Exam #8: Bacterial Infections of the Skin & Soft Tissue I

Question 1

What organisms is DM a risk factor for?

Answer 1

S. aureus
GBS
Anaerobes
Gram negative bacteria

Question 2

What organisms is neutropenia a risk factor for?

Answer 2

Pseudomonas aeruginosa

*Note that hot tub exposure is also a major risk factor for the development of P. aeruginosa infection

Question 3

What organisms is IV drug abuse a risk factor for?

Answer 3

MRSA

P. aeruginosa

Question 4

What is impetigo?

Answer 4

Most superficial of bacterial skin infections that causes:

• Honey crusted lesions*
• Bullae (blisters)
• Erosions

Often seen around the mouth

*Most common in children & S. aureus causes 80% of cases (2nd is GAS)

Question 5

What is Bullous Impetigo?

Answer 5

S. aureus that causes blisters initially filled with cloudy fluid that later rupture resulting in erosions and crusting

Question 6

What causes Bullous Impetigo?

Answer 6

Exfoliative toxin of S. aureus

Question 7

What is Ecthmya?

Answer 7

• Deeper form of impetigo that results in scarring and ulceration– this may result from neglected of inadequately treated impetigo
• Causes “punched-out” ulcerative lesions

*Seen in the homeless population

Question 8

What is a furuncle?

Answer 8

Deep-steated infection of the hair follicle caused by S. aureus

Question 9

What is a carbuncle?

Answer 9

• Multiple coalesced furuncles/boils that extend into deeper subcutaneous tissue caused mainly by S. aureus
• Extends deeper into subcutaneous tissue

The defining feature of a carbuncle is when a patient presents with chills and fever

Question 10

What is Folliculitis?

Answer 10

Infection/ Inflammation of the hair follicles that is most commonly caused by P. aerugenosa

Question 11

What are Erysipelas?

Answer 11

• Erysipelas is an acute infection of the skin that is characterized by distinct borders between the involved skin and the uninvolved skin.
• Most commonly caused by S. pyogenes

Question 12

What is cellulitis?

Answer 12

• Cellulitis is a skin infection that involves deeper subcutaneous tissue
• BOTH local inflammation & systemic inflammation are seen (fever)

*In contrast to erysipelas, in cellulitis it is v. difficult to differentiate between the involved & uninvolved areas

Question 13

What organisms cause cellulitis? Why is it important to differentiate between them?

Answer 13

• Cellulitis is most commonly caused by S. pyogenes (GAS), followed by S. aureus
• However:
  1) Dog or cat bite= Pasteurella multocida
  2) Saltwarer= Vibrio vulnificus

*Differentiation is important for treatment; remember to start broad and then narrow abx selection

Question 14

What is a Paronychia?

Answer 14

Localized & superficial infection or abscess of the nailfolds

Question 15

What is the difference between acute & chronic paronychia?

Answer 15

Acute= bacteria (S. aureus) 
Chronic= fungi (c. albicans)

Question 16

What is Necrotizing Fasciitis?

Answer 16

• Acute infection of subcutaneous tissue that leads to destruction of muscle & fat
• It presents as cellulitis followed by bullae & gangrene

Question 17

How is Necrotizing Fasciitis treated?

Answer 17

Surgical debridement & antibiotics

Question 18

List the characteristics of S. aureus.

Answer 18

• Gram (+) cocci in clusters “grape-like”
• Catalase +
• Coagulase +

Question 19

How is S. aureus transmitted?

Answer 19

Nasal Shedding

Question 20

List the diseases that are caused by S. aureus.

Answer 20

Skin infections 
Sepsis 
Endocarditis 
Pneumonia 
Osteomyelitis 
Septic Arthritis 
Purulent Meningitis 
Food Poisoning 
Toxic Shock

Question 21

Describe S. aureus in culture.

Answer 21

Aureus= “gold” i.e. golden colonies

B-hemolytic (complete hemolysis)

Question 22

What are the virulence factors associated with the structure of S. aureus?

Answer 22

Capsule= anti-phagocytic 
Biofilm 
Peptidoglycan (TLR)= endotoxin like acticity 
Teichoic Acid (Adhesion)
Protein A

Question 23

What is the mechanism of Protein A?

Answer 23

• Protein A inhibits antibody mediated clearance by binding to IgG Fc domain (links to the phagocyte)
• Basically turns the antibody around so that it can no longer link to a phagocyte

Question 24

What are the toxins associated with S. aureus?

Answer 24

Cytotoxins
Exfoliative toxins
Enterotoxins
TSST-1

Question 25

What cytotoxin is associated with S. aureus?

