Identification of zinc-dependent mechanisms used by Group B Streptococcus to overcome calprotectin-mediated stress

Nutritional immunity is an elegant host mechanism used to starve invading pathogens of necessary nutrient metals. Calprotectin, a metal binding protein, is produced abundantly by neutrophils and is found in high concentrations within inflammatory sites during infection. Group B Streptococcus (GBS) colonizes the gastrointestinal and female reproductive tracts and is commonly associated with severe invasive infections in newborns such as pneumonia, sepsis, and meningitis. Though GBS infections induce robust neutrophil recruitment and inflammation, the dynamics of GBS and calprotectin interactions remain unknown. Here we demonstrate that disease and colonizing isolate strains exhibit susceptibility to metal starvation by calprotectin. We constructed a mariner transposon (Krmit) mutant library in GBS and identified 258 genes that contribute to surviving calprotectin stress. Nearly 20% of all underrepresented mutants following treatment with calprotectin, are predicted metal transporters, including known zinc systems. As calprotectin binds zinc with picomolar affinity, we investigated the contribution of GBS zinc uptake to overcoming calprotectin-imposed starvation. Quantitative RT-PCR revealed a significant upregulation of genes encoding zinc-binding proteins, adcA, adcAII, and lmb, following calprotectin exposure, while growth in calprotectin revealed a significant defect for a global zinc acquisition mutant (ΔadcAΔadcAIIΔlmb) compared to the GBS WT strain. Further, mice challenged with the ΔadcAΔadcAIIΔlmb mutant exhibited decreased mortality and significantly reduced bacterial burden in the brain compared to mice infected with WT GBS; this difference was abrogated in calprotectin knockout mice. Collectively, these data suggest that GBS zinc transport machinery are important for combatting zinc-chelation by calprotectin and establishing invasive disease.