Molecular Characterization of Italian Isolates of Fluoroquinolone-Resistant Streptococcus agalactiae and Relationships with Chloramphenicol Resistance

A total number of 368 clinical isolates of Streptococcus agalactiae (group B Streptococcus, GBS) were collected in 2010–2016 from three hospitals in a region of central Italy. Fluoroquinolone (FQ)-resistant isolates were selected using levofloxacin. Levofloxacin-resistant (LR) strains (11/368, 2.99%) were characterized for several features, and their FQ resistance was analyzed phenotypically and genotypically using seven additional FQs. Their gyrA and parC quinolone resistance-determining regions were sequenced. Of the 11 LR isolates, 10 showed highlevel and 1 low-level resistance. The former isolates exhibited higher minimal inhibitory concentrations also of the other FQs and all shared one amino acid substitution in ParC (Ser79Phe) and one in GyrA (Ser81Leu); only Ser79Phe in ParC was detected in the low-level LR isolate. The 11 LR strains exhibited distinctive relationships between their susceptibilities to non-FQ antibiotics and typing data. Remarkably, despite the very rare occurrence of chloramphenicol resistance in S. agalactiae, no <4 of the 11 LR isolates were chloramphenicol-resistant. Studies of GBS resistance to FQs in Europe remain scarce, notwithstanding the emergence of multidrug-resistant isolates. The incidence of LR GBS isolates is still limited in Italy, consistent with the moderate (though growing) rates reported in Europe, and much lower than the very high rates reported in East Asia. The intriguing relationships between FQ and chloramphenicol resistance deserve further investigation.