This study reports the molecular characterization of colonizing strains of Group B Streptococcus in the vagina and rectum of pregnant women. Serotype V was the most frequent serotype in this population representing 33.3% of the population.
Sequence types ST19, 486, 182, 24 and 2 accounted for 69.7% of the total isolates. This diversity is different from other reports that have reported ST19, ST23 and ST1 more commonly 17. The highly virulent ST17 responsible for most invasive disease, especially late onset disease in neonates18, was rare in this study (3%) population which may suggest a better prognosis for neonates who may become colonized at birth in this environment. While ST182 was reported as a frequent clonal type by Medugu et al in another local study in Abuja, Nigeria2, ST486 has not previously been documented as a common sequence type among GBS isolates in Nigeria.
Alp1 (Epsilon) (33.3%) was the most common of all the alpha-like proteins among the isolates in this study, similar to findings reported in Egypt (27%)19. Other studies have documented Rib as the most frequently detected Alp gene in other geographic regions like China and Iran (37.5% and 53% respectively)20,21. The serine-rich repeat protein Srr-1 was identified in almost all isolates, however only a single isolate had the Srr-2 known to be associated with greater binding affinity and higher morbidity22. The same isolate was positive for the hypervirulent GBS adhesin (HvgA), another surface protein demonstrated to increase adhesion and virulence particularly in the ST17 lineage23.
Similar to other studies from Nigeria2,24, all isolates were determined to be susceptible to beta-lactam antibiotics including 3rd generation cephalosporins. This is quite reassuring, suggesting that penicillin and ampicillin remain as viable options for intrapartum antibiotic prophylaxis in this region despite concerns for antimicrobial resistance. For women allergic to penicillin, guidelines recommend clindamycin as an alternative option for prophylaxis25. In this study however, clindamycin and erythromycin resistance were relatively high at 24.2% and 30.3%, respectively. Data from an earlier study in Nigeria reported erythromycin and clindamycin resistance rates of 6.5%24 suggesting increased resistance rates within the country. However, all isolates were susceptible to vancomycin in this study, so patients with a high risk for penicillin anaphylaxis who are colonized with a clindamycin-resistant GBS strain, could receive vancomycin as the next best option25.
Although other antibiotic classes such as fluoroquinolones, chloramphenicol, and tetracycline are not recommended for intrapartum prophylaxis, determining their susceptibility is important for tracking of resistance at the pneumococcal population level and as useful options for other invasive group B streptococcal infections in other age groups. All isolates were predicted as tetracycline resistant which is consistent with several other reports documenting high levels of resistance to this antibiotic24,26,27. Resistance to chloramphenicol and levofloxacin were also high at 25.6%. While these antibiotics may not be recommended for neonatal sepsis, (for which we screen mothers for GBS to prevent), they are useful options for other invasive group B streptococcal infections in other age groups.
The only tetracycline gene present in this population was tetM. Of all the tetracycline resistance determinants, the tetM has the most spread geographically among GBS24,26,27. The presence of the double gyrA + parC mutants conferring full fluoroquinolone-resistance was high and were found predominantly in serotype V/ST-19 clone. The most frequently reported fluoroquinolone non-susceptible serotype/ST combinations are III/ST-19 and V/ST-1928,29 supporting the data from this study. Occurrence of either of these clonal complexes is likely a reflection of the circulating clones in those geographical areas in which they are found.
This study has demonstrated some salient characteristics of the prevalent colonizing strains of GBS in Nigeria, thus, contributing to the body of knowledge concerning GBS, as well as providing AMR data if IAP is considered in the future.