support of the laboratory for the diagnosis of skin ... symposium/dedis… · diagnosis of skin...
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SBIMC-BVIKM and Société belge de / Belgische Vereniging voor dermatologieMarch 11, 2006
Support of the laboratory for the diagnosis of skin infections :
The lab point of vue
Anne DedisteMicrobiology unitCHU Saint-Pierre
Bruxelles
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A few words about stool collection procedures for parasites
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Triple Faeces Test
•SAF preservation fluid for ova and trophozoites.
•Samples collected during 3 consecutive days.
•We request to provide the stool with clinical information.
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TRIPLE FAECES TEST
Bottle 1 with SAF
direct wet smear*
Empty bottle 2 Bottles 3a with SAF
Auramine Cryptosporidium
Ridley concentration
At Day 3 the complete set is returned to the laboratory
direct wet smear*
direct wet smear*
∗ If suspect: Permanent staining with Chlorazol black
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Recovery of parasites • comparison of one fresh stool specimen vs TFT :
from 10.33% to 28.07% positive cases, for both pathogenic and non-pathogenic protozoans.
Vandenberg et al. submitted to CMI. 2006
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Diagnosis of skin infection : is the bacteriology lab useful?
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NORMAL OR RESIDENT SKIN FLORA
• 1.5m2 skin normally colonized within the first few days after birth predominantly with
– Coagulase negative Staphylococci– Corynebacterium spp.– Micrococcus spp.– Propionibacerium spp.
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And less frequently
– Acinetobacter spp. (25% population)– Staphylococcus aureus (external nares of 30% pop.)– Saprophytic Mycobacteria– Malassesia – Candida spp.– Enterobacteriaceae– Bacillus spp.– …..
P. Murray et al. Clinical Microbiology Handbook. 8th ed. 2003.
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PREANALYTICAL STEPS
• The purpose of a sampling must be defined prior to its collection.
• Don’t swab to know IF there is a microbial skin infection.
• Don’t forget the microbiologist can be associated to the prescription.
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HELPFUL CLINICAL INFORMATION FOR THE MICROBIOLOGIST.
• Origin of the lesion : trauma, animal or human bite, gardening wound, decubitus ulcer,genital ulcer, post surgical infection, …
• Travel.• Duration of the illness : acute vs chronic.• Location of the lesion.• Underlying disease : diabetes, HIV,
immunocopromised patient…
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WHY?
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• To inoculate appropriate selective culture media and choose the specific growth conditions.
• To search the most probable involved pathogens including viruses, fungi, mycobacterias and parasites.
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HENCE
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Don’t hesitate to call the microbiologist to discuss the best approach to diagnosis prior to sending specimens to the laboratory.
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WHAT IS THE ROLE OF THE MICROBIOLOGIST?
• Trying to find the aetiological agent of an infection. • To discuss together with the clinician over the most
useful tests to reach this goal : bacterial and viral cultures but also serology, specific stainings, direct immunofluorescence, molecular biology, …
• To perform ID and AST on relevant isolates only :– many wound infection are polymicrobic, there is little need
to identify them all.– AST on non relevant specimens may lead to an
inappropriate (over)use of antbiotics.
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WOUND SAMPLES
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COLLECTION OF CLINICAL SPECIMENS (1)
• Superficial wounds are always colonized by commensal flora.
• Before swabbing, clean wound surface with 70% alcohol or non bacteriostatic sterile saline.
• Prefer pus or fluid aspirate in a syringe, deep swabbing or punch biopsy of the leading edge of the lesion.
JM. Miller. A guide to specimen management in clinical microbiology. 2nd ed. 1999. H. Isenberg. Clinical Microbiology Procedures Handbook, 2nd ed. 2004.
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COLLECTION OF CLINICAL SPECIMENS (2)
• Syringes must be capped with a sterile closure. Syringes with needles in place are inacceptables!
• Virus cultures require specific transport media.
• Don’t forget to specifically request yeasts and molds and mycobacterial cultures .
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Tinea corporis
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M. marinum
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COLLECTION OF CLINICAL SPECIMENS (3)
• For bacteriological cultures, always use swabs with transport medium (gel or wet sponge) to prevent drying of the specimen and to preserve the bacteria.
