Hemagglutinating properties of a Streptococcus gordonii strain expressing sialic acid-binding adhesin homolog with low binding site similarity to that of strain DL1.

Objectives: The Hsa adhesin of Streptococcus gordonii strain DL1 was previously identified as a hemagglutinin that binds specifically to sialoglycoconjugates. We recently found that among oral streptococcal species, S. gordonii strains most frequently express Hsa homologs on the bacterial cell surface. However, the effect of amino acid sequence diversity of nonrepetitive region 2 (NR2), a putative binding site of Hsa, on antigenicity and hemagglutinating (HA) properties is unclear due to difficulties in DNA sequencing the NR2 coding region. The aim of this study was to elucidate the similarity of the low NR2 antigenicity Hsa homolog of strain NDU1118 to that of strain DL1 and the association of the homolog with HA properties of the strain.

Methods: The hsa homolog of NDU1118 was sequenced using a long-read next-generation sequencer, and the Hsa homolog was assessed by alignment analysis of the deduced amino acid sequences. The hsa mutant of NDU1118 was generated by insertion of the erythromycin resistance gene. The HA properties of the wild type and the hsa mutant were assessed with human erythrocytes.

Results: The NR2 amino acid sequence of the NDU1118 Hsa homolog was almost identical to that of the S. gordonii M99 Hsa homolog, also known as GspB, and less similar to that of DL1 Hsa. The hsa mutation of NDU1118 induced reduction of HA activity in untreated erythrocytes, but surprisingly increased lactose-inhibitable HA activity in neuraminidase-treated erythrocytes.

Conclusions: The results suggest the existence of an adhesin other than the Hsa homolog on the cell surface of NDU1118.