Streptococcus agalactiae in Brazil: serotype distribution, virulence determinants and antimicrobial susceptibility

Abstract

Background: Group B Streptococcus (GBS) remains a major cause of neonatal sepsis and is also associated with invasive and noninvasive infections in pregnant women and non-pregnant adults, elderly and patients with underlying medical conditions. Ten capsular serotypes have been recognized, and determination of their distribution within a specific population or geographical region is important as they are major targets for the development of vaccine strategies. We have evaluated the characteristics of GBS isolates recovered from individuals with infections or colonization by this microorganism, living in different geographic regions of Brazil.

Methods: A total of 434 isolates were identified and serotyped by conventional phenotypic tests. The determination of antimicrobial susceptibility was performed by the disk diffusion method. Genes associated with resistance to erythromycin (ermA, ermB, mefA) and tetracycline (tetK, tetL, tetM, tetO) as well as virulence-associated genes (bac, bca, lmb, scpB) were investigated using PCR. Pulsed-field gel electrophoresis (PFGE) was used to examine the genetic diversity of macrolide-resistant and of a number of selected macrolide-susceptible isolates.

Results: Overall, serotypes Ia (27.6%), II (19.1%), Ib (18.7%) and V (13.6%) were the most predominant, followed by serotypes IV (8.1%) and III (6.7%). All the isolates were susceptible to the beta-lactam antimicrobials tested and 97% were resistant to tetracycline. Resistance to erythromycin and clindamycin were found in 4.1% and 3% of the isolates, respectively. Among the resistance genes investigated, tetM (99.3%) and tetO (1.8%) were detected among tetracycline-resistant isolates and ermA (39%) and ermB (27.6%) were found among macrolide-resistant isolates. The lmb and scpB virulence genes were detected in all isolates, while bac and bca were detected in 57 (13.1%) and 237 (54.6%) isolates, respectively. Molecular typing by PFGE showed that resistance to erythromycin was associated with a variety of clones.

Conclusion: These findings indicate that GBS isolates circulating in Brazil have a variety of phenotypic and genotypic characteristics, and suggest that macrolide-resistant isolates may arise by both clonal spread and independent acquisition of resistance genes.

Figure 1

Serotype distribution among the 249 Streptococcus agalactiae isolates recovered from colonization included in the present study, according to geographical region. NT, nontypeable by using antisera against Ia, Ib, II-VIII serotypes.

Figure 2

Dendrogram constructed by similarity and clustering analysis using the Dice coefficient and UPGMA of the digitalized PFGE profiles of 42 Streptococcus agalactiae isolates included in the present study. A total of 17 erythromycin-resistant isolates and 25 erythromycin-susceptible isolates were included. A tolerance of 1% was applied. The vertical line indicates the 70% level of similarity. The upper-case letters indicate the clonal complexes. cMLS_B: constitutive resistance to macrolide-lincosamide-streptogramin B; iMLS_B: inducible resistance to macrolide-lincosamide-streptogramin B; M: resistance to erythromycin only. (-): indicates the absence of resistance-associated genes.