Staphylococcus (Ditki Notes)

• Staphylococcus are catalase-positive, non-motile, and do not form spores
• Catalase converts hydrogen peroxide to water and oxygen, aiding resistance to oxidative stress
• Staphylococcus aureus is the most virulent strain and a leading cause of infectious disease
• Carotenoid pigments give S. aureus a distinctive golden color

Virulence Factors of S. aureus

• Capsule inhibits phagocytosis, disrupts chemotaxis, and prevents mononuclear cell proliferation
• Teichoic acids bind S. aureus to fibronectin
• Lipoteichoic acid and peptidoglycan layer trigger macrophage release of IL-1 and Tumor Necrosis Factor, causing hypotension and septic shock
• Cell wall Protein A binds antibodies, blocks complement activation, and inhibits phagocytosis
• Biofilm adheres bacterial colonies and debris to tissues, reinforcing adhesion and shielding from immune cells and antibiotics
• Cytotoxins (alpha, beta, delta, gamma, and Panton-Valentine leukocidin) lyse red and white blood cells
• Exfoliative toxins A & B are proteases that destroy the stratum granulosum of the epidermis
• Enterotoxins stimulate T-cell and macrophage release of cytokines and trigger Mast cell degranulation, resulting in peristalsis and vomiting
• Toxic shock syndrome toxin-1 stimulates T cell proliferation and T cell and macrophage release of IL1, IL-2, and TNF, causing blood vessel leakage
• Superantigens like Exfoliative toxin A, Enterotoxin, and TSS-1 induce massive immune responses that cause significant damage to the host
• Coagulase converts fibrinogen to fibrin to promote clot formation and clumping, protecting S. aureus from host defenses
• Fibrinolysin (staphylokinase) dissolves fibrin clots, potentially allowing spread to new niches
• Hyaluronidase degrades hyaluronic acids in host extracellular matrix
• Lipases free fatty acids; they are thought to inhibit host granulocytes, inactivate bactericidal lipids, and promote biofilm formation
• Nucleases hydrolyze DNA and aid in bacterial evasion of Neutrophil Extracellular Traps (NETS)

Purulent Infections

• Impetigo occurs on limbs and face, with flat, reddened areas and pustules that crust upon rupture
• Group A streptococci also cause some forms of impetigo
• Folliculitis is an infection of the hair follicle; styes are infections of eyelash follicles
• Furuncles (boils) are larger, raised pus-filled nodules; surgical drainage is sometimes necessary
• Carbuncles are furuncles that coalesce and affect deeper subcutaneous tissues; bacteremia leads to chills and fever
• Wound infections can also be caused by S. aureus, particularly in patients with compromised immune systems
• Acute endocarditis occurs when bacteria and cellular debris accumulate in vegetations and damage cardiac valves
• Impaired blood flow can occur, and vegetations that break free can embolize
• Pneumonia presents with infiltrates and consolidation or abscesses caused by cell-damaging toxins and enzymes
• Empyema is the accumulation of pus in the pleural cavity
• Osteomyelitis occurs when S. aureus infects the bones; early onset is characterized by pain and fever
  • In children, infection involves the metaphyseal area of long bones
  • In adults, infection tends to occur in the vertebral bodies
• In subacute osteomyelitis, localized infection within the bone can produce Brodie's abscesses

Toxin-Mediated Infections

• Scalded skin syndrome (Ritter's disease) is caused by exfoliative toxins, primarily affects newborns and young children
  • Disease onset is abrupt, beginning with perioral inflammation followed by superficial cutaneous blistering, then epithelial desquamation
  • Antibodies appear within 7-10 days, scarring is unusual, and mortality rate is low
• Food poisoning is caused by ingestion of enterotoxins, which produces nausea, vomiting, and diarrhea
• Toxic shock syndrome toxin-1 penetrates mucosal barriers and induces fever, hypotension and shock, and rash
  • Because it travels in the bloodstream, the toxin causes damage to multiple organ systems

Coagulase-Negative Staphylococcal Strains (CONS)

• CONS are often associated with infections of prosthetic joints and valves as well as catheters and shunts
• Staphylococcus epidermidis
• Staphylococcus saprophytic is associated with urinary tract infections in sexually active young women
• Staphylococcus lugdunensis is particularly associated with native valve endocarditis
• Staphylococcus haemolyticus