Longitudinal survey of Staphylococcus aureus in cystic fibrosis patients using a multiple-locus variable-number of tandem-repeats analysis method

BACKGROUND: Staphylococcus aureus infection in patients with cystic fibrosis (CF) is frequent and may be due to colonization by a few pathogenic lineages. Systematic genotyping of all isolates, methicillin-susceptible S. aureus (MSSA) as well as methicillin-resistant S. aureus (MRSA) is necessary to identify such lineages and follow their evolution in patients. Multiple-locus variable-number tandem repeat analysis (MLVA/VNTR) was used to survey S. aureus clinical isolates in a French paediatric CF centre. RESULTS: During a 30 months period, 108 patients, aged 2 to 21 years, regularly followed up at the centre, provided sputum for culture. From 79 patients, a total of 278 isolates were genotyped by MLVA, resolving into 110 genotypes and 19 clonal complexes (CC) composed of similar or closely related isolates. 71% of the strains were distributed into four main CCs, in term of number of isolates and number of genotypes. Spa (Staphylococcus protein A) typing was performed on representative samples, showing an excellent concordance with MLVA. In 17 patients, strains from two to four different CCs were recovered over time. On six occasions, S. aureus isolates with the same genotype were shared by 2 different patients and they belonged to one of the four main clusters. Methicillin-resistance was observed in 60% of the isolates, 90% of which belonged to the main clonal complexes CC8, CC45 and CC5. In 5 patients, methicillin-resistance of S. aureus isolates was not associated with the mecA gene: for four patients, it was due to overproduction of β-lactamase, leading to BOR-SA (borderline S. aureus) isolates, while a strain showing probably a new modified penicillin-binding capacity (MOD-SA) was observed from one patient. CONCLUSION: Systematic genotyping of S. aureus isolates recovered from sputum of CF children allows a thorough analysis of the strains responsible for sporadic as well as chronic colonization and the follow up of their evolution over time. We show here that more than 70% of these strains belong to 4 major CCs. MSSA as well as MRSA, BOR-SA and MOD-SA isolates can persist over several years, despite antibiotic treatments.