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Serotype IX, a Proposed New Streptococcus agalactiae Serotype

Neisseria and Streptococcus Reference Laboratory, Department for Bacteriology, Mycology and Parasitology, Statens Serum Institut, Copenhagen, Denmark,¹ Centre for Infectious Diseases and Microbiology-Public Health (CIDM-PH), Institute of Clinical Pathology and Medical Research (ICPMR), Westmead Hospital, Westmead, New South Wales 2145, Australia²

Received 17 January 2007/ Returned for modification 30 March 2007/ Accepted 6 July 2007

We identified three isolates of Streptococcus agalactiae (group B streptococcus [GBS]), of human origin, which failed to react with antisera against any of the nine known GBS serotypes. Polyclonal rabbit antisera raised against these isolates and standard GBS typing sera were used in capillary precipitation and Ouchterlony tests to compare the strains with known GBS serotype reference strains. All three previously nontypeable isolates reacted with all three new antisera, producing lines of identity in the Ouchterlony test. Weak cross-reactions with antisera against several GBS serotypes were observed but were removed by absorption with corresponding antigens. The new antisera were used to test 227 GBS isolates that had been nontypeable or difficult to type using standard antisera. Of these, five reacted with the new antisera. These results suggested that all eight isolates belong to the previously unrecognized GBS serotype. They were tested by Western blotting for the C and Cß proteins and by PCR to identify molecular serotypes and surface protein antigen genes. Two segments of the cps gene cluster (3' end of cpsE-cpsF and 5' end of cpsG, approximately 700 bp; 3' end of cpsH and 5' end of cpsM, approximately 560 bp) were sequenced. All eight isolates expressed C, and seven expressing the Cß protein and the corresponding genes, bca and bac, respectively, were identified. They all share the same, unique partial cps sequence. These results indicate that these eight isolates represent a new S. agalactiae serotype, which we propose should be designated serotype IX.

Published ahead of print on 18 July 2007.

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