[1] IST SUPER SANITA, EPIDEMIOL & BIOSTAT LAB, I-00161 ROME, ITALY
[2] IST RIC SCI, OSPED PEDIAT BAMBINO GESU, DIV NEFROL & DIALISI, ROME, ITALY
[3] UNIV MILAN, SERV IGIENE PUBBL REG LOMBARDIA, I-20122 MILAN, ITALY
[4] UNIV MILAN, DEPT PEDIAT 2, I-20122 MILAN, ITALY
[5] IST GIANNIAN GASLINI, GENOA, ITALY
[6] OSPED RIUNITI BERGAMO, SERV MICROBIOL, BERGAMO, ITALY