Background
CAMP test is part of the panel of microbiological tests for GBS identification. Accordingly, a highly conserved gene-fragment encompassing the CAMP-factor encoding cfb gene is the target of molecular diagnostic PCR-based systems for the rapid identification of GBS.
Methods
A total of six PCR negative, culture positive GBS strains were identified; four strains were collected as part of the universal antenatal screening program of pregnant women; the other two strains were from symptomatic vaginitis.
Results
The strains were serotype II, ST-22 (one strain), serotype V, ST-1 (two strains), serotype Ia, ST-23 (three strains). Initial PCR mapping confirmed that all strains lacked the cfb gene and presented deletions both upstream and downstream the target gene.
Whole genome sequencing of all GBS strains was performed by the Nanopore MinIon technology and genome assemblies were done by using as the reference the complete genome with the same serotype and ST of the tested genome.
All GBS genomes presented variably sized chromosomal deletions encompassing the cfb gene: the two serotype V, ST-1 GBS genomes presented a chromosomal deletion of 11845 and 16181 nt, respectively; the serotype II, ST-22 had a 7-kb deletion and the the three Ia, ST-23 strains presented deletions ranging from 10 to 30 Kb.
Conclusions
Molecular diagnostic-escape GBS strains are rare (about 0.5-1%); nevertheless the deletion of the chromosomal region encompassing of the cfb gene can cause false-negative results influencing medical decisions. The cfb gene-deleted chromosomal region is not linked to the clonal expansion of a particular GBS lineage.