Background. Invasive Group B streptococcus (GBS) is a major cause of serious neonatal infection. Current strategies to reduce early onset GBS disease have no impact on late onset disease (LOD). Although GBS is a normal part of the enteric microbiota in healthy term infants, LOD cases arising in the neonatal intensive care unit setting raise questions about mode of acquisition.

Methods. Enhanced surveillance for any case of late onset GBS sepsis admitted to a level 3, 24-bed neonatal intensive care unit over a 2 year period was instituted following a cluster of four cases. All late onset GBS isolates were serotyped and genomes sequenced. Rectal screening of neonates for GBS was undertaken weekly. Healthcare workers and parents were not screened.

Results. Over 24 months, a total of 12 late onset invasive GBS episodes were identified (incidence 0.6/1000 live births). Genomic analysis revealed that 11/12 GBS isolates (92%) were linked to at least one other LOD isolate. Four isolates from the first cluster were serotype V, resistant to macrolides and lincosamides, providing early evidence of a common source. Sequencing confirmed isolates were indistinguishable, or distinguishable by 1 SNP, from each other, and distinct from contemporary serotype V GBS. Although a common environmental source was not identified, prompt infection prevention interventions were instituted and no further serotype V GBS infections arose. Prospective surveillance identified three further clusters of LOD due to serotypes Ia, Ib, and III, leading to re-evaluation of interventions required for preventing GBS LOD.

Conclusion. Acquisition routes for LOD GBS in the neonatal unit are poorly understood; such cases may not necessarily be sporadic. Within this neonatal unit, our data suggest that a single case of LOD GBS sepsis should be considered a potential nosocomial transmission event warranting prompt investigation, heightened infection prevention vigilance and action where required.