Microbiology · MU Sofia

47. Staphylococci (genus Staphylococcus)

Covers the general features of staphylococci, including Gram-positive grape-like cocci that are catalase positive, and the main species with their coagulase status. It then sets out a table of S. aureus virulence factors grouped as structural, enzymes and toxins, followed by the pyogenic and toxin-mediated diseases it causes. Laboratory diagnosis, treatment of MSSA and MRSA infections, and prevention and control measures close the note.

Inside: 8 sections · 7 labelled pictures · 3 tables · 14 flashcards · 4 min

Topic 47 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 8 sections, about 380 words (4 min of reading), with 14 flashcards.

In this note

  1. Staphylococci (Genus Staphylococcus)
  2. I. General Characteristics
  3. II. Species
  4. III. Virulence Factors of S. aureus
  5. IV. Diseases Caused by S. aureus
  6. V. Laboratory Diagnosis
  7. VI. Treatment
  8. VII. Prevention and Control

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.

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