Methicillin-resistant Staphylococcus aureus: a three-year epidemiological and microbiological survey of high-risk patients

In order to assess the frequency, and epidemiological and microbiological features, of respiratory and blood stream infection due to methicillin-resistant Staphylococcus aureus in high-risk patients, all S. aureus strains cultured from reliable clinical specimens (respiratory secretions obtained by tracheo-bronchial aspirate or bronchoalveolar lavage, or blood cultures), were prospectively evaluated over a three-year period, in six inpatient wards selected on the ground of an elevated frequency of severe and/or nosocomially-acquired infections, because of the prevalence of immunocompromised patients, organ transplant recipients, or need of intensive care. Repeatedly positive cultures obtained from a single patient within 30 days were considered as one isolate. Of 507 S. aureus strains responsible for pneumonia or sepsis in the selected wards, 317 (62.5%) proved methicillin-resistant, in absence of significant variations throughout the study period, and according to the specimen origin. Methicillin-resistant S. aureus strains prevailed over sensitive ones in all examined wards (from a 95% rate of the respiratory intensive care unit, to 55.9% of the pneumology department), save the neonatal and pediatric intensive care unit (41.4%). Most of methicillin-resistant S. aureus isolates were recovered from lower airways, compared with blood cultures (p<.0001). The majority of the 317 methicillin-resistant strains were found in the general intensive care unit (42.6%), followed by the pneumology department (18%), and the respiratory intensive care unit (16.4%). Among methicillin-resistant S. aureus strains, a broad variation of sensitivity to other antimicrobial agents was observed: from 3.3% of erythromycin, to 76.9% of chloramphenicol, and 79.7% of cotrimoxazole; glycopeptide antibiotics remained effective against all cultured strains. In our three-year survey of more than 500 episodes of documented staphylococcal infection involving high-risk patients, methicillin resistance was a very common feature, observed at a greater frequency than that reported in literature studies focusing on surgical, pneumological, or intensive care settings. A long-term microbiological monitoring of high-risk inpatient wards may allow a continued update of local antimicrobial susceptibility maps, and significantly add to both chemoprophylaxis and empiric treatment strategies of patients which are either immunocompromised or hospitalized for a long period.