Close encounters: The bacterial kind

Touching idea

A new crystal structure reveals unprecedented detail of the interaction between bacterium and cell surface as the two come into close proximity during the infection process.

A better understanding of how bacteria infect cells could lead to better prevention and treatment of a vast range of human diseases as well as perhaps even allowing researchers to ward off emerging bacterial resistance to antibiotics. Now, an interdisciplinary team at the University of Bristol has developed a new approach to studying molecules in their natural environment that could open up new studies of bacterial interactions.

Conventional approaches to researching bacterial infection have homed in on details of individual components from the bacteria and target cells themselves and their biochemistry. This approach has led to many breakthroughs in treating bacterial infection but often fails to address why these individual components behave differently when they are part of a complete bacterium. These types of variations are often thought to account for the failure of many drugs that otherwise appear optimal when studied in the laboratory.

Now, biochemist Leo Brady and molecular biologist Mumtaz Virji have brought together these otherwise distinct approaches - the cellular biology and the biochemistry.

Infectious work

"Conventional antibiotics do not usually work by binding to cell surface proteins like UspA1," Brady told SpectroscopyNOW. "However, antibodies that are generated from vaccines do work in this way. In principle new forms of antibacterial drugs (as distinct from conventional antibiotics that are internalised and kill bacteria) could also do this." The team looked at the common bacterium Moraxella catarrhalis, which causes respiratory and middle ear infections in young children, as well as being a major cause of death in patients with heart disease. Until now, researchers had focused on isolating and studying proteins from the Moraxella cell surface that initiate infection. Blocking such proteins with either novel drugs or vaccines should block the spread of infection. However, new insights are needed, which is where the work of Brady and Virji comes to the fore.

The earlier studies have provided the foundations for the present team's overview of one of the key cell surface adhesion protein in Moraxella, known as UspA1. The surface of a bacterium is usually decorated with a layer of adhesin proteins, which initiate colonization and infection. However, the accumulated knowledge concerning UspA1 pertains only to the molecular biology of this protein in isolation. The Bristol team realised that a more useful view might be offered by capturing details in situ, i.e. in its natural setting on the bacterial cell surface. "An in situ model of UspA1 would hopefully provide a clearer understanding of how putative drugs or vaccines interact with the protein and prevent bacterial infection," adds Brady.

Brady and Virji teamed up with Bristol physicist Massimo Antognozzi, whose research team was developing a new type of atomic force microscopy, lateral molecular force microscopy (LMFM). The researchers have now optimised LMFM so that they can use it to observe changes in UspA1 at the Moraxella cell surface. LMFM taps the samples against the AFM cantilever rather than the cantilever "scanning" the sample surface. The technique allowed the team to measure fine molecular changes and forces on a living cell surface.

Lateral thinking

The team has now used data from LMFM and combined it with their X-ray crystal structures of two isolated, N-terminal fragments of UspA1 and a previous structure of a specific binding site. In so doing they could then correlate behaviour with previously unobservable physical changes of the bacterial cell as it binds to and infects its target human cells. The study has allowed the researchers to observe for the first time the changes that take place as the bacterial cell approaches and binds to a human cell.

"The findings have triggered the development of a novel technology that promises to open up a new approach for studying molecular medicine," says Brady. "This breakthrough will undoubtedly prove equally useful for the study of many other biological processes directly within their cellular environment, something that has long been needed in molecular medicine."

" This study therefore provides a rare direct demonstration of protein conformational change at the cell surface," the team concludes.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.