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Methicillin-resistant Staphylococcus aureus (MRSA) colonization in cystic fibrosis (CF) patients is an increasing problem in many countries. In our Respiratory Center at the Hospital de Niños "Dr. Ricardo Gutiérrez", Buenos Aires, Argentina, the prevalence has climbed from 23% in 1995 up to 32% in 2011. Our objective was to analyze the diversity of MRSA isolates recovered from respiratory samples of CF patients attending our center, characterizing their phenotypes and clonal distribution. Therefore, a prospective study was conducted on all CF patients attending the pediatric Respiratory Center between June 2012 and May 2013 to collect MRSA isolates. Antibiotic susceptibility testing, multilocus sequence typing, pulsed-field gel electrophoresis, spa typing, and agr genotyping were performed on collected isolates. The prevalence of MRSA during this period was 34.2%, and 71.9% of the patients were infected with isolates that carried SCCmec IV. High resistance rates were detected for gentamicin, erythromycin, clindamycin, ciprofloxacin, and rifampicin. Strains related to the community-associated MRSA clones, ST5-IV and ST30-IV, were the most frequently recovered. Remarkably, even though most of the isolates were related to these clones, the rate of multi-resistance shown in CF patients was higher than that reported for the same lineages recovered from other infections in our country.

BackgroundTigecycline (TIG) is an antibiotic belonging to the glycylcyclines class and appears to be a good choice to fight infections caused by Staphylococcus aureus. To date, TIG exhibits good activity against this microorganism. The aim of this work was to obtain in vitro mutants of S. aureus resistant to TIG and evaluate possible changes in their susceptibility patterns to other antibiotics.ResultsTwo mutants of S. aureus resistant to TIG (MIC = 16 µg/mL) were selected in vitro from clinical isolates of methicillin-resistant S. aureus. In both mutants, corresponding to different lineage (ST5 and ST239), an increase of efflux activity against TIG was detected. One mutant also showed a reduced susceptibility to vancomycin, corresponding to the VISA phenotype (MIC = 4 µg/mL), with a loss of functionality of the agr locus. The emergence of the VISA phenotype was accompanied by an increase in oxacillin and cefoxitin MICs.ConclusionsThis study demonstrates that, under selective pressure, the increase of efflux activity in S. aureus is one of the mechanisms that may be involved in the emergence of tigecycline resistance. The emergence of this phenotype may eventually be associated to changes in susceptibility to other antibiotics such oxacillin and vancomycin.