Medical microbiologist Liselot Dewachter: ‘What is surprising is that the combined effect of the two antibiotics is greater than the sum of their individual effects.’
Pneumococci resistant to antibiotics are a growing problem. With an ERC Starting Grant, microbiologist Liselot Dewachter is developing a new approach. The early results are promising.
‘Streptococcus pneumoniae, the pneumococcus, has my interest because it is an important pathogen. It can be dangerous, particularly for young children and older people. Doctors treat this bacterium with the antibiotic amoxicillin, but increasingly the bacterium is proving insensitive to it, and that resistance has so far been impossible or very difficult to overcome. We have developed a new approach that can make resistant bacteria susceptible to amoxicillin again’, says Liselot Dewachter. She delivered a keynote lecture on 6 November at the annual meeting of the Royal Netherlands Society for Microbiology in Groningen. Dewachter is one of the group leaders in health and microbes at the de Duve Institute in Brussels, part of the French-speaking Université Catholique de Louvain, where she holds a professorship. Earlier this year she received a Starting Grant from the European Research Council (ERC) and was able to appoint new staff. ‘We are going to look for new targets to combat S. pneumoniae by thoroughly unravelling its growth and replication at the molecular level.’
Screening strategy
Her interest in this bacterium arose when, after completing her degree in bioscience engineering and her doctorate in Leuven, she did a postdoc in Lausanne with Dutch researcher Jan-Willem Veening. There she developed a screening strategy based on CRISPR interference, a method that allowed her to scan the entire genome for genes – and thus proteins – that play important roles in the bacterium’s life cycle. Those genes represent vulnerabilities, and it is at those vulnerabilities that she tries to strike.
Indispensable
The research led to the synthesis of the molecule undecaprenyl phosphate (Und-P), which is indispensable for building the cell wall. Bacteria have a cell wall surrounding the cell membrane that provides rigidity; it consists of a network of long chains of amino sugars cross-linked to one another by peptide bridges. ‘The building blocks of that wall are produced inside the cell and have to be transported outward through the cell membrane. That is what Und-P does’, Dewachter explains. ‘That was already known, but with our screening we discovered just how enormous the consequences are when Und-P synthesis fails. Because the building blocks of the cell wall can no longer reach the places where they are needed, the bacterial cells can no longer divide. Instead, they elongate whilst the cell wall becomes weaker. The cells blow themselves up.’
Potentiation
There is an existing drug that inhibits Und-P synthesis: clomiphene. This compound can indeed kill S. pneumoniae, as it turned out. ‘But that is not all. In combination with the antibiotic amoxicillin, clomiphene is even more effective. Amoxicillin works differently from clomiphene; it attacks the cross-links between the amino sugar chains and thereby weakens the cell wall. A combination of the two compounds attacks the bacterium on two fronts. What is surprising is that the combined effect of the two agents is greater than the sum of their individual effects, even in bacteria that are insensitive to amoxicillin alone. We do not yet know the mechanism behind this mutual potentiation. In addition, the combination has the advantage that it is difficult for a bacterium to develop resistance to two compounds simultaneously.’
She wants to continue along this path – using genetic screening to identify where bacteria are most vulnerable in order to target those sites. ‘I focused first on cell wall synthesis because eukaryotes have no cell wall, so agents that weaken the cell wall will have no effect on human cells.’
Side effects
That happens not to be the case for clomiphene, a drug that can stimulate ovulation in women. This compound can have side effects. ‘But the principle of our approach has already proved itself. We still need to work on clomiphene to improve the compound, and we hope that a modification will eliminate the side effects.’ The combination of amoxicillin and clomiphene has only been tested in the laboratory so far and is not yet ready for clinical use, she emphasises. She also wants to examine other bacterial processes. ‘And not only under standard laboratory conditions, but also under the conditions that S. pneumoniae actually encounters in real life. Because there are major differences. The laboratory is like a five-star hotel, with everything optimally tuned for bacterial growth. During an infection, bacteria do not enjoy that luxury. They may be exposed to, among other things, an elevated temperature due to fever, a shortage of the right nutrients, and attacks from the immune system. All manner of processes then come into play that differ from those under optimal conditions, and we will unravel those too in order to find new targets for treatment.’
‘The academic world is trying to find solutions to the growing problem of bacteria that are resistant to antibiotics’, she concludes. ‘But new treatment strategies will need to be taken up and developed further by the pharmaceutical industry. Whether that will happen quickly remains to be seen.’