Genetic Determinants and Clonal Composition of Levofloxacin-Resistant Streptococcus agalactiae Isolates from Bulgaria

<b>Background</b>: Levofloxacin is a broad-spectrum third-generation fluoroquinolone with bactericidal activity against <i>Streptococcus</i> species. We aimed to investigate the susceptibility rates of levofloxacin, the genetic determinants contributing to resistance, the serotype distribution, and the population structure of levofloxacin-resistant <i>Streptococcus agalactiae</i> (GBS) isolates. <b>Methods</b>: Antibiotic susceptibility testing was conducted according to the EUCAST criteria. PCR-serotyping, determination of mutations in the quinolone resistance-determining regions (QRDRs), and multi-locus sequence typing (MLST) were performed on all levofloxacin-resistant strains. <b>Results</b>: Among the 328 GBS isolates, 11.9% exhibited resistance to levofloxacin. We categorized the samples into two main groups: vaginal (64.1%) and extra-vaginal. The latter was further subdivided into invasive (10.3%) and non-invasive (25.6%) ones. The most common serotypes identified were V (30.8%) and III (25.6%). All examined resistant strains possessed missense mutations in the QRDR of <i>parC</i> (Ser79Phe/Tyr and Asp83Asn), whereas 59.0% of them exhibited additional mutation in <i>gyrA</i> (Ser81Leu and Glu85Lys/Ala). The MLST results disclosed six clonal complexes: CC19(64.1%), followed by CC1 (10.3%), CC452 (7.7%), and CC459 (5.1%), and CC12 and CC23, represented by single strains. <b>Conclusions</b>: We observed a growing resistance to fluoroquinolones that appears to exceed the average in Europe. More than half of the isolates exhibited the accumulation of mutations within the QRDRs. Rigorous monitoring is needed to prevent the emergence of MDR GBS and preserve the effectiveness of the newer generations of fluoroquinolones.