Identification and molecular characterization of a S. agalactiae strain lacking the capsular locus.

During a national surveillance program on Group B streptococci (GBS) maternal carriage and neonatal infections, a GBS strain isolated from a pregnant woman's vagino-rectal swab was non typable by either serological or molecular methods. Further molecular characterization demonstrated that the strain lacked the entire capsular locus, possibly by a recombination event that excised a 14,1 Kbase pairs genomic fragment extending from the regulatory protein cpsX gene to the neuA gene. The natural loss of the capsular locus by GBS isolated from a human has never been described so far. Such an event, while possibly a dead-end from the evolutionary point of view, leaves a still able-to-colonize organism unrecognizable by the vaccines currently under development.