RESULTS AND DISCUSSION SPN NanA mutant and heterologous expression of NanA in Lactococcus lactis

Streptococcus pneumoniae (SPN; pneumococcus) accounts for 50% of bacterial meningitis in humans. This often devastating infection car­ ries a 30% mortality rate, and up to half of survivors experience neurological sequelae reflecting a wide spectrum of brain injuries, including cortical neuronal necrosis and hippo­ campal neuronal apoptosis (Koedel et al., 2002; van de Beek et al., 2004; Weisfelt et al., 2006; Weber and Tuomanen, 2007). To cause meningitis, bloodborne bacteria must first at­ tach to and penetrate human brain micro­ vascular endothelial cells (hBMECs), which is the single­cell layer comprising the majority of the blood–brain barrier (BBB; Tuomanen, 1996; Kim, 2003). The molecular mecha­ nisms underlying the distinct central nervous system (CNS) tropism of SPN are poorly un­ derstood. All SPN clinical isolates express the surface­anchored sialidase (neuraminidase) NanA (Cámara et al., 1994; Pettigrew et al., 2006) that targets sialic acid residues on host cells and bacterial flora to promote SPN mucosal colonization (Shakhnovich et al., 2002; Tong et al., 2002). Here, we use an isogenic SPN NanA­deficient mutant and heterologous expression of the NanA constructs to probe the contribution of this protein to SPN­ hBMEC interactions in vitro and BBB pene­ tration in vivo.