Background
Risk factors and mode of transmission in late-onset GBS disease (LOD-GBS) is still poorly understood.
Methods
Four cases of LOD-GBS were considered. They regarded at-term infants born to GBS colonised mothers and breast milk-fed. The onset of GBS-LOD was between ten and forty-seven days of life.
Each LOD-GBS case consisted of three GBS isolates: one from the blood or cerebrospinal fluid of the infant, one from the maternal vaginal-rectal swab and one from the breast milk, all collected at the time of the onset of symptoms. Then, in total 12 mother-infant pair samples were genome sequenced by using the Illumina technology.
Results
All 12 mother-infant pair samples were serotype III and belonged to the hypervirulent ST-17 lineage. Vaginal, breast-milk and infant GBS genomes were equally distant within each case, roughly accumulating the same number of single-nucleotide point mutations (SNPs) (ranging, in total cases, from 65 to 154 SNPs). The SNPs were not randomly distributed but clustered in discrete chromosomal regions, often encoding transposon and phage elements.
Conclusions
Maternal GBS colonisation is a risk factor for LOD-GBS. Unfortunately, the lack of maternal GBS samples collected before or at birth did not allow to determine a time course evolution, therefore the vertical or horizontal GBS acquisition by the infant could not be determined.