The pathogenetic mechanism of AB is poorly understood. A limited number of virulence factors reduce this bacterium to the role of an opportunist. Although growth in an environment with an acidic pH at lower temperatures may enhance its ability to invade devitalized tissue, no known cytotoxins are produced. Lipopolysaccharide is present in the cell wall, but little is known of its endotoxigenic potential in humans. Most experts think that it is an opportunistic organism. On the other hand, AB and Neisseria meningitidis belong to the same family of Neisseriaceae, and they both have lipopolysaccha-rides in the outer membrane. Excellent reviews are available that have addressed the general and immunologic aspects of meningococcal disease, the role of cytokines, and the role of N meningitidis lipopoly-saccharides in the pathophysiology. Clinically, there are a number of features that are similar between CAP-AB infection and infection caused by N meningitidis, including septic shock and DIC. Lipopolysaccharides may play an important role in the pathogenesis of CAP-AB, which may explain its fulminant nature. However, this does not explain why CAP-AB is more fulminant than HAP-AB, since both are caused by the same organism. in detail
It is also possible that the fulminant course of disease in CAP-AB patients is due to a selection bias, in which only the most severely affected patients with CAP-AB are admitted to hospital, while patients with milder cases are treated in the community. Acinetobacter sp is ubiquitous, and the source of infection is likely environmental. CAP-AB generally occurs in patients with diminished host defenses (eg, due to alcoholism, tobacco use, diabetes mellitus, renal failure, or underlying pulmonary disease). It is possible that subclinical infection may occur in people exposed to an environmental source of AB, but invasive diseases occur only in hosts with diminished local defense or general immunity, similar to infections caused by L pneumophila. L pneumophila infection can manifest in the following two different forms: Pontiac fever, which is a mild, self-limiting febrile illness without pneumonia; and Legionnaires disease, which is a highly lethal form of pneumonia.