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Swab with transport medium
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DRY SWABS
SHOULD NEVER BE USED FOR BACTERIOLOGICAL CULTURE
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COLLECTION OF CLINICAL SPECIMENS (4)
• If anaerobic bacteria are suspected to be involved in an infectious process, prefer fine needle aspiration or tissue biopsy since most ordinary swabs are not valid transport media for these pathogens.
• Samples for diagnostic and epidemiologic surveillance (eg MRSA) are to be differentiated.
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NEVER USE A SWAB IF PUS OR
LIQUID SECRETIONS ARE AVAILABLE
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PLEASE REMEMBER
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Homepathy is based on dilutions principle :
The more diluted, the more efficient!
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Microbiology is quite different :
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The greater amount of microorganisms,
the better recovery of them in culture
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• Insufficient material may yield false-negative results.
• The more different analysis you want, the greater amount of infected material you have to provide!
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REMEMBER ALSO
• Some microorganisms are not cultivable.
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2nd Syphilis
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Lyme disease
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Leprosy
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Myiasis
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REMEMBER ALSO
• Some microorganisms are not cultivable.
• Rare diseases are rare but yet do really exist!
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Anthrax :Bacillus anthracis
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Tularemie
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WORST SPECIMENS FOR BACTERIOLOGICAL CULTURE (1)
• Superficial swabbing of a wound :
– Yield many squamous epithelial cells and little PMNs on microscopic examination.
– Is of little diagnostic value because of presence of many skin commensals.
– Isolated organism(s) may only be colonizing ones not involved in the infective process.
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The presence of epithelial cells indicates contamination of the specimen with skin microbiota and compromises the significance of the culture results.
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Superficial swabbing of a wound
Usually no identification and no antimicrobial susceptibility testing on such samples.
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WORST SPECIMENS FOR BACTERIOLOGICAL CULTURE (2)
• Don’t submit decubitus ulcers swabs in order to establish an aetiological diagnosis.
These wounds are always colonized with resident, transient or fecal flora.
But this culture is useful to look for presence of MRSA.
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REJECTION CRITERIA FOR MICROBIOLOGICAL SPECIMENS :
SOME EXAMPLES
• Specimens recieved in fixative. Exception : stool for parasites.
• Dry swab.• Only one swab for multiple requests.• Anaerobic cultures on decubitus ulcer material.
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BEST SPECIMENS FOR BACTERIAL CULTURE
• From clinically infected or deteriorating wounds.• Whose result will influence therapy.• Specimens collected prior to initiation of therapy.
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BEST SPECIMENS FOR BACTERIAL CULTURE
Tissues collected during surgery.Aspirates through intact skin by needle and syringe or fine-needle biopsy eventually after irrigation with non bacteriostatic saline.Samples from viable infected tissues avoiding necrotic aeras.
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SOME EXAMPLES
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CLINICAL IMPRESSIONS AND MAIN POTENTIAL PATHOGENS
Impetigo, echhyma Strep. pyogenes, Staph. aureus
Folliculitis, furuncles Staph. aureus
Erysipelas Strep. pyogenes
Cutaneous abcess Staph. aureus, Staph. epidermidis, Streptococcus spp., Propionibacterium spp., Peptostreptococcus spp., Leishmania tropica or brasiliensis
Nodules, papules, subcutaneous tissue involvement
Various bacteria, mycobacteria, yeasts and molds, viruses, parasites
Tinea Various dermatophytes
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Impetigo : Staphylococcus aureus
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Impetigo : Streptococcus βA
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DIABETIC FOOT INFECTION
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DIABETIC FOOT INFECTION
Culture for infecting organisms remains problematic :• Deep tissue cultures (surgically obtained) seem to
provide the most accurate results.• Poor correlation was demonstrated between deep
tissue culture and other modalities (ulcer swabs, draining material, …).
• Most patient have several isolates, all of them are not involved in the infective process.
Sapico et al.JCM 12:413, 1980
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ERISYPELAS
Acute skin infection caused by Streptococcus pyogenes.Culture of the lesions
– lacks of sensitivity • Needle aspiration of the lesion 5% culture +• Punch biopsy of the skin 20% culture +
– is frequently contaminated with normal skin flora and Gram stain is not contributive.
– its interest is mainly epidemiological.Blood cultures
– seldom positive (5% in febrile patients).
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TAKE HOME MESSAGE
Overuse of microbiology lab can lead to its misuse
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CONCLUSION
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