Classification of wounds
Potential for infection
The best measure of wound contamination at the end of an operation, and the risk of developing infection, is to sample tissue in the wound edge. Bacteria will already be affected if antibiotic prophylaxis has been given, but the theoretical degree of contamination relates well to infection rates (Table 7.6). When wounds are heavily contaminated or an incision is made into an abscess, continuing prophylaxis as therapy is justified. Infection rates after nonprosthetic clean surgery may be higher if looked for. Antibiotic prophylaxis is controversial.
Bacteria involved in wound infection
Streptococci form chains and are Gram positive on staining (Fig. 7.16). The most important is the 13-haemobytic streptococcus which resides in the pharynx of 5—10 per cent of the population. It is in group A of the Lancefiebd A—G carbohydrate antigens. The alternative name of Streptococcus pyogenes is deserved because of its tendency to spread (cellubitis) and to cause tissue destruction through release of stretoptolysin, streptokinase and streptodornase. Strepococcus faecalis is the enterococcus in Lancefield group D (often found in synergy with other organisms) and the y-haemolytic (no haemobysis on blood agar) peptostreptococcus is an anaerobe. Both may be involved in wound infection after large bowel surgery. The ct-haemolytic Streptococcus viridans is not related to wound infections. The streptococci are still sensitive to penicillin; erythromycin and cephabosporins are alternatives in case of allergy.
Staphylococci form clumps and are Gram positive (Fig. 7.17). Staphylococcus aureus is the most important pathogen in this group and resides in the nasopharynx of up to 15 per cent of the population. It can cause exogenous suppuration in wounds (and implanted prosthetics), and MRSA can be involved in epidemics. Doctors and nurses may need to be swabbed and carriers identified and treated in an epidemic. Infections are usually localised (see Wound abscess above). Most hospital S. aureus strains are now f3-lactamase producers and are resistant to penicillin. Sensitivity to flucloxacillin, vancomycin, aminoglycosides, some cephabosporins and fusidic acid (used in osteomyelitis) is still high.
Staphylococcus epidermidis (syn. albus and most conventionally coagulase-negative Staphylococcus) was regarded as a commensal but is now recognised as a major threat in prosthetic (vascular and orthopaedic) surgery. It exists in hospitals as a nosocomially acquired organism MRCNS and is resistant to many antibiotics.
Clostridial organisms are Gram-positive, obligate anaerobes which produce resistant spores (Fig. 7.18). Clostridium perfringens is the cause of gas gangrene (Specific wound infections above). Clostridium tetani causes tetanus following implantation in the tissues or a wound by release of the exotoxin tetanospasmin. A short prodromal period is related to development of severe spasms including opsithotonus, respiratory arrest and death. A longer prodromal period of 4—5 weeks is associated with a much milder form of the disease. Prophybaxis with toxoid is the best preventative treatment but, once established, minor débridement with benzyl penicillin and rebaxants (even with ventilation) may be required. The use of antitoxin is controversial.
Clostridium difficile is the cause of pseudomembranous colitis but is not involved in wound infection.
Aerobic Gram-negative bacilli (AGNB) are normal inhabitants of large bowel. Escherichia coli and Klebsiella spp. are lactose fermenting; Proteus is nonlactose fermenting. Most organisms in this group act in synergy with Bacteroidesto cause wound infections after bowel operations (in particular appendicitis, diverticubitis and peritonitis). The pseudomonads tend to cobonise burns and tracheostomy wounds, as well as the urinary tract (all members of this group are a cause of urinary tract infection). Pseudomonads may be regarded as markers and cobonise wards and intensive care units from which they may be difficult to eradicate. Surveillance of cross-infection is important in outbreaks. Hospital strains become resistant to 3-lactamase which can be transferred by pbasmids and individual sensitivity testing may be needed. Wound infections only need antibiotic therapy when there is progressive or spreading infection with systemic signs. The aminogbycosides are effective but some cephalosporins and penicillin may not be. Many of the new quinolones, e.g. ciprofloxacin, or carbapenems, e.g. meropenem, are useful in severe infections.
Bacteroides are nonspore-bearing, strict anaerobes which cobonise the large bowel, vagina and oropharynx. Bacteroides fragilis is the principal organism which acts in synergy with AGNB to cause wound infection after colorectab or gynaecological surgery. They are sensitive to the imidazobes, e.g. metronidazole, and some cephalosporins, e.g. cefotaxime.
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