The most likely candidate responsible for the production of overwhelming septicemic shock complicating bacteremia with Neisseria meningitidis is:

Neisseria meningitidis is a gram-negative pathogen that causes 20% of meningitis cases, the second most common cause of the disease. Bacteremia produced by this organism (meningococcemia) is characterized by high fever, hemorrhagic rashes (petechiae) often culminating in disseminated intravascular coagulation, and circulatory collapse (Waterhouse-Friderichsen syndrome). Meningitis is the most common complication of meningococcemia. The endotoxin (lipopolysaccharide) associated with the outer membrane of the cell wall of this organism is responsible for these symptoms. Capsular polysaccharides and outer membrane proteins are useful antigenic markers for the classification of this organism but do not contribute to the development of septicemic shock. Ref: Ray C.G., Ryan K.J. (2010). Chapter 30. Neisseria. In C.G. Ray, K.J. Ryan (Eds),Sherris Medical Microbiology, 5e.