Answer 25

Alpha toxin= pore forming

Question 26

What are the Exfoliative toxins associated with S. aureus?

Answer 26

Serine proteases that split desmosoe junction in the epithelial layer *This is what mediates the symptoms that are seen in "Staph Scalded Skin Syndrome"

Question 27

What enzymes associated with S. aureus are virulence factors?

Answer 27

There are a variety of enzymes that can hydrolyze host tissue to aid in the spread of S. aureus throughout the body - Coagulase= converts fibirinogen to fibrin - Hyaluronidase= hydrolyzes HA in connective tissue, promoting spread - Fibrinolysin= Dissolves fibrin clots - Lipases= hydrolyzes lipids - Nucleases= hydrolyzes DNA

Question 28

Outline the pathogenesis of S. aureus.

Answer 28

1) Staph adheres to the cell via Tiechoic acid 2) Secretes alpha toxin that forms pores to enter the cell 3) Produces Protein A that "sops up antibody" and prevents macrophage mediated clearance

Question 29

List the toxin mediated diseases that are caused by S. aureus.

Answer 29

Scalded Skin Syndrome Food poisoning Toxic Shock

Question 30

List the infectious diseases that are caused by S. aureus.

Answer 30

``` Impetigo Folliculitis Furuncles Carbuncles Osteomyelitis Septic arthritis ```

Question 31

What is Scalded Skin Syndrome?

Answer 31

Sloughing off of the superficial layer of the skin that looks like a sunburn in the absence of sun exposure - Toxin mediated disease - Typically seen in infants - Nasal transmission - Starts on face and mouth and then spreads to the rest of the body *Note that you CANNNOT see S. aureus in culture in this syndrome

Question 32

What causes Scalded Skin Syndrome?

Answer 32

Exfoliative toxin (ETA or ETB) *Serine proteases that break apart desmosomes

Question 33

Can S. aureus be cultured from skin in Scalded Skin Syndrome?

Answer 33

No--this is toxin mediated *Note that there also is no leukocytosis

Question 34

How does Scalded Skin Sydrome recover?

Answer 34

Neutralizing Antibodies that are produced by the baby

Question 35

List the stages of Scalded Skin Sydrome.

Answer 35

1) Initial (red rash) 2) Exfoliative 3) Desquamative

Question 36

What is a Nikolsky sign?

Answer 36

Skin desquemation with minimal pressure from the examiner *This is pathognomonic for SSSS

Question 37

How is SSSS managed?

Answer 37

1) Fluid resuscitation 2) Correction of electrolyte imbalance 3) Abx including oxacillin or vancomycin

Question 38

How is SSSS diagnosed?

Answer 38

- Presentation | - Skin Biopsy

Question 39

Will a skin culture of folliculitis, impetigo or furuncle/ carbuncles be positive for S. aureus?

Answer 39

Yes--these diseases are caused by infection, NOT toxin

Question 40

How are furuncles & carbuncles treated?

Answer 40

- I&D/ remove foreign body | - Antibiotics--nafcillin, oxacillin, cefazolin

Question 41

What is MRSA? How did MRSA come to be?

Answer 41

Methecillin Resistant S. Aureus - B-Lactamase (hydrolyzes B-lactam ring of PCN) - MecA gene acquired on a mobile genetic element is a PBP with low affinity for the semi-synthetic PCNs

Question 42

Where are most community acquired MRSA infections acquired?

Answer 42

Skin

Question 43

How is MRSA treated?

Answer 43

Vancomycin

Question 44

What is Osteomyelitis?

Answer 44

Skin and soft tissue infection that has progressed into a bone

Question 45

What are the symptoms of Osteomyelitis?

Answer 45

Bone pain Fever Swelling Malaise

Question 46

What are the risk factors associated with Osteomyelitis?

Answer 46

Anything that is going to impair blood flow to the bone: - Sickle cell anemia - Injury - FB - IV drug use - DM - Kidney dialysis - Surgical procedures to bony areas - Untreated infections of tissue near a bone

Question 47

What is the most common causes of Osteomyelitis?

Answer 47

S. aureus is the most likely cause of osteomyelitis

Question 48

What is septic arthritis?

Answer 48

- Painful, erythematous joint with purulent material obtained on aspiration - Usually seen in the large joints (shoulder, knee, hip, elbow)

Question 49

What is the most common cause of septic arthritis in children & adults receiving intraarticular injections?

Answer 49

S. aureus

Question 50

What is the most common causative organism in sexually active women?

Answer 50

Neisseria gonorrheae

Question 51

Generally, how are non-MRSA S. aureus skin infections treated?

Answer 51

Penicillinase-resistant PCN (nafcillin & oxacillin) MRSA= Vancomycin