Microorganisms

The numbers and types of microorganisms involved in the development of sepsis are influenced by several factors, including site of intervention, immune and nutritional status of the patient, use of antibiotics and other comorbid states. Cutaneous infections are usually due to Gram-positive cocci. Staphylococci normally colonize the skin and integument and may lead to infection of surgical wounds, intravascu-lar catheters and graft prosthesis. The organism is usually implanted at the time of the procedure but presentation may be delayed. Group A streptococci may also be inoculated during invasive procedures which breach the skin. Infection with this organism usually presents as spreading cellulitis but a more virulent strain, microaerophilic non-haemolytic, may produce a synergistic infection with aerobic haemolytic staphylococci giving rise to a rapidly progressive gangrenous ulceration which requires urgent extensive debridement of all affected tissue together with large doses of penicillin. Hospitalization, critical illness and possibly antibiotic usage may alter the cutaneous microorganisms, replacing the resident Gram-positive with Gram-negative ones.

Other organisms that may cause wound site infection include enterococci, Escherichia coli and Pseudomonas aeruginosa. The latter is one of the commonest organisms involved in nosocomial pneumonia. Many factors predispose critically ill patients to nosocomial pneumonia (Fig. 5.3). First, there is an increase in oropharyngeal colonization in critically ill patients due to the lack of mastication and salivation. Secondly, the routine use of therapies (e.g. H2 antagonist and antacids) to neutralize gastric acid allows bacteria to colonize in the stomach. These organisms are found in increasing numbers within 24-96 h of admission to the ICU. The presence of a nasogastric tube leads to loss of lower oesophageal sphincter competence, allowing bacteria to ascend from the stomach to the upper airways where they may be aspirated. Thirdly, the presence of an endotracheal tube allows environmental organisms to gain entry into the airways and lungs. An association has been found between the duration

Malnutrition Mild ischaemia Hypoxia

Ischaemia-reperfusion injury

Figure 5.3. Factors leading to the development of pneumonia in the critically ill patient.

of endotracheal intubation and the incidence of pneumonia. Finally, the cough reflex is impaired in patients with a decreased level of consciousness and endotracheal intubation.

The Gram-negative bacteria, Escherichia coli, Klebsiella, Enterobacter and Proteus spp are normally associated with infections of the gastrointestinal and urinary tracts. These together with Bacteroides spp are found mainly in the distal ileum and colon and have been incriminated in Gramnegative sepsis. Although sepsis is predominantly caused by Gram-negative organisms, Gram-positive bacteria, viruses and fungi are becoming commoner. The incidence of Grampositive bacterial sepsis in particular is gradually increasing and may be related to the increasing use of invasive monitoring devices in older and immunosuppressed patients and the development of resistant strains of organisms to routine antibiotics, for example methicillin-resistant Staphylococcus aureas (MRSA). Concurrent diseases such as cardiovascular, respiratory and renal disease may also contribute to the rise. Coagulase-negative staphylococci is the single commonest cause.

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