GM000287 is a conserved determinant of serum resistance and virulence in Klebsiella pneumoniae.

Klebsiella pneumoniae (Kp) is an opportunistic Gram-negative pathogen responsible for severe community- and healthcare-associated infections. Resistance to complement-mediated killing is an important determinant of Kp survival in human serum, yet mechanisms beyond capsular polysaccharide-mediated protection remain incompletely understood. Here, using comparative proteomic approaches, we identified a previously uncharacterized protein, GM000287, as a conserved determinant of serum resistance and virulence in Kp. Human serum exposure induced expression of the bacterial type II secretion system (T2SS), and deletion of the essential T2SS components GspD/GspE increased susceptibility to human serum killing. Importantly, disruption of GspD/GspE attenuated virulence in a murine pneumonia model. Comparative secretome analysis identified GM000287 as a T2SS-associated envelope protein that contributes to serum survival. Loss of GM000287 increased C3b production and factor B deposition on the bacterial surface during human serum exposure, consistent with enhanced alternative pathway amplification. In a murine pneumonia model, deletion of GM000287 reduced bacterial persistence and dissemination and attenuated virulence. Given the minimal bactericidal activity of mouse serum against Kp, indicating an additional role for GM000287 during infection. Importantly, GM000287 was highly conserved among Klebsiella species and broadly distributed among members of the Enterobacteriaceae family, and its contribution to serum resistance was validated in genetically diverse Kp clinical isolates. Together, these findings identify GM000287 as a conserved determinant of human serum resistance and in vivo virulence and reveal distinct contributions of GM000287 to K. pneumoniae fitness across different host-associated environments.