Dhritiman Samanta, Ph.D.

Coxiella burnetii is a gram-negative bacterium and the causative agent of human Q fever. In nature, Coxiella transmits from livestock animals to humans via direct contact or via contaminated aerosols. While acute Q fever commonly manifests as a non-specific febrile illness, it could lead to life-threatening endocarditis in chronic cases. Patients at the highest risk of developing chronic Q fever are those with pre-existing cardiac valve defects, immunocompromised patients, and pregnant women. Chronic Q fever treatment requires at least 18 months of antibiotic combination therapy and there is currently no licensed vaccine available in the United States. The lack of effective antibiotics for chronic Q fever or a vaccine warrants further research into Coxiella pathogenesis.



Coxiella 's success in multiplying in human cells and evading host immune response is attributed to its specialized replication compartment. Upon entering the host cell, the bacteria reside in a small vacuole that grows over time to become a large acidic compartment termed the Coxiella- containing vacuole (CCV). Besides providing a replication environment, the CCV protects Coxiella from host immune response and antibiotics, thereby facilitating disease progression. Our long-term goal in the Samanta Lab is to identify key host and bacterial proteins that regulate CCV biogenesis. Unlike many other intracellular bacteria, Coxiella allows CCV to interact with lysosomes that deliver acids and enzymes into the CCV. However, our previous data show that over-acidification of the CCV is lethal for Coxiella, and therefore, to prevent CCV over-acidification Coxiella blocks host lysosome maturation. In the Samanta lab, we culture mammalian cells in the tissue culture incubators and infect the cells with a BSL-2 strain of Coxiella, and study endosomal maturation pathways during infection using quantitative microscopy.



We also showed previously that Coxiella- injected bacterial proteins inhibit Rab5-Rab7 transition on host endosomes, a key molecular switch that carries out endosomal maturation. In one project, we are studying the molecular mechanisms behind this regulation .

One outcome of Coxiella's inhibition of endosomal maturation is presumably the generation of a population of "immature" cellular compartments. However, the characteristics of these "immature" compartments in Coxiella- infected cells remained unclear. Normally host cells tightly regulate the trafficking of the cellular compartments and recycle a subset of these to the plasma membrane. Our previous data showed that Coxiella -infected cells have a comparable number of starting compartments but significantly fewer mature lysosomes than mock-infected cells (Fig 1). This led us to hypothesize that Coxiella induces the recycling of a large portion of cellular compartments in the infected cells. To test this hypothesis, we used fluorescent microscopy to measure the quantity of Rab35 and Rab11, two markers of the recycled compartments in mock and Coxiella -infected cells. So far, we observed significantly higher Rab35 content in the infected cells compared to the mock, suggesting Coxiella induces compartmental recycling during infection. Our studies with Rab11 are ongoing. To our surprise, both Rab35 and Rab11 also localized to a subset of CCVs, suggesting Coxiella also directly interacts with recycling endosomes. Therefore in the second project (currently the main focus of the lab), We are analyzing the role of recycling endosomal proteins Rab35 and Rab11 in Coxiella pathogenesis and we aim to identify the molecular mechanisms of Coxiella's interactions with recycling endosomal proteins.



Our data, for the first time, show that Coxiella induces compartmental recycling and interacts with the recycling compartments. Our current and future studies will further elucidate the molecular mechanisms of these interactions and potentially identify novel bacterial and host proteins essential for Coxiella survival, which can be targeted to develop vaccines and antibiotics against Coxiella infections in the future.

Below is a summary of our generalized workflow of studying Coxiella pathogenesis in the lab:



We are looking for enthusiastic student researchers!! If our projects and mammalian cell cultures interest you, contact us at dsaman@midwestern.edu.
Manipulation of endosomal recycling by Coxiella burnetii
Mechanisms of endosomal maturation inhibition by C. burnetii
Antimicrobial effects of Short-Chain Fatty Acids on C. burnetii