I. General Characteristics
- Staphylococcus are Gram-positive cocci arranged in grape-like clusters due to division in multiple planes.
- They are non-motile, non-spore forming, and facultative anaerobes.
- They grow well on ordinary nutrient media like blood agar and selective media such as Mannitol Salt Agar (MSA).
- They are catalase positive, which differentiates them from Streptococci (catalase negative).
- Among them, Staphylococcus aureus is coagulase-positive and is the most pathogenic species.
Catalase helps break down host-derived hydrogen peroxide, helping them survive oxidative stress. Coagulase helps in forming fibrin clots, which shields bacteria from immune detection.
IV. Diseases Caused by S. aureus
1. Pyogenic (pus-forming) Infections
These result from local invasion and destruction of tissue, usually with pus accumulation.
- Impetigo: Honey-colored crusted lesions, often on the face.
- Folliculitis: Infection of hair follicles; superficial.
- Furuncle (boil): Deep-seated nodular infection.
- Carbuncle: Cluster of furuncles, extensive inflammation, systemic symptoms.
- Wound and surgical site infections: Especially with sutures, catheters.
- Osteomyelitis: Bone infection, may arise hematogenously or from trauma.
- Septic arthritis: Common in children or after joint injury.
- Pneumonia: Post-viral (esp. influenza), can be necrotizing.
- Endocarditis: Especially in IV drug users; right-sided tricuspid valve.
- Bacteremia and Abscesses: Bacterial spread to multiple organs.
2. Toxin-Mediated Diseases
- Food poisoning: Rapid onset (1–6 hrs) vomiting due to enterotoxin A.
- Toxic Shock Syndrome (TSS): High fever, diffuse rash, desquamation, hypotension.
- Staphylococcal Scalded Skin Syndrome (SSSS): Mostly infants; widespread desquamation without scarring.