Spring-like bacterial hairs initiate infection

First cryo-EM paper shows how bacteria cling to host tissue and facilitate urinary tract infections

One of the first users of Diamond Light Source’s new cryo-electron microscope (cryo-EM) has elucidated the structure of the hair-like appendages found on Escherichia coli (E.coli). The remarkable atomic model generated from a 3.8 Å resolution cryo-EM reconstruction has recently been published in the high-profile journal, Cell (Fig. 1).

Bacteria, such as E.coli, naturally possess hair-like structures exposed at the cell surface, which are known as pili. The pili play an essential role in infection as they are responsible for seeking out and latching onto specific host tissues. By fully understanding the structure and function of these unassuming bacterial appendages, it is thought that new medicines could be developed to prevent infection.

A team of scientists from University College London (UCL) and Birkbeck, University of London, the University of Washington, and the University of Virginia, took on the challenge to delve into the inner workings of the pili. By using the cutting edge cryo-EM facility at Diamond, the group discovered that the pili were shaped like springs, which allowed the bacteria to cling on to host tissues even in adverse environments.

Figure 1. A) Surface diagram of the pilus in its coiled state, showing each subunit in a different colour. The subunit coloured cyan is the reference subunit and is numbered ‘0’. Subunits above or below this reference are numbered accordingly. B) Surface diagram of the pilus in its uncoiled state, showing only the N-terminal extension sections to clearly illustrate the helical organisation of the pilus.

Shear forces

The pili studied by the team were known as type P, which were isolated from pathogenic E.coli. The bacteria are responsible for urinary tract infections, so these pili have to work extra hard to maintain adhesion during urine flow. The shear forces exerted on the bacteria from the flow of urine would ordinarily dislodge them, so understanding how the pili are able to resist these forces is of great interest.
The pili were made in vitro and applied to electron microscopy (EM) grids, which were flash frozen prior to analysis at the new electron-Bioimaging Centre (eBIC) at Diamond. Professor Waksman, director of the Institute of Structural and Molecular Biology at Birkbeck and UCL, and principal investigator of the study highlighted the importance of this new national facility: “There is clearly a revolution in EM, which has been driven by improvements in three key areas: better microscopes, better detectors and better software, and all of this latest technology is available at eBIC.”
The data were collected in two days at eBIC, and the atomic structure was finalised two weeks later. Never before has the structure of the P pilus been resolved, so this study has given scientists an important insight into mechanisms employed by bacteria to promote infection.
Cryo-EM showed that the pili were coiled into springs, and it was this flexibility that provided the strength needed to resist the shear forces in the urinary tract (Fig. 1). Prof Waksman elaborated: “The pili uncoil during urine flow and recoil when the forces recede. By interfering with the pili using small molecules, we might be able to prevent infections. These pili are found on other pathogens beside E.coli, so a new class of antibiotics could be developed.”
The group next plan to explore how the pili are formed and intend to disrupt this process. Prof Waksman explains: “As the pilus forms, protein subunits are translocated and polymerise into a helical shape. We want to see what drives this helix formation, so we will introduce a series of mutations into the pili to potentially weaken their spring-like property and see how their growth is affected.”
To find out more about using the cryo-EM facility at eBIC, or to discuss potential applications, please contact Diamond’s Life Sciences Coordinator, Dr Martin Walsh: martin.walsh@diamond.ac.uk.

Related publication (Open Access):

Hospenthal MK, Redzej A, Dodson K, Ukleja M, Frenz B, Rodrigues C, Hultgre SJ, DiMaio F, Egelman EH & Waksman G. Structure of a chaperone-usher pilus reveals the molecular basis of rod uncoiling. Cell 164, 1–10. (2016) doi:10.1016/j.cell.2015.11.049


Related content: From the University of Virginia

Watch the University of Virginia's video with Professor Edward H. Egelman describing how the research team explored the structure of the bacterial pili.


Image credit:

The image that appears on the homepage and the Science Highlights page is adapted from Graphical Abstract of Hospenthal et al. 2015. This image is used under the creative commons Attribution 4.0 International licence.