Laboratory diagnosis of skin and soft tissue infections

 

Skin and soft tissue infections

Learning objectives:

Different types of Skin & soft tissue infections & their aetiological agents

Laboratory methods to identify the aetiology of various SSTI to guide management          

 

Skin and soft tissue infection (SSTI) is a comprehensive term which includes all the kinds of infections which involve skin, muscle and connective tissues. These may be associated with involvement of deeper parts of body like bones and joints.

Various types of SSTI and their aetiology

Type of infection	Description	Aetiological agents
1. Folliculitis	Infection and inflammation of hair follicle	Bacteria: Staphylococcus aureus, Pseudomonas aeruginosa Fungus: Candida albicans, T.rubrum, Malassezia
2. Furuncles (Boil)	Skin infection with painful swelling and collection of pus.	Staphylococcus aureus
3. Carbuncles	Individual boils clustered together leading to large infected mass filled with fluid, pus and dead tissue	Staphylococcus aureus
4. Cellulitis	Diffuse inflammation of connective tissue with involvement of dermal and subcutaneous layers of skin	Staphylococcus aureus & other staphylococci Streptococcus pyogenes
5. Impetigo	Highly infectious bacterial skin infection commonly seen in pre-school children	Streptococcus pyogenes Staphylococcus aureus
6. Erysipelas	Acute infection of skin and superficial lymphatics. Lesions are typically raised and demarcated.	Streptococcus pyogenes
7. Necrotizing infections (Necrotising fascitis)	Also known as flesh eating disease. It is a rapidly progressing infection of skin and subcutaneous tissue which can spread across the fascial plane.	Streptococcus pyogenes, Staphylococcus aureus, Clostridium perfringens ( may lead to gas gangrene). Majority times it is a polymicrobial infection.
8. Surgical site infections	Infection at the site of surgery which occurs within 30 days after operation.	Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, members of enterobacteriaecae family, etc.
9. Pustule / Abscess	Collection of pus due to inflammatory processes	Staphylococcus aureus is most common. M.tuberculosis is associated with cold abscess.
10. Myositis and pyomyositis	Infection of muscle with or without pus collection.	Only myositis in absence of other surrounding site involvement is more commonly associated with autoimmune disorders rather than directly due to infection. Pyomyositis is usually caused by Staphylococcus aureus
11. Infection following animal contacts		B. anthracis (cutaneous anthrax), Bartonella henslae (Cat scratch disease), E.rhusiopathiae (Erysipeloid)
12. Infection after animal bite		Mixed aerobic and anaerobic bacteria, Rabies virus
13. Infection after human bite		Mixed aerobic and anaerobic bacteria
14. Mycetoma	Chronic subcutaneous infection characteristically associated with multiple sinuses which discharge typical grains	Eumycetoma: Caused by fungi (e.g. Madurella mycetomatis)   Actinomycetoma: Caused by bacteria (e.g. Nocardia sp.)
15. Cutaneous ulcers	Loss of epidermal & part of dermal tissues.	B. anthracis, M. marinum,
16. Burn wounds	Generally associated with bacteremia	Pseudomonas aeruginosa, members of Enterobacteriaceae family.
17. Systemic infections with skin manifestations		a) Petechiae in meningococcaemia. b) Cutaneous ulcers & bullae in V. vulnificus bacteremia.
18. Leprosy	Systemic disease primarily involving skin, peripheral nerves and nasal mucosa.	Mycobacterium leprae
19. Macule	Flat, non-palpable discoloration of skin (<5 cm size). If size exceeds 5 cm, is called as patch	Dermatophytes Viral rashes (e.g. enterovirus, measles)
20. Papule	Elevated lesions usually <5 mm in size that can be felt or palpated	·       Virus:  Molluscum contagiosum,  Warts (Human Papilloma virus) ·       Parasite scabies (Sarcoptes scabiei)
21. Plaque	Multiple papules my become confluent to form plaque which are palpable lesions >5 mm
22. Nodule	Firm lesions >5 cm size	·       Bacteria Staphylococcus aureus, Mycobacterium marinum ·       Fungi sporothrix
23. Vesicle	Fluid-filled lesions with a diameter less than 0.5 cm	Herpes simplex virus,  varicella-zoster virus
24. Bulla	Fluid-filled lesions with a diameter more than 0.5 cm	·       Bacteria Clostridium Staphylococcus aureus ·       Virus Herpes simplex virus
25. Scale	Excess dead epidermal layer	Dermatophytes Streptococcus pyogenes

 

Laboratory diagnosis:

Sample collection:

            1.        If there is subcutaneous or deep pus collection and pus is to be collected through intact skin, It should be done with sterile syringes and needle. Prior to collection, the surface area should be cleaned with alcohol. Sample collected in syringe should be immediately transported to laboratory in it.

            2.        If the pus is discharged through the skin, it should be collected with sterile swab. Prior to collection of sample, the surface area should always be cleaned with sterile saline (never use antiseptics). The pus is squeezed and collected with swab.

            3. Skin scrapings or biopsy may be required to collect particularly in dry lesion. In case of leprosy, skin smear with nasal smear is required or biopsy from nodular skin lesion and thickened nerves is recommended in some cases.

 

Laboratory procedures:

 

1. Microscopic examination:

            a. Gram stain: To screen for bacteria

            b. KOH examination: To screen for fungus

            c. Z N stain: For demonstration of M. tuberculosis Atypical mycobacteria. Modified acid fast   stain required for diagnosis of suspected case of leprosy.

 

3. Culture examination:

            According to clinical presentation & type of lesion, the Samples should be cultured in one or more of following culture media to identify the aetiology of lesion

                        a. Simple media: Nutrient agar

                        b. Differential media: Mac-Conkey agar

                        c. Enriched media: Chocolate agar / Blood agar

                        d. Anaerobic media: Robertson's cooked meat broth

                        e. L J medium: For atypical Mycobacteria

                        f. Sabouraud's dextrose agar

 

Exercise

A 5 year child presented with multiple painful swellings over the leg and thigh. Pus is discharging from few lesions.

Q.1     How the samples are collected? (In practical / exam students should identify the materials used for sample collection and transportation)

Q.2     Which culture media are used for isolation of possible aerobic and anaerobic bacterial pathogen (In practical / exam students may be asked to identify these media and bacterial growth)

Q.3     Which antibiotics should be tested for its sensitivity against isolated bacteria? (In practical / exam students may be asked for selection of appropriate antibiotic combination circle from the tray; and materials required for sensitivity testing)