Flashcards in Exam #8: Bacterial Infections of the Skin & Soft Tissue I Deck (51):
What organisms is DM a risk factor for?
Gram negative bacteria
What organisms is neutropenia a risk factor for?
*Note that hot tub exposure is also a major risk factor for the development of P. aeruginosa infection
What organisms is IV drug abuse a risk factor for?
What is impetigo?
Most superficial of bacterial skin infections that causes:
- Honey crusted lesions*
- Bullae (blisters)
Often seen around the mouth
*Most common in children & S. aureus causes 80% of cases (2nd is GAS)
What is Bullous Impetigo?
S. aureus that causes blisters initially filled with cloudy fluid that later rupture resulting in erosions and crusting
What causes Bullous Impetigo?
Exfoliative toxin of S. aureus
What is Ecthmya?
- 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
What is a furuncle?
Deep-steated infection of the hair follicle caused by S. aureus
What is a carbuncle?
- 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*
What is Folliculitis?
Infection/ Inflammation of the hair follicles that is most commonly caused by P. aerugenosa
What are Erysipelas?
- 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
What is cellulitis?
- 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
What organisms cause cellulitis? Why is it important to differentiate between them?
- Cellulitis is most commonly caused by S. pyogenes (GAS), followed by S. aureus
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
What is a Paronychia?
Localized & superficial infection or abscess of the nailfolds
What is the difference between acute & chronic paronychia?
Acute= bacteria (S. aureus)
Chronic= fungi (c. albicans)
What is Necrotizing Fasciitis?
- Acute infection of subcutaneous tissue that leads to destruction of muscle & fat
- It presents as cellulitis followed by bullae & gangrene
How is Necrotizing Fasciitis treated?
Surgical debridement & antibiotics
List the characteristics of S. aureus.
- Gram (+) cocci in clusters "grape-like"
- Catalase +
- Coagulase +
How is S. aureus transmitted?
List the diseases that are caused by S. aureus.
Describe S. aureus in culture.
Aureus= "gold" i.e. golden colonies
B-hemolytic (complete hemolysis)
What are the virulence factors associated with the structure of S. aureus?
Peptidoglycan (TLR)= endotoxin like acticity
Teichoic Acid (Adhesion)
What is the mechanism of Protein A?
- 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
What are the toxins associated with S. aureus?
What cytotoxin is associated with S. aureus?
Alpha toxin= pore forming
What are the Exfoliative toxins associated with S. aureus?
Serine proteases that split desmosoe junction in the epithelial layer
*This is what mediates the symptoms that are seen in "Staph Scalded Skin Syndrome"
What enzymes associated with S. aureus are virulence factors?
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
Outline the pathogenesis of S. aureus.
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
List the toxin mediated diseases that are caused by S. aureus.
Scalded Skin Syndrome
List the infectious diseases that are caused by S. aureus.
What is Scalded Skin Syndrome?
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
What causes Scalded Skin Syndrome?
Exfoliative toxin (ETA or ETB)
*Serine proteases that break apart desmosomes
Can S. aureus be cultured from skin in Scalded Skin Syndrome?
No--this is toxin mediated
*Note that there also is no leukocytosis
How does Scalded Skin Sydrome recover?
Neutralizing Antibodies that are produced by the baby
List the stages of Scalded Skin Sydrome.
1) Initial (red rash)
What is a Nikolsky sign?
Skin desquemation with minimal pressure from the examiner
*This is pathognomonic for SSSS
How is SSSS managed?
1) Fluid resuscitation
2) Correction of electrolyte imbalance
3) Abx including oxacillin or vancomycin
How is SSSS diagnosed?
- Skin Biopsy
Will a skin culture of folliculitis, impetigo or furuncle/ carbuncles be positive for S. aureus?
Yes--these diseases are caused by infection, NOT toxin
How are furuncles & carbuncles treated?
- I&D/ remove foreign body
- Antibiotics--nafcillin, oxacillin, cefazolin
What is MRSA? How did MRSA come to be?
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
Where are most community acquired MRSA infections acquired?
How is MRSA treated?
What is Osteomyelitis?
Skin and soft tissue infection that has progressed into a bone
What are the symptoms of Osteomyelitis?
What are the risk factors associated with Osteomyelitis?
Anything that is going to impair blood flow to the bone:
- Sickle cell anemia
- IV drug use
- Kidney dialysis
- Surgical procedures to bony areas
- Untreated infections of tissue near a bone
What is the most common causes of Osteomyelitis?
S. aureus is the most likely cause of osteomyelitis
What is septic arthritis?
- Painful, erythematous joint with purulent material obtained on aspiration
- Usually seen in the large joints (shoulder, knee, hip, elbow)
What is the most common cause of septic arthritis in children & adults receiving intraarticular injections?
What is the most common causative organism in sexually active women?