The biology of bacterial type III secretion systems is a fast-moving field and rapidly accumulates knowledge about the function of the complex nanomachines that pathogens developed for injecting host cells with the toxins necessary to trigger host-cell invasion. However, mechanistic understanding of this complex process—which ultimately may allow exploitation of type III secretion systems for drug delivery—requires insights into the three-dimensional structure and structural dynamics of the highly sophisticated type III machinery.

“...continued work on this project may pave the way for the development of a new type of antimicrobial agents, or the capacity to exploit “tamed” pathogens to deliver therapeutic agents to the body.”

Gaining this insight has been hampered by the fact that fully assembled type III secretion systems integrate more than 20 protein components into a highly organized structure and that some of these components are membrane-embedded proteins whose structure determination remains notoriously difficult. Our study provides the first detailed picture of a substructure of Salmonella’s type III system, known as the "needle complex."

Moreover, we also solved the structure of an assembly intermediate of the needle complex (view image), known as the "base," that lacks the long, needle-like extension used to transfer toxins from the pathogen to the host cell. By comparing the two structures, we were able to visualize large conformational changes that are associated with switching the complex from an "assembly mode" into a "delivery mode"—a feat that had not been accomplished before.

Taken together, this study provides a powerful template to generate and test new hypotheses about structure-function relationships in the complicated molecular assemblies, and also to understand the results of biochemical studies in the context of the molecular structure of type III secretion complexes.

Many bacteria that are able to cause disease have evolved an ingenious device or nanomachine known as the "type III secretion injectisome" that allows them to "inject" bacterial proteins into human cells. These bacterial proteins can then take control of key cellular functions of the cell for the benefit of the pathogen. One of these bacteria is Salmonella enterica, the cause of food poisoning and typhoid fever, which uses one of these machines to coax the cell into taking it in. Utilizing high-resolution electron cryomicroscopy, we have been able to visualize this nanomachine in great detail. This information will help understand how this device works and design drugs to interfere with its function.

My research group became involved after I was approached by Jorge Galán, chairman of the Department of Microbial Pathogenesis at the Yale School of Medicine. Galán, who has spent many years on understanding the genetics and biochemistry of type III secretion systems, showed me a few electron micrographs that his group had generated from preparations of the needle complex.

Seeing these images, I was immediately convinced that this macromolecular complex would make a wonderful sample for the type of high-resolution molecular microscopy that we do in my laboratory. From this initial meeting, we have formed a close collaboration with the Galán lab and in the course of this collaboration our different fields of expertise have provided a mutually beneficial, stimulating, and very exciting environment.

Nevertheless, the way was paved with some obstacles—the most serious being that needle complexes are oligomeric structures and that they can assemble with different rotational symmetries. It took about two years to realize this and to sort it out, but once that was out of the way, exciting results came about very quickly.

This is not an easy question to answer, but in principle, the answer is "yes." As noted above, continued work on this project may pave the way for the development of a new type of antimicrobial agents, or the capacity to exploit "tamed" pathogens to deliver therapeutic agents to the body. Moreover, fully understanding the precise molecular mechanisms by which pathogens trick susceptible host cells into letting themselves being invaded may also prove essential to counteract and prevent the use of engineered pathogens as biological weapons.