The Staphylococci: Morphology and Identification, Antigenic Structure, Enzymes and Toxins.

Question 1

Morphology and identification

Answer 1

Staphylococci are gram-positive cocci that appear in grape-like clusters. They are non-motile, non-spore-forming, and catalase-positive. Staphylococcus aureus is coagulase-positive and forms golden-yellow colonies on nutrient agar. It grows on most culture media, including blood agar (producing beta-hemolysis) and mannitol salt agar (ferments mannitol, turning the medium yellow). Other species like S. epidermidis and S. saprophyticus are coagulase-negative. They are facultative anaerobes and tolerant of high salt concentrations.

Question 2

Antigenic structure

Answer 2

Staphylococci have several key antigens and surface structures: 1) Peptidoglycan: provides rigidity, can activate complement and induce cytokine production. 2) Teichoic acids: contribute to adherence to mucosal surfaces and elicit antibody responses. 3) Protein A (in S. aureus): binds the Fc portion of IgG, preventing opsonization and phagocytosis. 4) Capsule and slime layer: help in evading phagocytosis and biofilm formation (especially in S. epidermidis). These structures are important for immune evasion and colonization.

Question 3

Enzymes and toxins

Answer 3

S. aureus produces numerous enzymes and toxins that contribute to its pathogenicity: 1) Coagulase: clots plasma, distinguishing S. aureus from other staphylococci. 2) Catalase: breaks down hydrogen peroxide, allowing survival inside phagocytes. 3) Hyaluronidase: hydrolyzes connective tissue. 4) Staphylokinase (fibrinolysin): dissolves fibrin clots. 5) Lipases and proteases: aid tissue invasion. Toxins include: a) Hemolysins (alpha, beta, gamma, delta): lyse red and white blood cells. b) Leukocidins (e.g., Panton-Valentine toxin): destroy leukocytes. c) Exfoliative toxins (ETA, ETB): cause scalded skin syndrome by cleaving desmoglein-1. d) Enterotoxins (A–E, G–I): heat-stable superantigens causing food poisoning. e) Toxic shock syndrome toxin-1 (TSST-1): a superantigen causing fever, rash, hypotension, and multiorgan failure. These factors enable staphylococci to colonize, evade immune responses, and damage host tissues.