Micro: Greenberg

Question 1

What are the virulence factors of strep?

Answer 1

adherence: *M protein - bind to skin cells, capsule, ECM binding proteins
immune evasion: *hyaluronate capsule (anti-phagocytic), M protein anti-complement, Ig binding proteins, C5a peptidase
invasion and spread: DNAase B, SpeB, streptolysins
toxicity: superantigens, pyrogenic exotoxins (SpeA & C)

Question 2

What is the microbiology of clostridium?

Answer 2

gram positive rods, form spores
most strict anaerobes, some microaerotolerant
habitat = soil, intestines of animals and humans (feces)

Question 3

What are the toxin mediated syndromes of c. perfringens?

Answer 3

*alpha toxin is most important: phospholipase C cleaves phosphatidyl choline, hemolytic (RBCs, WBCs, platelets, endothelial cells)

Question 4

What are the skin diseases caused by c. perfringens?

Answer 4

cellulitis
necrotizing faciitis
*suppurative myositis and myonecrosis

Question 5

What is the gastroenteritis dz caused by c. perfringens?

Answer 5

mild self-limited diarrhea

from storing meats too long at too warm after cooking - gives spores that survived cooking time to germinate

Question 6

What are key differences in uncomplicated vs. complicated skin infections?

Answer 6

*monomicrobial, gram + (strep, staph) vs. polymicrobial
superficial layers w no underlying medical illness vs. deeper soft tissue layers w immunocompromised or vascular insufficiency
antibiotics alone vs. antibiotics and possible surgery
erysipelas, impetigo, cellulitis vs. abscesses, ulcers, necrotizing faciitis

Question 7

Which forms of pyoderma are usually caused by strep and which by staph?*

Answer 7

strep - impetigo (*honey colored crusts, no scar), ecthyma (inf penetrates through epidermis, may ulcerate and leave scar)
staph - folliculitis, furuncle, carbuncle

Question 8

What are the basics of impetigo?

Answer 8

associated w poor hygiene
year round in tropical environments, summer peak in temperate climates
*caused by both strep and staph
M serotypes more common
transmission by direct contact, fomites, insects - *person-to-person spread is common

Question 9

What is the clinical manifestation of impetigo?

Answer 9

incubation 10-14 days
papule –> vesicle –> pustule (breaks open in 4-6 days)
superficial localized lesions w surrounding erythema
*honey colored exudate and crust - contains bacteria

Question 10

What is the treatment for impetigo?

Answer 10

often self limited, or can resolve w soap and water
topical treatments: mupirocin gets strep and staph, bacitracin against group A strep, retapamulin for strep and staph inc MRSA
penicillin for strep, broader for staph
good hygiene! = prevention

Question 11

What is the clinical presentation of erysipelas and how is it generally treated?

Answer 11

acute inflammation of skin w lymphatic involvement
advancing red *well-demarcated margins
facial often associated w prior pharyngitis and resolves spontaneously
trunk often surgical wound inf - can be fatal
penicillin is effective

Question 12

What is cellulitis?

Answer 12

inf of dermis and subcutaneous tissue
usually following trauma
painful, red, swollen, tender
bacteremia in some cases

Question 13

What is the clinical presentation of cellulitis?

Answer 13

*different than erysipelas - margins unclear, lesions may not be raised
more common in leg than arm
predisposing factors - DM, drug use, bypass
*staph and strep most common

Question 14

What is lymphangitis?

Answer 14

spread of inf up the lymphatics
get red linear streak
progresses to bacteremia in 24-48 hrs
often lymphadenitis
fatal if not treated

Question 15

What skin inf associations are there for psuedomonas, vibrio vulnificus, and pasteurella multocida?

Answer 15

• ecthyma gangrenosum
• diabetics and cirrhotics particularly vulnerable
• dog or cat bites

Question 16

Aeromonas hydrophila

Answer 16

fresh water habitat, fish pathogen
usually causes gastroenteritis, can cause cellulitis
3rd generation cephalosporins

Question 17

vibrio vulnificus

Answer 17

curved GNR, estuary reservoir
usually immunocompromised - *liver dz common
rapidly progressive - symptoms w/i 18 hrs of exposure
high mortality
treat w tetracyclines, cephalosporins, fluoroquinolones

Question 18

pasteurella multocida

Answer 18

natural inhabitant of many animals, GN coccobacillus
causes pneumonia in rabbits, lethal for lots of animals
rapidly progressive cellulitis after *cat > dog bite
treat w augmentin (covers anaerobes in bite too)

Question 19

erysipelothrix rhusiopathiae

Answer 19

GPR, found in many animals and fish
occupational illness - fish handlers, slaughterhouse workers
chronic and indolent

Question 20

What is necrotizing fasciitis?

Answer 20

gangrenous destruction of skin, fascia and sometimes muscle
usually at site of trauma - *but may be inapparent
bacterial toxins prominent virulence factors - cause tissue separation and cell death
vascular thrombosis leads to ischemia and infarction

Question 21

What is synergistic necrotizing cellulitis?

Answer 21

gram negative enteric flora and anaerobes
proximity to GI and GU tract
*Fournier’s gangrene - scrotum and external genitalia

Question 22

What are the subtypes of necrotizing fasciitis?

Answer 22

progressive bacterial synergistic gangrene - often post-op
synergistic necrotizing cellulitis
streptococcal gangrene = flesh-eating bacteria (group A)
clostridial myonecrosis (gas gangrene) - follows trauma, war injuries

Question 23

What are the clinical manifestations of necrotizing fasciitis?

Answer 23

described as “woody”
tender, *pain out of proportion to findings
red then dusky then purple, skin may slough
bullae may form, serous fluid in fascia
few PMNs, many bacteria

Question 24

What are the features distinguishing necrotizing fasciitis from cellulitis?

Answer 24

systemic toxicity
prostration
rapidly spreading lesions
bullae

Question 25

What can radiology of necrotizing fasciitis show?

Answer 25

gas in the tissues

Question 26

What is the treatment of necrotizing fasciitis?

Answer 26

no delivery of antibiotics if vascular occlusion
*requires surgical debridement w adjunctive Abx, resection of muscle and fascia, sometimes amputate
hyperbaric oxygen may be adjunctive - toxic to strep and clostridia and some toxins, increases neutrophil respiratory burst

Question 27

How do antibiotics work in necrotizing fasciitis?

Answer 27

usually need combos

penicillin and clindamycin effective for strep and clostridium

Question 28

What is myositis?

Answer 28

may follow trauma

caused by strep (toxic shock) and c. perfringens

Question 29

What is streptococcal toxic shock?

Answer 29

invasive inf initiates on skin or throat
secretion of superantigens SpeA and C
all age groups, person to person spread, small outbreaks reported
M1 and M3 genotypes

Question 30

What are the criteria for diagnosing strep toxic shock?

Answer 30

isolated group A strep

hypotension plus 2 or more: renal impairment, coagulopathy, liver dysfunction, ARDS, rash, soft tissue necrosis

Question 31

What is pyomyositis?

Answer 31

pyogenic inf w/i body of muscle itself
almost always *s. aureus
in IVDA, HIV, immunocompromised, diabetics
usually follows blunt trauma
3 stages of inf (muscle only w no abscess, abscess, systemic)

Question 32

How is pyomyositis diagnosed?

Answer 32

*imaging to locate abscess - CT, MRI, ultrasound

aspiration for diagnosis