Phenotypic and genotypic characterization of serogroup 6 Streptococcus pneumoniae isolates collected during 10-valent pneumococcal conjugate vaccine era in Bulgaria

Abstract

Serogroup 6 remains common in the pneumococcal-conjugated vaccine era in Bulgaria; therefore, we investigated its clonal and serotype dynamics. The antibiotic susceptibilities were assessed by broth microdilution. Strains identified as serogroup 6 with latex agglutination method were subjected to serotype-specific PCRs. Erythromycin-resistant strains were analyzed by PCR for presence of ermB and mefE genes. MLST was performed to define clonal composition of the sequence types (STs). Serogroup 6 was represented by 40 (13.3%) from 301 invasive and non-invasive Streptococcus pneumoniae isolates. Molecular serotyping revealed new emerging serotype 6C (6.6%), not detected in pre-vaccine era. Among unvaccinated patients, mostly we observed serotypes 6А (57.1%) and 6В (28.6%). Serotype 6C was distinctive for vaccinated children (64%), followed by 6A (24%). Penicillin and ceftriaxone nonsusceptible serogroup 6 strains were 65% and 5%, respectively; erythromycin-and clindamycin-resistant were 70.0% and 52.5%, respectively. Multidrug-resistant strains were 57.5%. Prevalent genetic determinant for macrolide resistance was ermB gene (75%). MLST revealed 17 STs into 5 clonal complexes and 7 singletons. Predominant genetic lineage was CC386, represented by MDR-6C non-invasive strains. Serotype 6B, principally responsible for invasive diseases in the pre-vaccine era, retreated this position to serotype 6A.

Synthetic Library of Oligosaccharides Derived from the Capsular Polysaccharide of Streptococcus pneumoniae Serotypes 6A and 6B and Their Immunological Studies

“…Severe pneumococcal infection causes life-threatening invasive pneumococcal diseases (IPD), such as sepsis, meningitis, pericarditis, and others, not only in infants and the elderly but also in adults with weak immune defenses. , Serotypes 6A (ST6A) and 6B (ST6B) pneumococci are two of the major causes of IPD . Due to the wide use of antibiotics and vaccines, , cases of severe pneumococcal disease have gradually decreased; , however, protection from the vaccines are serotype-specific, , and the bacteria have acquired antibiotic resistance. − Moreover, prevalent SPn serotypes are not permanent and vary between regions, , bringing the increase of pneumococcal conjugate vaccine (PCV) valency from 7 (prevenar 7, Pfizer, initial approval at 2000) to 20 (prevnar 20, Pfizer, initial approval at 2021). Although ST6A and ST6B antigens are included in PCV13, SPn6A and SPn6B still cause mortality. , Also, to consistently acquire quality phosphate group-bearing CPSs of serogroup 6 from natural sources remains a major challenge .…”