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Current molecular techniques fail to detect bacterial source of cellulitis
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Researchers were unable to determine the bacterial etiology of cellulitis using current molecular techniques to compare pathogen prevalence and quantity in infected and uninfected skin biopsy specimens, according to recently published data.
Researchers examined the biopsy specimens of 50 adult patients with acute cellulitis and no drainage who presented to an ED research network. Along with demographic and case information, specimens from each patient’s infected site and an uninfected site on the opposite side of the body were obtained for each patient. Microbiology of these samples was determined using quantitative PCR, pyrosequencing and standard culture techniques. The predominant bacterial species from both sites then were compared to determine etiology.
Culture findings revealed that in 50 paired specimens, 84% had no growth, and most isolated organisms were skin colonizers or contaminants. Using culture, researchers identified methicillin-susceptible Staphylococcus aureus (MSSA) in 4%, but neither MRSA nor S. pyogenes was identified by culture.
Among 49 patients tested by PCR, researchers identified MSSA in 41% of the infected-site specimens and in 35% of the uninfected-site specimens. MRSA and S. pyogenes were undetectable in any infected-site specimen using PCR.
In 49 patients tested by pyrosequencing, researchers found S. aureus in 24% of infected-site specimens and 33% of uninfected-site specimens as well as streptococci in 31% of infected-site specimens and 41% of uninfected-site specimens. S. pyogenes was not detectable using pyrosequencing.
“Pyrosequencing identified abundant atypical organisms, including staphylococci and streptococci, yet no outstanding representation of any particular species,” the researchers wrote. “From infected site specimens, S. pyogenes was not identified by any technique, and MRSA was not identified by techniques that could discriminate it from MSSA, ie, culture or PCR.”
According to the researchers, clinical trials currently should be used for optimal treatment of cellulitis, and noninfectious causes also should be explored. – by Tina DiMarcantonio
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Adam Friedman, MD, FAAD
The nonspecific physical examination findings of cellulitis can make diagnosis a challenge, especially when trying to distinguish cellulitis from a cellulitis mimicker (pseudocellulitis), such as stasis and contact dermatitis. Given the depth of inflammation and infection associated with cellulitis, samples are not as readily available for diagnostic evaluation as more superficial bacterial cutaneous infections, including impetigo (bullous or nonbullous) or viral infections such as those caused by human herpes virus 1 and 2.
In the current study by Crisp and colleagues, this diagnostic difficulty was further highlighted when even complex, expensive research tools generally not available in practice were utilized to detect the infectious etiology of cellulitis and still fell short. In clinical dermatology practice, we tend not to biopsy cellulitis, and will only do tissue cultures if we suspect a less common pathogen, whether it be due to the clinical presentation, patient's history (including travel and past injuries/surgeries) or other medical problems.
There are two take home messages from this study:
1. Confirm your diagnosis — make sure you are actually dealing with cellulitis and not treating an allergic contact dermatitis with broad spectrum antibiotics. Our diagnostic armament according to Crisp and colleagues’ study is clearly insufficient, so you may ask, “How?” Consult your dermatologist. In a recent study by Strazzula and colleagues, more than 70% of cases evaluated by the dermatology consult service that were first diagnosed as cellulitis were actually mimickers, aka pseudocellulitis (Strazzula, et al. J Am Acad Dermatol. 2015;doi: 10.1016/j.jaad.2014.11.012). The "red leg" differential diagnosis can be broad and difficult to discern, which is why recruiting the help of a dermatologist can be crucial in both initiating the correct treatment and avoiding overuse of antibiotics, which would ultimately limit the persistent rise in antimicrobial resistance.
2. Know your bugs — it is important to have access to or be aware of which organisms commonly cause cellulitis as well as to know antibiotic resistance patterns in the area in order to empirically select the right antibiotic. Though MRSA gets more press, group A streptococci more commonly cause cellulitis, as well as erysipelas (a more superficial form of cellulitis). Strep is less sensitive to trimethoprim-sulfamethoxazole than MRSA, and therefore selecting this antibiotic due to the assumption that everything is MRSA, and not clindamycin for example, results in greater treatment failures (Elliot DJ, et al. Pediatrics. 2009;doi:10.1542/peds.2008-2428).
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