Skin and Soft tissue infections
Peter Gilligan
Feb 15, 2008
Skin and soft tissue infections can be caused by a wide variety of organisms.
Clinical clues based on travel history, animal exposure, specific trauma all are
important in helping the laboratory determine the etiology of these infections. In
this presentation, we will concentrate on organisms that we have actually seen at
UNC Hospital in the past 20 years. More esoteric organisms will be saved for
your personal board review.
Acute infections of skin and soft tissue
Cellulitis, superficial abscess (impetigo, carbuncle etc)
Key points for laboratory diagnosisSpecimen collection for cellulitis diagnosis is almost always inadequate- need
aspirate from leading edge of wound or punch biopsy (very rarely done)- Both
have low yield (<50%)- organism number are low so gram stains are rarely
informative with cellulitis: organisms on list generally do not need special culture
conditions beyond the standard work-up that we do. Blood cultures are also of
low yield in cellulitis.
Key organismsGroup A streptococci
1. always pencillin susceptible but macrolide resistance is increasing-2007
UNC data (7.5% erythromycin resistant); clindamycin resistance, both
inducible and constitutive (as detected by D-test) is 6% with 2/3 of
resistance being inducible. TMP/SMX is not active against GAS.
2. Cellulitis can progress to GAS necrotizing fasciitis so we try to diagnosis it
if we can early so the patient does not progress; mortality with GAS
necrotizing fasciitis is approximately 50% at UNC.
Staphylococcus aureus
1. CA-MRSA is the most common strain causing wound infection in
outpatients and is common in inpatients as well.
2. These organism continue to be susceptible to TMP/SMX and doxy but
fluorquinolone resistant is becoming more common
Group B streptococci
1. Will be seen in cellulitis in diabetics as well as a chronic infection in these
patients
2. As with GAS, organism remains susceptible to penicillin G but both
macrolide (37%) resistance and clindamycin (31%; 25% constitutive)
resistance is high.
Cellulitis secondary to animal bites
Pasteurella multicoda
1. oxidase positive organism; short, gram negative rod that does not grow on
MacConkey agar
2. more common following cat bites than dog bites although dog bites are
much more common
3. bacteremia uncommon
Capnocytophaga canimorsus
1. oxidase positive organism; long, thin gram negative rod; does not grow on
MacConkey
2. More fastidious than Pasteurella
3. Splenectomized patients and those who abuse alcohol are at increased
risk for fulminant septicemia with DIC
Cellulitis secondary to human bites
1. Because of the complexity of the human oral microflora, these tend to be
complicated cultures; always assume anaerobes are present
2. need to figure out if either Eikenella corrodens or Actinobacillus are
present because they are difficult to treat; both have distinctive phenotype
with Eikenella producing a strong bleach odor and pitting of the agar while
Actinobacillus colonies have a cross-appearance in the center
Cellulitis/skin abscess secondary to skin popping black tar heroin
Clostridium botulinum
1. Spore forming organism survive in the drug; areas of repeated skin
popping may be necrotic and allow organism to germinate.
2. Organism may be recovered but demonstration of toxin production of the
isolate would be needed to identify as C. botulinum; test for toxin in serum
has a sensitivity of 50% in one large study; culture is positive in 75% of
patients with toxin in serum.
Clostridium tetani
1. Much less common in this setting than C. botulinum but it has been
described.
2. Organism is extremely oxygen sensitive so we will not isolate it: don’t
know of a clinical lab that has ever isolated this organism.
3. diagnosis then is made on clinical grounds.
Cellulitis associated with water exposure
Warm salt water- (NC in the summer time)
Vibrio vulnificus
1. halophile but will grow on primary isolation media; oxidase positive
2. cellulitis associatd with trauma after exposure to salt water in summer
months; can progress to necrotizing faciiitis although I can only remember
one case in 20+ years here
3. systemic disease seen most commonly in individuals with iron over load
disease including cirrhosis; usually associated with consumption of raw
oyster
Aeromonas hydrophilia
1. Usually associated with fresh water or brackish water infections secondary
to trauma; grows on MacConkey; glucose fermenter /oxidase positive
2. typically not as aggressive an infection as V. vulnificus but may also
progress to myonecrosis
3. organism is resistant to ampicillin
Necrotizing fasciitis/myonecrosis/gas gangrene/pyomyositis
Gram stains can be very valuable in the diagnosis of these conditions. What is
seen on gram stain and what you should consider:
Gram positive cocci in chains- Group A streptococci- add clindamycin for
toxic shock syndrome;
Gram negative rods only- Vibrio vulnificus and Aeromonas hydrophiliashould be considered- doxycycline is used to treat Vibrio vulnificus
Mixed gram positive and negative organisms- mixture of aerobic and
anaerobic bacteria-usually contiguous with GU or GI tract- work up of cultures
will need to be directed because multiple organisms will be present
Gram positive rods- no spores- no wbcs- Clostridium perfringensGram positive rods-subterminal spores- few or no white blood cells-C.
septicum- major cause of gas gangrene in the absence of prior trauma- patients
have a high risk of GI malignancy.
