, increased work of breathing, and adventitious breath sounds (ie, rales/crackles, rhonchi)
  • Treatment: Antibiotics, supportive
• Viridans streptococci


  • Habitat: normal flora of the mouth
  • Gram ⊕, α-hemolytic
  • Optochin resistant, bile insoluble
  • Streptococcus mutans and S mitis cause dental caries
  • S sanguinis makes dextrans that bind to fibrin-platelet aggregates on damaged heart valves, causing infective endocarditis