L45: Medically Important Cocci

Question 1

Gram positive cocci

Answer 1

1. Staphylococcus (cocci in clusters, catalase +ve, facultative anaerobes, salt tolerant: mannitol salt agar)
   —> Coagulase +ve (bound/extracellular):
   - Aureus —> fibrinogen to fibrin —> hindering phagocytosis
   —> Coagulase -ve:
   - Epidermidis —> prosthesis infection
   - Saprophyticus —> cystitis
2. Streptococcus (cocci in chains, catalase -ve)
   - Beta-haemolytic (Lancefield grouping A, B, C, G) —> S. Pyogenes
   - Alpha-haemolytic —> S. Pneumoniae / Viridans strept
3. Enterococcus
   - Non-haemolytic —> E. Faecalis

Question 2

Gram negative cocci

Answer 2

• Diplococci
• Strictly aerobic
• Cytochrome oxidase
  Neisseria gonorrhoeae
  Neisseria meningitidis

Question 3

Virulence factor of Staphylococcus aureus

Answer 3

1. Attachment
   - Teichoic acid
2. Evasion
   - Coagulase
   - Catalase
   - Protein A (anti-phagocytic)
3. Invasion
   - hyaluronidase
   - DNase
   - protease
   - lipase
4. Toxins
   - epidermolytic toxins (scalded skin syndrome)
   - toxic shock syndrome toxin (toxic shock syndrome)
   - enterotoxin (food poisoning)
   - Panton-Valentine Leukocidin (dermonecrotic)

Question 4

Clinical disease of S. Aureus

Answer 4

1. Pyogenic
   - Skin and soft tissue (Folliculitis, furuncle, carbuncle)
   - Wound and surgical site
   - Arthritis
   - Infective endocarditis
2. Toxin-mediated
   - Scalded skin syndrome
   - Toxic shock syndrome
   - Food poisoning

Question 5

Risk factors for S. Aureus

Answer 5

1. Diabetes mellitus
2. Haemodialysis
3. IV drug users

Question 6

Antibiotic sensitivity

Answer 6

• Penicillin resistance (almost all)
• Methicillin / Cloxacillin stable to penicillinase (MRSA)
• Vancomycin (VRSA: not prevalent currently)

Question 7

Virulence factor of Neisseria

Answer 7

1. Pili: attachment
2. Capsule: anti-phagocytic
3. LPS: endotoxin

Question 8

Risk factors for N. gonorrhoeae

Answer 8

• Sexual transmission (mucosal contact)

- Sexual promiscuity

Question 9

Clinical disease of N. gonorrhoeae

Answer 9

1. Urethritis (men)
2. Endocervicitis (women)
3. Pelvic inflammatory disease
4. Proctitis