Med School Atlas

    Gram Positive Cocci

    Laboratory Techniques

    Gram (+) bacteria stain purple, whereas gram (-) bacteria stain pink. Liquid media are pure cultures (monocultures) from sterile body locations. Solid media have mixed populations of bacteria.

    Alpha (partial) hemolysis produces greenish-gray discoloration. Beta (complete) hemolysis is distinct, clear, and colorless. Gamma (none) hemolysis is transparent.

    Catalase (+): Staphylococci

    • Coagulase (+): Staphylococcus aureus
    • Coagulase (-): CONS

    Catalase (-): Streptococci

    Alpha hemolysis

    • Esculin (+), NaCl (+): Enterococci
    • Esculin (+), NaCl (-): Group D non-Enterococci
    • Esculin (-), NaCl (-): S. pneumoniae

    Beta hemolysis

    • Taxo A sensitive: Group A Streptococcus pyogenes
    • Taxo A resistant, CAMP (+): S. agalactiae
    • Taxo A resistant, CAMP (-), NaCl (+): Enterococci
    • Taxo A resistant, CAMP (-), NaCl (-): Group D non-Enterococci

    Gamma hemolysis

    • NaCl (+): Enterococci
    • NaCl (-): Group D non-Enterococci

    Virulence Factors

    Ability to invade tissue, cause disease, and evade host defense.

    Common factors

    • Adherence: Pili, fimbriae, capsule
    • Cell/tissue damage: Exotoxins, cytotoxins, etc.
    • Anti-complement: Capsule, proteases
    • Anti-phagocytic: Capsule, leukotoxins
    • Humoral immunity stealth: Fc receptors, Ig proteases, endotoxin, LPS/LTA, cell wall components
    • Cellular immunity stealth: Superantigens

    Group A Streptococcus pyogenes

    Invades tissues using hyaluronidase, which breaks down connective tissue, and streptokinase, which activates plasmin to release additional bacteria. Disease is caused by toxin production and superantigens.

    It evades host defense using capsules, hyaluronate as a protective layer, and M protein which is anti-opsonization and inhibits complement.

    See here for clinical presentation: Head and Neck Infections > Streptococcus Pyrogenes

    Group B Streptococcus agalactiae

    Invades tissues using lipoteichoic acid for adherence and C5a peptidase for adhesion and invasion. Disease caused by hyaluronic acid lyase which spreads the infection.

    Host defense

    • Polysaccharide capsular antigen: Resists phagocytosis
    • C5a peptidase: Blocks neutrophil chemotaxis
    • Cell surface C antigen: Prevents phagocytosis
    • Cell surface PBP 1a: Resists phagocytosis

    Staphylococcus aureus

    Invades tissues using collagen binding protein and fibronectin binding protein which promotes biofilm.

    Disease is caused by several toxins

    • Cytolytic toxins
    • Enterotoxin: Heat stable for food poisoning
    • Exfoliative toxin: Split skins in the upper epidermis
    • TSST-1: Staph superantigen which triggers cytokine release, enhances other toxic effects, increases endothelial leakage, and causes menstrual toxic shock syndrome

    It evades host defense using capsules, coagulase (clots blood), hyaluronidase, and protein A which is an antiphagocytic forms a biofilm, adheres to surfaces (e.g., catheters), and stimulates lung inflammation.

    Clinical Scenarios

    Pharyngitis from Streptococcus pyogenes

    Because pharyngitis can have several causes, a culture is required. Will be beta hemolytic, bacitracin-sensitive, and PYR (+).

    Syndromes

    • Strep. throat
    • Cellulitis
    • Impetigo
    • Necrotizing fasciitis (flesh eating disease)
    • Scarlet fever
    • Streptococcal toxic shock syndrome
    • Rheumatic fever
    • Acute post-strep glomerulonephritis

    Scarlet fever from Streptococcus pharyngitis

    There are three types of Strep. pyrogenic exotoxin (SPE): A, B and C.

    Syndromes

    • Strawberry tongue
    • Circumoral pallor (white area between lips and skin)
    • Acute rheumatic fever (ARF)
      • Polyarthritis
      • Subcutaneous nodules (deep-seated lesions)
      • Pericarditis (swelling of heart pericardium)
      • Erythema marginatum (reddening of skin)
      • Sydenham cholera (neurologic condition with rapid movements and personality changes)
    • Post-streptococcal glomerulonephritis (PSGN)
      • Follows skin or pharyngeal infections
      • Immune complex disease
      • Common in children
      • Kidney issues

    Meningitis from Streptococcus agalactiae

    Will be gram (+) in pairs and chains, capsulated, facultative anaerobe, beta hemolytic (3-4mm grey-white colonies), CAMP (+)

    Syndromes

    • Neonatal disease
    • In pregnant women: UTI, chorioamnionitis, postpartum endometritis, bacteremia
    • In adults: Sepsis, soft tissue infections, immunocompromised (DM, HIV, malignancy)

    Endocarditis from Streptococcus viridans

    Infection of the heart valve. Diagnose with blood cultures, serologic tests, and echocardiography (oscillating mass or perivalvar mass). Can result in heart failure, conduction abnormalities, stroke or aneurysm, or renal disease.

    Subacute endocarditis will take longer to show symptoms and is low virulence, while acute endocarditis has a short incubation and high virulence.

    Syndromes

    • Fever and murmur
    • Weakness
    • Dyspnea
    • Sweats
    • Weight loss
    • Embolic phenomena
    • Osler's node (painful)
    • Janeway lesion
    • Splinter hemorrhage

    Staphylococcus aureus infection

    Usually found on the skin. Teichoic acid is an important virulence factor. Methicillin-resistant Staphylococcus aureus (MRSA) has resistance encoded by the mecA gene.

    Skin and soft tissue

    • Boils
    • Cellulitis
    • Impetigo
    • Folliculitis
    • Furuncles, carbuncles, abscess
    • Hidradenitis suppurative (lumps in armpit or groin)
    • Pyomyositis (infection of skeletal muscle)
    • Mastitis
    • Bacteremia (often in catheter, skin, and pulmonary sources)
    • Endocarditis

    Respiratory infections

    • Ventilator-associated pneumonia
    • Post-viral pneumonia
    • CA-MRSA necrotizing pneumonia
    • Empyema (pus collection in pleural cavity)

    Bone and joint infections

    • Osteomyelitis (bone inflammation)
    • Native join septic arthritis
    • Prosthetic joint infections

    Associated toxins

    • Toxic shock syndrome toxin-1 (TSST-1): Superantigen that results in T cell activation
    • Preformed heat stable enterotoxin (food poisoning)
    • Exfoliative toxin (scaled skin syndrome, desquamating rash)
    ← Enterics
    Head And Neck Infections →