Gram positive cocci in clusters-Staphylococcus aureus- will be seen in
association with pyomyositis
Blood cultures are frequently positive in these more invasive infections.
Patients with gas gangrene may have positive blood cultures after only six hours
of incubation; complete lysis of all the blood cells in the culture bottle is
characteristic of Clostidium perfringens; C. perfringens is aerotolerant so may
also grow in aerobic blood culture bottle so don’t be fouled into thinking it is a
Bacillus if you hear the culture is growing aerobically.
Miscellanous agents of acute skin and soft tissue infections
Cat scratch disease-Bartonella henselae or quintana- will grow on chocolate
agar after 2 to 3 weeks incubation-we have grown it once from blood and never
have had success from a lymph node and know of few folks who have had
success.
Erysipelothrix rhusiopathiae-gram positive rod-can be confused with
lactobacilli because it is vancomycin resistant and has a similar gram stain
morphology and is catalase negative-is H2S positive which is a distinctive
characteristic of the organism associated with pigs and fish.
Chronic Infections- These infections are typically found either as a result of
infection with an organism of low virulence or the result of poor wound healing
secondary to peripheral vascular disease with diabetes being most frequent.
Organisms of low virulence that cause chronic infection
Mycobacterium marinum1. photochromogen(produces pigment only in the presence of light)
2. typically associated with freshwater and brackish water exposures
3.infections typically on the extremities- grows best at 30 C so if you are consider
this organism in your differential you need to tell use so we can use appropriate
growth conditions
4. draining sinuses may occur.
Rapidly growing mycobacterium-M. chelonae-abscessus group- will be the
topic of an entire Friday morning lecture so it will not be discussed today but to
say it is a problem following cosmetic surgery and has been associated with the
post-injection infections with certain alternative medicines and secondary to
acupuncture.
Nocardia and Actinomyces1. Both may appear as branching gram positive rods but we think that
Nocarida is more likely to be branching.
2. Nocardia is a strict aerobe; Actinomyces can be aerotolerant;
3. Nocardia grows both on fungal and mycobacterial media as well as bacti
media so if you think of it afterwards and want us to hold plates that is OK;
also like BCYE (the legionella medium) w/o antibiotics as a primary
isolation medium
4. Actinomyces usually associated with chronic head and neck infection; it is
typically an endogenous infection
5. Nocardia- infections anywhere on the body-environmental organismdiscuss being associated with draining sinuses and sulfur granules but we
just never see them; frequently will have nodules to go with draining
sinuses
6. Treatment is different and prolonged so indentifying properly is important;
After a week Nocardia will have an earthy odor which can stink up the
whole incubator; they are identified by sequencing; susceptibility done at
reference lab. This phenotype makes it fairly easy to differentiate from
Actinomyces.
Fungal agents
In immunocompromised patients, organisms to consider include dimorphics,
Trichosporon (can give a false positive crypto antigen), Cryptococcus,
Aspergillus, zygomycetes, Candida, and Fusarium
In immunocompetent with draining sinus consider Sporothrix (again must
incubate at lower temperature to isolate) and dematiceous fungi
Chronic wounds as result of poor wound healing
Most common in individuals with vascular disease, secondary to diabetes.
Infections typical found in the extremities where there is poor perfusion.
Stages in the development of chronic infection
Contamination- organisms are present but are not replicating
Colonization- organism present in the wound but are not causing tissue
damage; colonization does not slow wound healing
Local infection/critical colonization-believed to be a transition state between
colonization and infection- traditional signs of infection are absent but wound
healing is delayed
Infection-presence of > 105 cfu/ml (exceptions are GAS and GBS were lower
numbers cause infection) of replicating bacteria that are causing tissue injury
The presence of 4 or more different organism also correlates with non-healing
If biopsies are done, they should include culture for anaerobes; predominant flora
include S. aureus, Pseudomonas aeruginosa and Peptostreptococcus.
Organisms are believed to grow in a mixed bioflim mode of growth making
antibiotic therapy more difficult.