Phenotypic and molecular analysis of nontypeable Group B streptococci: identification of cps2a and hybrid cps2a/cps5 Group B streptococcal capsule gene clusters

作者：Alhhazmi Areej; Tyrrell Gregory J^*

来源：Emerging Microbes & Infections, 2018, 7(1): 137.

DOI：10.1038/s41426-018-0138-6

The Group B streptococcus (GBS) can express a capsular polysaccharide (CPS). There are ten recognized CPSs (la, Ib, and II-IX). A GBS isolate is considered nontypeable (NT) when CPS cannot be identified as one of ten types. Two groups of GBS NT isolates were studied, isolates without surface sialic acid (sia(-)) and isolates with surface sialic acid (sia(+)). The first objective was to characterize NT sia(-) isolates that failed CPS identification by an immunodiffusion antisera typing assay and a RT-PCR capsule typing assay. NT sia(-) isolates were characterized by assaying phenotypic changes and identifying covR/S mutations that may potentially have a role in the altered phenotypes. The second objective was to characterize NT sia(+) isolates that failed to identify as one of the ten CPS types by an immundiffusion antisera-based typing assay and a RT-PCR capsule typing assy yet expressed capsule. Fifteen NT sia(-) isolated displayed increased beta hemolysis/orange pigmentation, decreased CAMP activity, inability to form biofilm, and susceptibility to phagocytosis by human blood. DNA sequence analysis of the covR/S genes in the sia(-) isolates found mutations in 14 of 15 isolates assayed. These mutations in the covR/S genes may potentially contribute to lack of expression of phenotypic traits assayed in vitro. For the three NT sia(+) isolates, whole-genome sequence analyses identified two isolates with cps gene clusters identical to the recently described and uncommon CPSIIa type. The third isolate possessed a hybrid cluster containing cps genes for both CPSIIa and CPSV suggesting recombination between these two gene clusters.

• 出版日期2018-8-8