The Microbiology of Skin Infections

Question 1

Resistance of skin to infection

Answer 1

Keratizined layer

Inhibitory surface lipids and free FAs

AMPs

Innate immune system and complement
Adaptive immune system

Question 2

Folliculitis…what is it caused by and what it looks like

Answer 2

Infection of hair follicles

Base of follicle becomes red, swollen, pus filled

S. aureus

Question 3

Cellulitis vs. erysipelas

Answer 3

Both has erythema, edema, and warmth

Erysipelas involves upper dermis and superficial ;ymphatics

Cellulitis involves deeper dermis and subq fat

Erysipelas has more distinctive features, raised above level of skin, and clear demaraction

Question 4

Cellulitis..organizms, CM, Rx

Answer 4

S. aureus and Strep. pyo

Any part of skin…fever, redness, swelling, pain and tenderness, may get lymphagnatic stewaking…margins not distinct

Clindamycin/dicloxacillin for non-purulent

Question 5

Erysipelas…pathogenesis, CM, Tx

Answer 5

S. pyogenes spreading into lymph nodes triggering pain

Reddening of face, arm, or legs, and has distinct margens

Caused by toxins that stimulate macrophages and T lymphocytes to release cytokines…fever shokc and rash …warm to tocuh

Clindamycin/dicloxacillin for non-purulent

Question 6

Skin anatomy of cellulits, erysipelas, skin absecess

Answer 6

All the wya to subQ

Only to dermis

Only to dermis

Question 7

Skin abscess cause, CM, Rx

Answer 7

S. aureus collection of pus within dermis and deeper…assumed MRSA until proven

Painful, tender, fluctuant and erythematous nodules

Drain, clindamycin/bactrim if sent home…vanc if hospitalized

Question 8

Nonbullous impetigo Cause, CM, Rx

Answer 8

Normally Strep pyo but some S aureus

Small, flat, honeycombed red pathces that develop into oozing and pus-filled vesicles on a red base…vesicles bust and form honey-colored crust and can causes itching

Mupirocin for single…dicloxacillin for multiple

Question 9

Necrotizing fascitis, cause, CM, pathophys, Tx

Answer 9

Strep pyo

On surface, hot and very painful…may get fever, nausea, and low BP as it goes lower

Strep enters through wound and spreads rapidly and secretes enzymes that destory

May get moderate gas if mixed

Debridement** then clindamycin/bactrim

Question 10

Clostridium perfirnges morphology, what is causes

Answer 10

Gram positive, encapsulated, spore forming aneraobe

Myonecrosis - spores into tissue…toxins destory cells…fermentation of tissue….gas accumulates insubq space (crepitus)

Question 11

Clostridium perfringes grwoth and toxin

Answer 11

Requires additional factors…need necrosis OR nutritional excess

Alpha toxin - lecithinase that degrades lecitihin in mammla cell membranes

Question 12

Pseudomonas aeruginosa morphology, when to see it and CM

Answer 12

Gram neg, motile, aerobic rod

Opportunistic (burns, wounds, CF)

Can form biolfils of alginate of green and fruity smell

Question 13

P aeruginosa virulence

Answer 13

Exotoxin A - inactivates EF-2 via ADP-ribosylation

Question 14

Ecthyma gangrenosum cause, CM, pathophys

Answer 14

Skin lesion following spetic infection with pseudo aeruginosa

Beghin as red macules…slightly elevated papuler…hemorrhage and centrally necrotic with puple to black ocloriton…lesions can occur all over

Breakdown of skin allows dissemination…perivascular bact invasion or arteries and veins deep produce necortizing vasculitis…impredes blood supply…leads to necrosis of epidermis and dermis…nodular lesions develop

Question 15

Bartonella henselae morphology, CM in healthy

Answer 15

Gram-neg bacillus, aerobe

Cat scratch

Papule or lesion at site and for 1-3…2-3 weeks after infeciton get fever, headache, lymphadenopay…Lipid A endotoxin triggers inflam

Question 16

Bartonalla in HIV patients

Answer 16

Bacillary angiomatosis…unique vascular lesions that most frequently involve skin…may have other sx

Cutaneous lesions most common are papular, nodular, pedunculated or verrucous forms…lesions due to VASCULAR PROLIF of Bartonella

Question 17

Pasteruella multicoda morphology, CM

Answer 17

Gram-neg bipolar staingins rods, facultative

Same presentation as cat sratch

Question 18

Human bites

Answer 18

Eikenlla….gram neg, facultative rod

Question 19

Scabies tramssion, CM, Dx, Rx

Answer 19

Direct or indirect contact with mite (sarcoptes scabei)

Generalized and intense itching…usually spares head nad face…worse at night…finger webs, flexors of wrists

Dx - need to ID mites or eggs from lesions…initialy dx with light microscopy

Permethrin inhibits sodium ion influx through nerve cell membrane channels resultiing in paralysis

Question 20

Head lice cause
Transmission
CM
Dx 
Rx

Answer 20

Pediculus humanus capitis

Direct contact iwth head of infected

Female lays eggs..adults can suck blood and grasp to heairs and feed on scalp

Pruritis occurs as a result of allergic reaction to lice saliva

Suspicion on patient iwth scalp pruritis…then visualize live lice or nits

Permethrin

Question 21

Sprotorix schneckii morphology, route, Cm, Rx

Answer 21

Dimorphic soil saprophyte
Direct inoculation through skin

Initially painless pink, purple, black papule…enlarge and ulcerate…severeal painless red-to-violet ulcerated nodeules develop along channels that drain intial lesion

Itraconazole