Research Interests

Our laboratory studies the molecular mechanisms of pathogenesis of Salmonella spp. and Campylobacter spp. Together, these bacterial pathogens are responsible for the majority of food-borne illnesses in the world. Although most often these bacteria cause self-limiting gastroenteritis, they can also cause life-threatening disease such as typhoid fever. To study these bacterial pathogens, we take a multidisciplinary approach that include bacterial genetics, animal models, cell biology, immunology, and structural biology approaches. We aim to define the functional and, when possible, the atomic interface between these pathogens and their hosts, and in the process, to provide the bases for the development of novel prevention and therapeutic strategies to combat infectious diseases that are a global public health concern. 


Type III protein secretion systems: remarkable nanomachines for protein injection into mammalian cells

An essential feature of the pathogenicity of Salmonella is their ability to engage host cells in two-way biochemical interactions. Central to these interactions are two type III secretion systems (T3SS), which have the remarkable capacity to “inject” bacterial proteins into eukaryotic host cells with the capacity to modulate cellular functions for the pathogen’s benefit. Our laboratory is interested in characterizing the structure and function of T3SSs as well as the activities of the bacterial “effector proteins” they deliver to host target cells. We are particularly interested in defining the structure, function, and assembly pathway of the type III secretion nanomachine or injectisome  (Fig. 1, Video 1).  We are also interested in understanding how this machine senses the host, recognizes its effector substrates, and delivers them to target host cells (Video 2). To pursue these aims we make use of a variety of imaging approaches including cryo Electron Tomography (CryoET, Video 1) and super-resolution fluorescence microscopy (Video 2). It is our ultimate goal to understand all aspects of the function of this remarkable nanomachine, and with this knowledge, develop novel therapeutic and prevention strategies to combat many important infectious diseases

Fig. 1.   Molecular model of the organization of the entire T3SS machine  in situ .  The available (or modeled) atomic structures of different components have been fitted into the structure. The location of the outer membrane (OM), inner membrane (IM), and peptidoglycan (PG) of the bacterial envelope are indicated.

Fig. 1. Molecular model of the organization of the entire T3SS machine in situ. The available (or modeled) atomic structures of different components have been fitted into the structure. The location of the outer membrane (OM), inner membrane (IM), and peptidoglycan (PG) of the bacterial envelope are indicated.

Video 1: Depicting the Remodeling of the Secretion Machine during Its Assembly

In situ molecular architecture of the Salmonella type III secretion machine. Bo Hu, Maria Lara-Tejero, Qingke Kong, Jorge. E. Galán, and Jun Liu. Cell. 2017 March 09. doi:10.1016/j.cell.2017.02.022


The cell biology of Salmonella infection: modulation of host functions to facilitate pathogen replication and survival

In order to survive and replicate within the host, Salmonella has evolved the ability to modulate all aspects of host cell physiology. Through the delivery of more than 50 bacterial effector proteins by its two type III secretin machines, Salmonella can modulate many cellular processes including cytoskeletal dynamics, signal transduction, vesicle trafficking, and transcriptional responses.  These bacterial effectors accomplish this remarkable task by operating as effective “mimics” of eukaryotic proteins.  Our laboratory is interested in characterizing the activities of the various effector proteins encoded by Salmonella.  Over the years, we have identified unique activities associated with these effector proteins, which include exchange factors (GEFs) and GTPase activating proteins (GAPs) for various Rho and Rab-family GTPases, actin nucleators, phosphoinositide and tyrosine phosphatases, E3 ubiquitine ligases, and a variety of remarkably specific proteases that target specific host proteins. The carefully coordinated activity of these effector proteins allow Salmonella to enter mammalian cells, build an intracellular niche, and avoid innate immune defense mechanisms.  In addition, they stimulate transcriptional responses that lead to inflammation (Fig. 2), which is critical for Salmonella to replicate within the gut. In addition to providing major insight into the mechanisms of pathogenesis, the study of the bacterial effector proteins can provide unique insight into basic mechanisms of cell biology and innate immunity.

