Agents of Bacterial Skin Infections: Staphlococci

Question 1

laboratory tests that distinguish Staphylococcus sp.

Answer 1

• Colony Morphology on blood agar
  
  • Color and hemolysis
• Gram stain: positive
• Catalase: positive
• Coagulase: positive in S. aureus
• Coagulase negative:
  
  • Anaerobic: S. epidermidis
  • Aerobic: S. saprophyticus

Question 2

Skin and Soft Tissue Infections caused by S. aureus

Answer 2

• Furuncle
• Carbuncle
• Mastitis
• Cellulitis
• Impetigo
• Wound infections
• Invasive Infections

Question 3

Furuncle

Answer 3

• A boil is a skin infection involving an entire hair follicle and nearby skin tissue
• begin as a tender, pinkish-red, swollen, firm area in the skin.
• yellow or white point at the center is seen when the boil is ready to discharge pus.
• Tx: heat and drainage
  
  • Antibiotics if invades subcutaneously

Question 4

Hydradentis Suppurativa

Answer 4

• Infection of the sweat gland
• causes chronic scarring and pus formation of the underarms (axilla) and groin/inner thigh areas
• Tx: antibiotics and corticosteroids

Question 5

Folliculitis

Answer 5

• Chronic furunculosis
• can be stye near eye
• Tx: topical mucopirocin to eliminate nasal colony

Question 6

Staphylococcus Cellulitis

Answer 6

• area which is red, hot, and tender

- Can be secondary infection from break in skin

Question 7

Impetigo

Answer 7

• Usually due to GAS, but 30% from staphylococci

- Same as seen in Strep

Question 8

Ecthyma grangrenosum

Answer 8

• wound infection
• caused by Pseudomonas aeruginosa
• environmental reservoir
• round or oval lesion
• Necrotic and less suppurative
• A necrotic center is usually present with a surrounding erythematous edge

Question 9

Vibrio vulnificus

Answer 9

• wound infection
• fluid filled blisters
• from salt water exposure

Question 10

MRSA

Answer 10

• Methicillin resistant S. aureus

- skin infection seen in steroid injections and tattoos

Question 11

S. aureus Virulence Factors: anti-phagocytic

Answer 11

• Protein A: acts as an immunological disguise
• Catalase: breaks down toxic H2O2 to H2O and O2
• Leukocidin: pore forming toxin

Question 12

S. aureus Virulence Factors: Riobotechoic & Techoic Acid

Answer 12

• Bind Fibronectin

- Induces shock similar to endotoxin

Question 13

S. aureus Virulence Factors: Coagulase

Answer 13

• extracellular and surface bound

- Initiates conversion of fibrinogen to fibrin

Question 14

S. aureus Virulence Factors: capsule

Answer 14

• polysaccharide
• max expression in vivo
• 8 differnt serological types

Question 15

S. aureus Virulence Factors: Hyaluronidase

Answer 15

• Acts on Hyaluronic acid in CT

- Facilitates dissemination through subcutaneous tissues

Question 16

S. aureus Virulence Factors: Cytotoxins (5)

Answer 16

1. Alpha hemolysin: potent pore former; toxic to many cell types
2. Beta toxin: sphingomyelinase C: kills cells via hydrolysis of membrane phospholipids
3. Delta toxin: cytolytic for many cells; non-specfic detergent action
4. Gamma toxin: pore forming toxin; lysis of neutrophils and macrophages
5. Panton Valentine Leukocidin: pore forming toxin; lysis of neutrophils and macrophages

Question 17

Bullous Impetigo

Answer 17

• appear around the diaper region, axilla, or neck
• exfoliative toxins cause for the epidermis and dermis to separate
• Exfoliating toxins are serine proteases that specifically bind to and cleave desmoglein 1

Question 18

Scalded skin syndrome

Answer 18

• Localized bullous impetigo
• widespread formation of fluid filled blisters that are thin walled and easily ruptured
• positive for Nikolsky’s sign: slight rubbing of the skin results in exfoliation
• Exfoliatins (serine proteases) cause splitting of desmosomes in the stratum granulosum

Question 19

Toxic Shock Syndrome

Answer 19

• Caused by toxic shock syndrome toxin-1
• Sudden onset fever chills, vomiting, diarrhea, muscle pains and rash
• Later: hypotension, mucus membrane involvement, multiple system failure, desqumation
• Risk: menstruating women and women w/ barrier contraceptive devices, and nasal surgery

Question 20

S. aureus Resistance to Penicillin & Methicillin

Answer 20

• Penicillinase producing S. aureus
  
  • plasmid encoded
  • transfer by conjugation or transduction
• Methicillin: semi-synthetic penicillin
  
  • resistant to penicillinase
  • Developed resistance due to mecA gene

Question 21

S. aureus Vancomycin Resistance

Answer 21

• Due to acquisition of vanA gene

- Peptidoglycan precursor contains D-ala-D-lactate rather than D-ala-D-ala

Question 22

S. epidermidis

Answer 22

• UTIs
• Osteomyelitis
• Endocarditis
• Bacteremia
• Endophthalmitis
• Infections of indwelling devices (catheters, pacemakers, etc)
  
  • Biofilm causes irreversble adherence that protects against antibiotics

Question 23

S. intermedius

Answer 23

• Coagulase positive
• Invasive zoonotic pathogen
  
  • dog bites