Characterization of Penicillin-Non-Susceptible, Multidrug-Resistant Streptococcus agalactiae Using Whole-Genome Sequencing.

Streptococcus agalactiae (group B Streptococcus, GBS) is a major pathogen in neonates and adults causing neonatal sepsis, pneumonia, and meningitis. Although penicillin is the first-line treatment, GBS infections have developed multidrug resistance, including non-susceptibility to penicillin. Given a lack of molecular genetic research, further characterization of clinical isolates is essential. Therefore, in this study, we aimed to advance our understanding of their resistance profiles and molecular epidemiology of penicillin-non-susceptible GBS isolates. We conducted a comprehensive analysis of the genetic characteristics of penicillin-non-susceptible, multidrug-resistant (MDR) GBS and antimicrobial susceptibility testing. Four penicillin-non-susceptible GBS isolates were obtained from clinical specimens. Antimicrobial susceptibility was assessed using VITEK2, MicroScan, broth microdilution, and Etest. Whole-genome sequencing was conducted using the Illumina MiSeq platform, followed by sequence analysis of resistance-related genes, including pbp, ciaH, rpo, and quinolone resistance determinants. The clinical data of patients were also reviewed. All four isolates exhibited penicillin minimum inhibitory concentrations (MICs) of 0.25 μg/mL and were resistant to levofloxacin, erythromycin, clindamycin, and tetracycline, as determined by antimicrobial susceptibility testing, confirming an MDR phenotype. The MICs of cefotaxime and ceftriaxone were close to the susceptibility breakpoints, indicating borderline susceptibility. Several amino acid substitutions in penicillin-binding proteins were associated with penicillin non-susceptibility and additional substitutions in CiaH and RpoD. Substitutions associated with fluoroquinolone resistance were also identified in GyrA and ParC. All isolates carried the resistance genes erm(B), mre(A), and tet(M). Although rare, the detection of these isolates underscores the need for continued surveillance and further research to inform effective treatment and prevention strategies.