This study showed that recombinant properdin, which promotes activation of the complement pathway, could protect mice against infection with lethal-dose Streptococcus pneumoniae and Neisseria meningitidis. In vitro, properdin caused deposition of C3b fragments on bacterial cell surfaces — which is a key step in promoting a host immune response against bacteria — and boosted cell lysis. Importantly, the majority of mice given properdin did not develop sepsis. So properdin might be useful for combating infections caused by drug-resistant neisserial or streptococcal strains of bacteria.