M12 - Infections of skin, soft tissues, and the musculoskeletal system (handout).pdf

Infections of the skin, soft tissues, and musculoskeletal system Dr Samson Wong Department of Microbiology BMSC3104 BMSN2603 Dr Samson Wong Department of Microbiology Learning objectives • Describe the defence mechanisms of the normal skin against infection. • Recognize the different types of sk...

Infections of the skin, soft tissues, and musculoskeletal system Dr Samson Wong Department of Microbiology BMSC3104 BMSN2603 Dr Samson Wong Department of Microbiology Learning objectives • Describe the defence mechanisms of the normal skin against infection. • Recognize the different types of skin and soft tissue infections with respect to the anatomical site of involvement. • Understand the predisposing factors for surgical wound infection. • Describe the pathology, aetiology, clinical manifestations of osteomyelitis. • Describe the pathology, aetiology, clinical manifestations of infective arthritis. Hairs Hair follicles Nails Sebaceous glands Epidermis Dermis Deep fascia Subcutaneous tissue Muscles Tendons Bones Joints • Common organisms. • Staphylococcus spp. • Corynebacterium spp. • Micrococcus spp. • Propionibacterium acnes. Resident skin flora • Gram negative bacilli (e.g. Acinetobacter). • Yeasts (e.g. Candida, Malassezia furfur). • Predominant skin flora varies with: • Age. • Body location. • Occupation. • Hospitalization. • Medications. • Diseases (local and systemic). • Normal integrity of the skin. • Rapid cell turnover. • Normal flora of the skin. • Antimicrobial effect of the lipid layer (sebumderived) of normal skin. Defence at the skin • Breach of the normal integrity of the skin. • Alteration of normal skin flora. • Changes in the local environment of the tissues, e.g. devitalized tissues, haematoma, foreign bodies, etc. Pathogenesis of skin and soft tissue infections • Introduction of exogenous microbial flora. • Colonization or infection of the keratinized tissues by a dermatophytic fungus (keratinolytic). • Trichophyton spp. Microsporum spp. Epidermophyton floccosum • Diagnosis Dermatophytosis (ringworm) • KOH wet mount. • Culture. Tinea faciei caused by Trichophyton rubrum Hay RJ. Dermatophytosis (ringworm) and other superficial mycoses. In: Bennett JE, Dolin R, Blaser MJ (ed). Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 8th ed. Elsevier. Impetigo Stevens Dl. Cellulitis, pyoderma, abscesses, and other skin and subcutaneous infections. In: Cohen J, Powderly WG, Opal SM (ed). Infectious Diseases, 4th ed. Erysipelas Cellulitis Stevens Dl. Cellulitis, pyoderma, abscesses, and other skin and subcutaneous infections. In: Cohen J, Powderly WG, Opal SM (ed). Infectious Diseases, 4th ed. Rigopoulos D, Larios G, Gregoriou S, Alevizos A. Acute and chronic paronychia. Am Fam Physician 2008;77:339–346. Leggit JC. Acute and chronic paronychia. Am Fam Physician 2017;96:44–51. Carbuncle of the buttock caused by Staphylococcus aureus. Stevens Dl. Cellulitis, pyoderma, abscesses, and other skin and subcutaneous infections. In: Cohen J, Powderly WG, Opal SM (ed). Infectious Diseases, 4th ed. Pyoderma Normal skin Folliculitis Furuncle Carbuncle • An acute spreading infection of the skin extending to involve the subcutaneous tissues. • Causative agents • Streptococcus pyogenes • (Staphylococcus aureus) Cellulitis • Vibrio, Enterobacteriaceae, other Gram negative bacilli. • Mostly preceded by trauma or an underlying skin lesions. • Local signs of inflammation, ill-defined margin of inflammation, local abscess, fever, chills, bacteraemia. • Deep and multiple tissue levels (dermis, subcutaneous fat, deep fascia, muscle). • Various forms • Myonecrosis (e.g. gas gangrene). • Necrotizing fasciitis. Necrotizing soft tissue infections • Thrombosis of blood vessels perforating the fascial envelope. • Extension of area of necrosis under the skin. • Type I • Anaerobic bacteria (e.g. Bacteroides) + facultatively anaerobic bacteria (e.g. streptococci, Enterobacteriaceae). • Type II Aetiology of necrotizing fasciitis • Streptococcus pyogenes ± other organisms, e.g. Staphylococcus aureus. • Type III • Vibrio and related species (e.g. Aeromonas spp). • Commonly: Vibrio vulnificus. • • • • Pyomyositis Abscess in skeletal muscles. Uncommon. Staphylococcus aureus, other bacteria. Psoas abscess. • Spectrum of pathogens depends on the type of operation. • Examples • Abdominal and pelvic surgery • E. coli, Proteus spp., Klebsiella spp., etc. Surgical wound infection • Streptococci, enterococci; anaerobes. • Orthropaedic and neurosurgery • S. aureus, coagulase-negative staphylococci. • Hospital-acquired • Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia; Candida albicans. • Osteomyelitis • An infectious process involving the various components of bone, viz. periosteum, medullary cavity, and cortical bone. • Acute osteomyelitis. • Chronic osteomyelitis. Infection of the bone • Osteomyelitis associated with prosthetic implants. • Acute osteomyelitis. • Haematogenous. • Contiguous focus. • Chronic osteomyelitis. • Uncontrolled acute osteomyelitis. Osteomyelitis: routes of infection • Haematogenous (e.g. Mycobacterium tuberculosis). • Prosthesis-related infection. • Early: introduced during operation or from postoperative wound infection. • Late: haematogenous. Acute inflammation Obliteration of vascular channels Sequestrum Ischaemia and necrosis Lifting of periosteum from bone Lew DP, Waldvogel FA. Osteomyelitis. Lancet 2004;364:369–379. • Generally monomicrobial infection. • Staphylococcus aureus. • Acute vertebral osteomyelitis • Usually haematogenous. Acute haematogenous osteomyelitis • Usually involves 2 adjacent vertebrae and the intervertebral disk. Mandell’s Colour Atlas of Infectious Diseases • Predisposing factors • Trauma, surgical operations of bones, open fractures, chronic soft tissue infections. • Vascular insufficiency, e.g. diabetes mellitus. • Often a polymicrobial infection. Contiguous focus osteomyelitis • Clinical • Radiological procedures • Plain X rays. • Changes on X rays lag at least 2 weeks behind the evolution of disease. • Computed tomography (CT). Osteomyelitis: diagnosis • Magnetic resonance imaging (MRI). • Radionuclide imaging • Microbiological diagnosis • Blood culture. • Biopsy of the bone lesion. • In chronic osteomyelitis, culture of sinus tract is not reliable for predicting the organisms causing osteomyelitis. Osteomyelitis: diagnosis • Infective arthritis • Acute vs. chronic arthritis. • Monoarthritis (90%) vs. polyarthritis (10%). • Native vs. diseased vs. prosthetic joints. • Age of patient and underlying diseases. Infective arthritis Circulation Adjacent soft tissue infection Subchondral bone Penetration through the skin Mandell’s Colour Atlas of Infectious Diseases • Pain, limitation of movement at the joint. • Fever, joint swelling; leukocytosis. • Usually affects larger joints: knee, hip, shoulder, ankle, elbow; but any joint can be involved. Infective arthritis: clinical features • Polyarthritis • Rheumatoid arthritis, systemic lupus erythematosus, diabetes mellitus, steroid therapy. • Viruses, gonococcus. • In most age groups, Staphylococcus aureus is the commonest pathogen. • Neisseria gonorrhoeae. • Gram negative bacilli. Acute bacterial arthritis • Elderly, chronic debilitating disease, underlying chronic arthritis. • Intravenous drug addicts (sometimes Pseudomonas aeruginosa). Russ S, Wrenn K. Disseminated gonococcal infection. N Eng J Med 2005;352:e15. • Differential diagnosis. • Rheumatoid arthritis. • Crystal-induced arthritis. • Gouty arthritis: monosodium urate. • Pseudogout: calcium pyrophosphate. Infective arthritis: diagnosis • Trauma; osteoarthritis; tumour. • Blood culture. • Diagnostic synovial fluid aspirate. • Leukocyte count, crystals, Gram stain, culture. • Synovial biopsy. Monosodium urate crystal Pascual E, Sivera F, Andrés M. Synovial fluid analysis for crystals. Curr Opin Rheumatol 2011;23:161–169.