Describe the laboratory diagnosis of pyogenic wound infection and abscess.

Collection

• Pus or tissue is far superior to a swab, and this is the single most important point. Aspirate pus into a syringe, expel the air, cap the needle or transfer into a sterile container, and send several millilitres; a swab carries a fraction of the material, dries out, and preferentially samples the surface
• Sample the deep part of the wound or the advancing edge, after cleaning and debriding the surface; a swab of the surface of a chronic ulcer grows colonising flora and is essentially uninterpretable
• Tissue from the base of a diabetic foot ulcer or from bone is the correct specimen in osteomyelitis, and superficial swabs correlate poorly with what is found in bone
• For anaerobes — send pus in a capped syringe or an oxygen-free transport vial, promptly and without refrigeration; anaerobes die on exposure to air, and a swab in ordinary transport medium is the commonest reason anaerobic culture fails
• Take specimens before antibiotics and before antiseptic application

Examination

• Macroscopic — colour, consistency and smell, which are genuinely informative: a foul, putrid odour indicates anaerobes; blue-green pus with a sweetish smell suggests pseudomonas; thick creamy yellow pus is typical of S. Aureus; anchovy-sauce pus indicates an amoebic abscess; and grains indicate mycetoma, whose colour and microscopy distinguish actinomycetoma from eumycetoma. sulphur granules indicate Actinomyces
• Gram stain — rapid and directive: clusters of gram-positive cocci (staphylococci), chains (streptococci and enterococci), gram-negative bacilli, and a mixed flora with pus cells suggesting anaerobes. Pus cells without organisms may indicate prior antibiotics, tuberculosis, or an anaerobic or fastidious organism
• Other stains — ZIEHL-neelsen (and a modified method for Nocardia, which is weakly acid-fast); KOH and calcofluor for fungi; and Giemsa
• Culture — blood agar, MacConkey and an enrichment broth aerobically; anaerobic culture on blood agar with neomycin in an anaerobic jar or cabinet for 48 hours or longer; Sabouraud agar for fungi; and mycobacterial culture where indicated
• Common organisms — Staphylococcus aureus (the commonest in skin and soft tissue), Streptococcus pyogenes (spreading cellulitis and necrotising infection), gram-negative bacilli and anaerobes in abdominal, perineal and diabetic foot infection, Pseudomonas in burns and moist wounds, and rapidly growing mycobacteria in post-surgical and post-injection abscesses

Special Situations and Applied Aspects

• Necrotising soft tissue infection is a surgical emergency in which pain out of proportion to the signs, systemic toxicity, crepitus and rapid progression demand immediate exploration; the diagnosis is made in theatre and delaying for imaging or culture costs limbs and lives
• Gas gangrene — Clostridium perfringens; a Gram stain showing large gram-positive bacilli with few pus cells (the alpha-toxin destroys leucocytes) is highly suggestive and available in minutes
• Post-surgical and post-injection abscess clusters in India are characteristically caused by rapidly growing mycobacteria (M. Chelonae, M. Abscessus, M. Fortuitum) from contaminated disinfectant, instruments or injectable solutions; they are acid-fast, grow in a week rather than the usual weeks, and are missed unless specifically sought
• The primary treatment of an abscess is drainage; antibiotics are adjunctive, and no antibiotic penetrates a collection reliably
• Send pus, not a swab — the single change that would most improve the quality of wound microbiology in routine practice
• Request anaerobic culture explicitly and provide a suitable specimen; anaerobes are involved in most abdominal, pelvic, dental and diabetic foot infections and are routinely under-reported because the specimen never permitted their growth
• Smell the specimen — a putrid odour is strong evidence of anaerobes and is available before any test, and blue-green pus points to Pseudomonas
• Culture bone, not the overlying ulcer, in suspected osteomyelitis of the diabetic foot; superficial swabs correlate poorly with the organisms actually in the bone and lead to the wrong antibiotic
• Consider rapidly growing mycobacteria in a cluster of post-operative or post-injection abscesses that fail to respond, and request acid-fast staining and culture specifically
• Pus cells without organisms on Gram stain suggest prior antibiotics, tuberculosis, an anaerobe or a fastidious organism, and should prompt further staining rather than a report of "no growth"
• Do not delay surgery for microbiology in necrotising infection; the diagnosis is made in theatre and specimens are taken there
• Send several millilitres where pus is available; volume matters here as in blood culture, and a swab dipped into an abscess wastes what has already been obtained