THE ROLE OF THE GRAM-NEGATIVE CELL ENVELOPE IN THE PATHOGENESIS AND TREATMENT OF PNEUMONIA

Abstract

Bacterial pneumonia is a leading cause of morbidity and mortality worldwide, and both host- and pathogen-derived factors contribute to pathogenesis. Acinetobacter baumannii is a Gram-negative pathogen that causes hospital-acquired and ventilator-associated pneumonia. Virulence factors implicated in A. baumannii pathogenesis include carbohydrates, proteins, and lipids. How bacterial lipids other than lipopolysaccharide/lipo-oligosaccharide, such as the anionic lipid cardiolipin, contribute to pneumonia pathogenesis is incompletely understood. Type strains such as A. baumannii 17978 are used to study A. baumannii pneumonia pathogenesis, but to what extent long-term laboratory maintenance of A. baumannii type strains has contributed to genetic divergence is currently unknown. Antibiotics that can be used for the treatment of bacterial pneumonia include aminoglycosides (AGs): a class of polycationic antibiotics. AGs appear to be uniquely effective when administered directly to the lung, however, the mechanism for this is not known. In this dissertation, the discovery that Gram-negative bacteria retained AG antibiotics following exposure is described. The bacterial outer membrane was implicated as the predominant AG reservoir. In a mouse model of pneumonia, AG-bound bacteria interacted with host-derived pulmonary surfactant to affect killing of co-infecting bacteria. Additionally, the discovery of two variants of A. baumannii strain 17978 with distinct genotypes and infection-associated phenotypes is described. The two variants (VU and UN) differed based on the presence of a 44-kb accessory locus, which encompasses several putative pathogenesis genes. The roles of two of these putative pathogenesis genes (an accessory catalase gene: katX; and an accessory cardiolipin synthase gene: clsC2) were interrogated. The presence of katX enhanced bacterial resistance to oxidative stress and neutrophil-mediated killing in vitro, whereas the presence of clsC2 promoted bacterial cell envelope stress resistance and altered effector functions of infected macrophages. Finally, preliminary inquiries into the effects of cardiolipin overproduction by A. baumannii on pneumonia pathogenesis are described. Expression of clsC2 by A. baumannii enhanced macrophage effector functions in vitro, and increased A. baumannii cell envelope cardiolipin content may be a driver of neutrophilic inflammation in a mouse model of pneumonia. Overall, this work furthers our understanding of the molecular interactions at the host-microbe interface in the setting of bacterial pneumonia.