High prevalence in cystic fibrosis patients of multiresistant hospital-acquired methicillin-resistant Staphylococcus aureus ST228-SCCmecI capable of biofilm formation

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Abstract
Although methicillin-resistant Staphylococcus aureus (MRSA) is increasingly recognized in cystic fibrosis (CF) patients, it has not been sufficiently studied in large series. We analysed all MRSA isolates recovered from respiratory secretions of patients attending our CF unit (1994–2006). Antibiotic susceptibilities were determined using both planktonic and sessile bacteria as inocula. Genetic relationships were determined by PFGE and multilocus sequence typing (MLST). SCCmec type and the presence of the pvl gene were also investigated. A total of 93 MRSA isolates (1–20 isolates per patient) were recovered from 18 of 77 CF patients with positive staphylococcal culture. Mean prevalence (4.4%) increased (P < 0.001) over time. All isolates were susceptible to linezolid, quinupristin/dalfopristin and co-trimoxazole but presented high resistance rates to amikacin (90%), gentamicin (85%), levofloxacin (81%) and erythromycin (69%). Except for macrolides and gentamicin, isolates were less susceptible growing in biofilms than in planktonic cultures. Fifteen different PFGE patterns were found, one of them consistently recovered for 6 years in the same patient. Identical clones were detected in several unrelated patients. MLST demonstrated that the international ST228 was the most frequent (67%) clone. The pvl gene was negative in all isolates and the SCCmec corresponded to types I (97%) and IV (3%). Strong mutators were not detected, but a considerable number were considered weak mutators. Distinct microbiological and molecular features were detected among CF-MRSA isolates, probably due to adaptation to specific conditions in CF patients. Prevalence of MRSA increased among these patients, most of them colonized with a multiresistant biofilm-forming clone belonging to ST228-SSCmecI, suggesting cross-transmission or a common source.

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