Physiopathology of MRSA Infections in Surgical Patients

MRSA and MSSA infections share common pathogenic mechanisms. Although asymptomatic nasal colonization with S. aureus is common, it appears to be an important factor in the development of most infections due to this organism (vonEiff etal. 2001).

Binding of S. aureus cell-surface components (e.g., teichoic acids) with either carbohydrate-rich surface components of mucosal epithelial cells or nasal mucus secretions provides a suitable explanation for initial colonization. Long-term carriage, however, is less understood. Inverted confocal laser scan fluorescence and electron microscopic examination of intranasal biopsy specimens from patients suffering from recurrent S. aureus rhinosinusitis revealed foci of intracellular reservoirs of S. aureus in the epithelium, glandular, and myofibroblastic cells (Clement et al. 2005).

Of 450 university student volunteers from North Carolina, 29% were S. aureus carriers. Two percent of the S. aureus were resistant to methicillin. Independent risk factors for carriage in this setting included older age, male gender, and chronic sinusitis (Bischoff et al. 2004). Carriage is most often clonal, although one observation suggested that about 7% of S. aureus-colonized individuals carry more than one strain (Cespedes et al. 2005).

Conflicting results have been published on risk factors for nasal carriage in patients. In the context of a clinical trial evaluating whether mupirocin prevented surgical site infections due to S. aureus, Herwaldt et al. prospectively collected data on 70 characteristics in a population of 4030 patients before surgery. Twenty-two percent of these patients carried S. aureus in their nares; the proportion of MRSA was not specified. Independent risk factors for S. aureus nasal carriage were obesity, male gender, and a history of cerebrovascular accident (Herwaldt et al. 2004).

An illustration of the relationship between S. aureus carriage and the subsequent occurrence of S. aureus infection was published by Wertheim et al. (2004). They screened 14,008 nonsurgical patients at hospital admission for S. aureus nasal carriage, and monitored them for development of bacteremia. Nosocomial S. aureus bacteremia was three times more frequent in S. aureus carriers (1.2%) than in non-carriers (0.4%), a significant increase (95% CI for the relative risk: 2.0-4.7). Eighty percent of the strains that caused bacteremia in carriers were endogenous.

The role of S. aureus carriage was also observed in the surgical setting. Numerous studies have shown that surgical patients who carry S. aureus in their anterior nares were at increased risk for S. aureus surgical site infections (SSIs), and that those infections are usually caused by the same strains that were carried by these patients prior to surgery (Herwaldt 2003).

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