Emergence of Multidrug-Resistant Streptococcus agalactiae in late pregnancy: virulence, resistance patterns, and interaction with protective Lactobacilli

Introduction. Streptococcus agalactiae (Group B Streptococcus, GBS) is a gram-positive bacterium that colonizes the human gastrointestinal and genitourinary tracts that represents the leading cause of severe neonatal infections. Maternal transmission can result in early- and late-onset disease, often leading to invasive infections, long-term sequelae, or fatalities. GBS is also an opportunistic pathogen in the elderly and immunocompromised. The rise of antibiotic-resistant strains complicates prevention and treatment strategies. This study aims at investigating antimicrobial resistance patterns, virulence gene profiles, and interactions with protective Lactobacillus species in GBS strains isolated during late pregnancy screening. Materials and Methods. Thirty-one clinical GBS isolates were collected from private healthcare facilities in Rome, mainly from vaginal swabs (93%), with few isolates from urine, and feces. Hemolytic activity was tested on blood agar; antimicrobial susceptibility was assessed with the VITEK system; virulence genes were detected by simplex PCR; and interactions with Lactobacilli were evaluated through co-culture assays. Results. Resistance rates were 77% for tetracycline, 73% for rifampicin, 40% for erythromycin, and 20% for clindamycin. All strains remained susceptible to penicillin, linezolid, moxifloxacin, teicoplanin, tigecycline, trimethoprim/sulfamethoxazole, and vancomycin. Moderate susceptibility to levofloxacin was observed in all but one strain, raising concerns for future resistance. Notably, 33% of isolates displaying a multidrug-resistant (MDR) phenotype were collected during the last weeks of pregnancy. Molecular analysis showed a high prevalence of virulence genes. Adhesion genes (fbsA, fbsB, lmb) were present in 90–100% of isolates. Hemolysis genes (cylB, hylB) were detected in 97%, while immune evasion genes (scpB, bac, bca) appeared in up to 58%. The capsular protein gene (rib) was found in 50% of cases. Discussion and Conclusions. These results highlight the clinical relevance of monitoring GBS colonization in late pregnancy, especially with the emergence of MDR strains carrying conserved virulence profiles. The prevalence of adhesion and hemolytic factors suggests their key role in perinatal transmission. Ongoing studies are examining GBS serotypes and the inhibitory potential of Lactobacilli through competition assays, thereby promising microbiota-based strategies for reducing GBS colonization and transmission risk.