Background: Streptococcus agalactiae (Group B Streptococcus, GBS) is a leading cause of neonatal infections, exhibiting remarkable adaptability. This study aimed to characterize the virulence and resistance profiles of GBS isolates obtained from pregnant women in Porto Velho, Rondônia, within the Brazilian Amazon. GBS strains were isolated and identified from rectovaginal cultures using phenotypic and genotypic methods. Capsule typing and the detection of virulence and resistance genes were performed through polymerase chain reaction (PCR). Antimicrobial susceptibility profiles and in vitro biofilm formation were also assessed.

Results: A total of 85 GBS strains were characterized. The most prevalent serotypes were Ia (36.5%) and V (15.3%), followed by Ib (14.1%), II (14.1%), III (11.8%), and VI (8.2%). A high prevalence of virulence genes was observed, including scpB (100%), lmb (97.6%), bca (84.7%), hylB (64.7%), cylE (60%), bac (57.6%), and hvgA (11.8%). All strains were susceptible to penicillin, ampicillin, cefazolin, ceftriaxone, and vancomycin. Non-susceptibility was identified for tetracycline (77.6%), erythromycin (18.9%), chloramphenicol (4.7%), clindamycin (3.5%), and levofloxacin (1.2%). Among the 17 strains non-susceptible to erythromycin and/or clindamycin, the detected phenotypes included M (82.4%), cMLSB (11.8%), and L (5.9%), with associated resistance genes erm(A) (5.9%), erm(B) (11.8%), and mef(A/E) (64.7%). Additionally, over 90% of the strains demonstrated strong biofilm formation capacity.

Conclusions: This is the first characterization of GBS in this region, revealing notable virulence and high susceptibility to first-line antibiotics. Six serotypes were identified, including Brazil's second report of serotype VI, emphasizing the genetic and epidemiological diversity of the Amazonian region. Moreover, given that the serotype distribution in this population corresponds to the most prevalent types worldwide, the hexavalent GBS conjugate vaccine could represent a promising strategy for this population, as it would theoretically cover 91.8% of the analyzed strains, if proven effective and made available in Brazil.