Definitions
Infection is the process whereby organisms (e.g. bacteria, viruses, fungi) capable of causing disease gain access and cause injury or damage to the body or its tissues. Pus is a yellow–green, foul-smelling, viscous fluid containing dead leucocytes, bacteria, tissue and protein. An abscess is a localized collection of pus, usually surrounded by an intense inflammatory reaction. Cellulitis is a spreading infection of subcutaneous tissue. Necrotizing fasciitis is progressive, infection located in the deep fascia, which spreads rapidly with secondary necrosis of the subcutaneous tissues.
Cleansing is the removal of gross surface contamination of an item, tissue or environment (e.g. simple hand washing). Disinfection is the reduction of infectious particles from an item or environment (e.g. surgical scrubbing). Sterilization is the removal/destruction of all infectious particles (spore and vegetative) from an item or environment (e.g. instrument autoclaving).
- An inoculum of >100 000 bacteria/ml is required to establish an infection.
- Many features of gram-negative infection (fever, elevated WBC, hypotension and intravascular coagulation) are mediated by endotoxin.
- Narrow spectrum antibiotics are preferred where possible as they are less likely to induce resistance or Clostridium difficile infection.
- Abscesses should be drained either radiologically or surgically.
Pathophysiology of Bacterial Infection
Establishing a bacterial infection requires:
- An inoculum of bacteria.
- A bacteria-friendly environment (water, electrolytes, carbohydrate, protein digests, blood, warmth, oxygen rich (except anaerobic/microaerophilic organisms)).
- Diminished host resistance to infection (impaired physical barriers, reduced biochemical/humoral response, reduced cellular response).
Bacterial Secretions
Bacteria cause some of their ill effects by releasing compounds:
- Enzymes (e.g. haemolysin, streptokinase, hyaluronidase).
- Exotoxin (released from intact bacteria, mostly gram-positive, e.g. tetanus, diphtheria).
- Endotoxin (LPS released from cell wall on death of bacterium).
Natural History of Infection
- Inflammatory response is established (rubor/redness, tumor/swelling, dolor/pain, calor/heat).
- Resolution: inflammatory reaction settles and infection disappears.
- Spreading infection:
- Abscess formation: localized collection of pus.
- Organization: granulation tissue, fibrosis, scarring.
- Chronic infection: persistence of organism in the tissues elicits a chronic inflammatory response.
- is present in all patients with the disease
- must be isolated from lesions in pure culture
- must reproduce the disease in susceptible animals
- must be re-isolated from lesions in the experimentally infected animals.
Management of Surgical Infection
Preventive Measures
- Short operations.
- Skin disinfection with antibacterial chemicals and detergents (patients’, surgeons’ and nurses’ skin).
- Filtering of air in operating theatre.
- Occlusive surgical masks and gowns.
- Prophylactic antibiotics:
Management of Established Infection
Diagnosis:
made by culture of appropriate specimens (pus, urine, sputum, blood, CSF, stool). Obtain appropriate specimens before giving antibiotics.
Antibiotics:
- Prescribe on basis of culture results and ‘most likely organism’ for initial empirical treatment while waiting for results.
- Certain antibiotics are reserved for serious infections – use the hospital policy wherever possible.
- Therapeutic monitoring of drug levels may be required, e.g. aminoglycosides.
- Synergistic combinations may be required in some infections, e.g. aminoglycoside, cephalosporin and metronidazole for faecal peritonitis.
- In serious, atypical or unresponsive infections seek advice from clinical microbiologist.
- Barrier nursing and isolation of patients with MRSA or VRE.
Drainage:
surgical or radiological – is the most important treatment modality for an abscess or collections of infected fluid.
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