Lipoprotein PsaA in Virulence of Streptococcus pneumoniae : Surface Accessibility and Role in Protection from Superoxide

Abstract

ABSTRACT PsaA of Streptococcus pneumoniae , originally believed to be an adhesin, is the lipoprotein component of an Mn 2+ transporter. Mutations in psaA cause deficiencies in growth, virulence, adherence, and the oxidative stress response. Immunofluorescence microscopy shows that PsaA is hidden beneath the cell wall and the polysaccharide capsule and only exposed to antibodies upon cell wall removal. A psaBC deletion mutant, expressing PsaA normally, was as deficient in adherence to Detroit 562 cells as were strains lacking PsaA. Thus, PsaA does not appear to act directly as an adhesin, but rather, psaA mutations indirectly affect this process through the disruption of Mn 2+ transport. The deficiency in Mn 2+ transport also causes hypersensitivity to oxidative stress from H 2 O 2 and superoxide. In a chemically defined medium, growth of the wild-type strain was possible in the absence of Fe 2+ and Mn 2+ cations after a lag of about 15 h. Addition of Mn 2+ alone or together with Fe 2+ allowed prompt and rapid growth. In the absence of Mn 2+ , the addition of Fe 2+ alone extended the 15-h lag phase to 25 h. Thus, while Fe 2+ adversely affects the transition from lag phase to log phase, perhaps through increasing oxidative stress, this effect is relieved by the presence of Mn 2+ . A scavenger specific for superoxides but not those specific for hydroxyl radicals or H 2 O 2 was able to eliminate the inhibition of growth caused by iron supplementation in the absence of Mn 2+ . This implies that superoxides are a key player in oxidative stress generated in the presence of iron.