The small world of bacteria never ceases to amaze us. Prof. Yves Dufrêne’s team has just demonstrated that a staphylococcus is capable of clinging fiercely to a host cell in the event of mechanical stress and then ‘letting go’ beyond a certain threshold. To infect more efficiently elsewhere?
A postdoctoral student at the Louvain Institute of Biomolecular Science and Technology (LIBST), Marion Mathelié-Guinlet sets the scene like a physicist: ‘We’re trying to understand how physical forces influence cell behaviour.’ She’s a member of the team of FNRS Research Director Prof. Yves Dufrêne, which is studying the interactions that occur when a bacterium adheres to an abiotic surface (such as a catheter) or to a cell in order to colonise and infect its host (see ‘Stressed out staph’). It first attaches itself to the host by means of adhesins, adhesion proteins that it expresses on its surface, which bind tightly to extracellular host proteins such as fibrinogen, a protein in blood plasma. During such interactions, bacteria are known to be subject to shear flow conditions, particularly in blood vessels or urinary tracts. You might think the harsher these conditions, the greater the shear and the more the bacteria will tend to detach from its host, losing its pathogenicity. This isn’t the case: studies on the scale of a population of cells have shown that when the shear is strong, bacterial adhesion can in fact be reinforced. This ability of the bacterium to strengthen its adhesion (it becomes stronger and lasts longer) when the mechanical stress to which it is subjected increases is potentially due to the formation of a so-called catch bond. This paradoxical behaviour was demonstrated at the molecular level in the 2000s by the Gram-negative bacterium E. coli (in simple terms, Gram-negative bacteria are those with two membranes separated by a thin wall). But what about other bacteria, particularly Gram-positive bacteria (which have only one membrane and a thick wall)? This is where the recent publication by Prof. Dufrêne’s team takes on its full significance.
Dr Mathelié-Guinlet explains, ‘This time we chose to study not the well-known Staphylococcus aureus but rather Staphylococcus pseudintermedius, a staphylococcus which is pathogenic for dogs, causing dermatitis and ear infections, for example. We were interested in it not only because it can be transmitted to humans but because it expresses adhesion proteins, notably the SpsD protein, which are similar to those of Staphylococcus aureus in terms of structure and interactions with certain proteins, such as fibrinogen, that human cells express.’ These interactions involve a mechanism called the dock, lock, and latch (DLL), which has been identified between the adhesins of Staphylococcus aureus and human fibrinogens. The adhesion forces brought into play between two proteins bound by this mechanism are very strong, on the order of one nanonewton (1 nN = 10-9 N), whereas in general, the interactions that bind adhesins and host proteins are on the order of a few tens to hundreds of piconewtons (1 pN = 10-12 N).
Prof. Dufrêne’s team then asked two questions: Why are these bonds so strong? Are they catch bonds?
Atomic force microscope
To study the interactions between a bacterium (or rather its adhesion proteins) and a host, researchers use an atomic force microscope (AFM). ‘It’s like a vinyl record player, but instead of producing sound it measures force,’ Dr Mathelié-Guinlet says. ‘A very fine tip scans the surface of the sample, which makes it possible to map the surface pixel by pixel, but also to extract its nanomechanical properties. Even more amazing, it’s possible to graft biomolecules onto the AFM tip and thus probe interactions, such as adhesion strength or rupture, between the molecules and a sample, such as proteins on the bacterial surface.’ In the case of Staphylococcus pseudintermedius, the researchers found a key-lock system. The key would be the fibrinogen (on the AFM tip), the lock the bacterium’s adhesion protein, SpsD. But here’s the trick: the key’s insertion, which takes place between two domains of the adhesin, causes a change in the adhesin’s three-dimensional structure. As a result, the fibrinogen becomes trapped within the adhesin. ‘It's a bit like turning a key in a lock: if you don’t turn it all the way, you can't remove it.’ Unless you exert significant force, roughly 2 nN.
Hence the second question: Is this intense interaction further reinforced in the event of greater stress – in other words, is it a catch-bond type relationship? ‘We subjected the bonds to different forces to mimic stresses to which the bacterium could be subjected,’ Dr Mathelié-Guinlet continues. ‘And we saw the emergence of a catch-bond phenomenon: when we increased the force exerted on the adhesion complex, the elapsed time for the bond to break increased. But if the force exceeded a certain critical threshold, the duration of the bond decreased. This transition is typical of a catch bond.’ This is the first time that a catch-bond phenomenon has been observed in a Gram-positive bacterium and in a living cell: the UCLouvain experiment is being carried out on living cells and not on purified proteins, as was the case in the past.
Path to other treatments
For Dr Mathelié-Guinlet, this discovery is important. ‘It shows a genuine strategy by bacteria. First of all, in stressful situations, for example in the blood vessels or urinary tract, they attach themselves to their host in a very strong, effective and prolonged way. But if the stress increases beyond a critical threshold, they weaken their bond and detach, most likely to infect other organs. Because that’s their job: to spread infection.’
This better understanding of the behaviour of bacteria is obviously a step towards the development of alternative antibacterial treatments – not to cure an infection, as is the case today with antibiotic treatment, but to prevent it by blocking bacteria from bonding and spreading throughout the body.
A glance at Marion Mathelié-Guinlet's bio
If Marion Mathelié-Guinlet’s career path is somewhat atypical, it’s destined to become less so: the Covid-19 pandemic shows that medicine needs researchers from different disciplines, among them physicists. Dr Mathelié-Guinlet is convinced of this. Originally from south-western France, she earned a master’s degree in physical chemistry engineering from the Ecole Centrale de Lyon (2014). It’s a highly multidisciplinary degree, which reflects her family background. ‘I lived in a family with varied backgrounds,’ she says, ‘from law to medicine, which actually sharpened my ‘opposing’ interest in the basic sciences. So I was attracted very early on to the mysteries of biology, but I wanted to bring a physicist’s eye to it and analyse problems at the molecular level.’ That didn’t stop her from complementing her education with a University of Manchester master’s degree in … astrophysics. After this escape into the infinite, she returned to the microscopic to pursue a PhD on the interactions between bacteria and nanoparticles, which she defended at the Laboratoire Ondes et matières d’Aquitaine (LOMA) in Bordeaux (2017). The subject led her to Prof. Yves Dufrêne’s team as a postdoctoral researcher in 2018..
Force-clamp spectroscopy identifies a catch bond mechanism in a Gram-positive pathogen, Marion Mathelié-Guinlet et al., Nature Communications, https://www.nature.com/articles/s41467-020-19216-8