Interactions Between Chlamydiae and The Mammalian Host
Research Areas: Chlamydia, Pathogenesis, Sexually Transmitted Disease, Trachoma, Cell Biology
The four species of chlamydiae are obligate intracellular bacteria that cause disease in a wide variety of animal species. These conditions affect millions of people worldwide and lead to billions of dollars in medical expenses yearly in the U.S. alone. Although the host range and diseases caused by the chlamydiae are diverse, the infectious process used by all chlamydiae is very similar. All chlamydiae have an alternating life cycle consisting of two distinct developmental forms. Attachment to and entry into the host cell is mediated through a non-metabolic life stage, the elementary body (EB), while intracellular multiplication progresses through the reticulate body (RB), a metabolically active, non-infectious form. After EB attachment and entry, chlamydiae remain separated from the cytoplasm in a membrane-bound vesicle, which grows in size until cell lysis and bacterial release. Fusion of the inclusion with host cell lysosomes is specifically inhibited during infection by chlamydiae- a process that protects the invading bacterium from the toxic environment within the lysosome. Conversely, fusion of the inclusion with Golgi derived vesicles is an integral part of the developmental process and it is thought this pathway may deliver nutrients to the growing chlamydiae. A final twist in this story is that early chlamydial protein synthesis is required for the placement of the inclusion into the appropriate vesicle trafficking pathway. Very little is known of the molecular events leading to these processes. Much is still unknown regarding the basic biology of chlamydial development, their interaction with host cells, and the means by which they cause serious disease.
Our laboratory focuses on three main areas of chlamydial research. First, we investigate the mechanisms used by chlamydiae to develop and maintain their intracellular environment (the inclusion) within infected cells. We (and others) have identified a collection of proteins- termed Inc proteins- that are localized to the inclusion membrane. Each of these proteins is present in Chlamydia -infected tissue culture cells but absent in the purified elementary bodies. Additionally, IncA is exposed to the cytoplasm of infected cells and is phosphorylated by host cell protein kinases. These proteins contact the cytosol in the infected cell and are positioned to directly interact with host cell proteins. These data are exciting because they show Inc proteins to be potentially very important in the unique interactions between host cells and chlamydiae. Continued studies in this area will further our understanding of the cellular processes parasitized by chlamydiae, and may identify ways to interfere with this parasitism.
Second, we conduct genomics analyses of clinical C. trachomatis isolates. A transformation system has just been developed for the chlamydiae, and the analysis of individual genes within this system remains challenging. We approach this problem through the study of a large library of clinical isolates assembled by collaborators at the University of Washington. We have identified strains within this library that have unique properties in vitro and in vivo, and we are currently using a genomics approach to investigate these unusual strains. Our approach has thus far been successful in associating IncA with a nonfusogenic phenotype, and in the identification of a unique property of secondary inclusion formation by some strains.
We also investigate antibiotic resistance and antibiotic design in this system. Our group participated in the first identification of an antibiotic resistant chlamydial species, Chlamydia suis, in which resistance is mediated via a TetC efflux pump. We have also shown in the laboratory that the resistance gene can be transferred to the human pathogen Chlamydia trachomatis via a poorly characterized horizontal transfer system. This is significant because tetracycline is a frontline drug of choice in treatment of both human and veterinary chlamydial infections, and the spread of this resistance in clinical settings would be a serious problem. This is also the first example of any horizontally acquired antibiotic resistance gene in any chlamydial species.
We continue our study of antibiotics in chlamydial biology by using chlamydiae as model organisms for antibiotic design, through collaborative efforts with a local biotechnology company. Novel antimicrobials are a critical need in the world today, as more and more clinical conditions are being met with significant resistance problems. Our efforts in this area have led to the identification of several lead compounds that may have utility against a variety of important bacterial species.