Fig. 2. Model for the interaction of  Salmonella  with the intestinal epithelium.  Through its type III secretion system encoded within its pathogenicity island 1,  Salmonella  delivers the effector proteins SopE, SopE2, and SopB, which in a functionally redundant manner activate Cdc42. Activation of Cdc42 leads to the formation of a PAK1/TRAF6/TAK1 complex, the phosphorylation of PAK1 at Ser223, and the subsequent activation of NF-κB and production of pro-inflammatory cytokines. The ensuing intestinal inflammation allows  Salmonella  to out-compete the resident microbiota and replicate within the lumen of the intestinal tract. At the same time, the inflammatory response controls the spread of  Salmonella  to deeper tissues.

Fig. 2. Model for the interaction of Salmonella with the intestinal epithelium. Through its type III secretion system encoded within its pathogenicity island 1, Salmonella delivers the effector proteins SopE, SopE2, and SopB, which in a functionally redundant manner activate Cdc42. Activation of Cdc42 leads to the formation of a PAK1/TRAF6/TAK1 complex, the phosphorylation of PAK1 at Ser223, and the subsequent activation of NF-κB and production of pro-inflammatory cytokines. The ensuing intestinal inflammation allows Salmonella to out-compete the resident microbiota and replicate within the lumen of the intestinal tract. At the same time, the inflammatory response controls the spread of Salmonella to deeper tissues.

Video 2: The T3SS-mediated Salmonella-host interaction and a plausible pathway of effector translocation

Visualization of the type III secretion mediated Salmonella-host cell interface using cryo-electron tomography. Donghyun Park, Maria Lara-Tejero, M Neal Waxham, Wenwei Li, Bo Hu, Jorge E Galán, Jun Liu. Elife. 2018 Oct 3.;7. pii: e39514. doi: 10.7554/eLife.39514.

Video 3: 4Pi-SMSN images of mEos3.2-PrgH, mEos3.2-SpaO, and SipD-AF647

Visualization and characterization of individual type III protein secretion machines in live bacteria. Yongdeng Zhang, María Lara-Tejero, Jörg Bewersdorf, and Jorge E. Galán. PNAS. 2017 May 22. doi:10.1073/pnas.1705823114.


Typhoid toxin: a window into the unique biology of Salmonella Typhi

Unlike most Salmonella enterica serovars, which cause self limiting gastroenteritis (i. e. “food poisoning”) and can infect a broad range of hosts, Salmonella enterica serovar Typhi (S. Typhi), is a unique human pathogen that causes “typhoid fever”, a life threatening systemic disease that kills more than 200,000 people every year. We are interested in defining the unique features that make this pathogen so lethal. One of the specific areas of emphasis is the study of “typhoid toxin”, a unique A2B5 toxin present in S. Typhi and the related serovar Paratyphi,  which can also cause typhoid fever (Fig. 3). S. Typhi produces typhoid toxin only within mammalian cells. After its production and secretion from the bacteria, the toxin is transported to the extracellular environment by a unique mechanism that involves vesicle carrier intermediates. Systemic administration of the toxin to experimental animals can, by itself, reproduce many of the life-threatening symptoms of typhoid fever. We are interested in defining the mechanisms by which the toxin is produced at the appropriate place, transported out of the bacteria and out of the infected cells, as well as the mechanisms by which it causes typhoid fever. We are also interested in defining in molecular detail the mechanisms that restrict S. Typhi replication in non-permissive hosts with the ultimate goal of defining novel mechanisms of pathogen control by the innate immune system.

Fig. 3. Atomic structure of typhoid toxin.


Campylobacter jejuni: decoding strategies to compete against the intestinal microbiota 

Campylobacter jejuni can colonize the intestinal track of many animals, particularly birds, without causing disease. Occasionally, however, C. jejuni can replicate within the human gut and cause serious illness. We are interested in defining the mechanisms by which the host’s intestinal microbiota can restrict C. jejuni replication in the gut and how C. jejuni can occasionally overcome this restriction barrier and cause disease. We make use of animal models, genomic and bacterial genetic approaches to identify factors involved in this complex interaction. The ultimate goal is to understand the mechanisms by which the host intestinal microbiota restricts the replication of an intestinal pathogen, which could lead to novel prevention strategies against food-borne infections.