In a particular and worrying health context, viruses are not the only microorganisms representing a serious threat. Bacteria (and fungi to a lesser extent) are responsible for many infections, particularly in healthcare establishments where one in twenty patients contracts a nosocomial infection.
Although most of these infections can be treated today, this may not be the case shortly. In fact, pathogens gradually and inevitably acquire resistance to the antibiotics that are opposed to them. The misuse of antibiotics (overuse, wrong dosage, etc.) selects the most resistant bacteria, which will therefore be able to survive and then transmit their resistance genes to their congeners.
Therefore, resistance to antibiotics can accumulate in certain bacterial species, and multi-resistant bacteria have already been emerging for many years. Now, the risk of developing pan-resistant bacteria (that is to say, resistant to all known antibiotics) causing incurable infections is a sword of Damocles, which threatens human health. If such a scenario were to occur, these bacterial infections could once again become the leading cause of death by 2050.
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The enemy in the shadows: the biofilm
To this phenomenon of resistance is added another mechanism developed by bacteria. When these are under stress, most bacteria change their behavior by sticking to a surface. They will then produce a matrix, a set of polymers that will consolidate and protect the bacteria from external attacks.
This set of adhered bacteria and matrix constitutes what is called a biofilm. Although they exist in many ecosystems, biofilms attract particular attention at the medical level, as their involvement in infections was greatly underestimated until recently.
The famous dental plaque is probably the most colorful example of biofilm-forming on our own tissues, and it remains safe as long as it is under control. On the other hand, biofilms frequently occur in bone infections by attaching themselves to the surface of the bone or prostheses (hips, knees, etc.) and in pulmonary infections, particularly in patients with cystic fibrosis, or even in heart infections.
In a biofilm, bacteria are better protected from attacks by the immune system and the action of antibiotics. Indeed, many bacteria in the biofilm are in a slower metabolic state (or even stopped), and the antibiotics are effective against the active bacteria, so the bacteria “avoid” them. Antibiotic treatments are then ineffective, or even worse, they can stress the bacteria (instead of killing them), and they will produce more biofilm. The only possible remedy is then to remove the prosthesis and/or to remove the tissues colonized by the biofilm. Therefore, the current medical challenge is to develop antibiofilm solutions and, in particular, methods for preventing the formation of biofilms.
So, where to look for these new molecules? Several possibilities exist because antimicrobial molecules can be of synthetic, biological, or hybrid origin. Among the possible biological resources, one of them draws particular attention: tree bark.
First of all, the bark represents the first physical and chemical line of defense of the tree against pathogens. Therefore, it is possible to envisage that antimicrobial molecules are present in certain forest species’ bark. Second, the bark is hardly exploited by the forest industry and is, at best, used as fuel when it is not simply considered waste and disposed of. They, therefore, represent valuable and non-polluting products that can be recovered in large quantities.
Tree bark against germs
It is with this in mind for recovery that the Biomaterials and Inflammation in Bone Site (BIOS) and Institute of Molecular Chemistry of Reims (ICMR) laboratories of the University of Reims Champagne-Ardenne have joined forces to study a panel of 10 species of ‘characteristic trees of north-eastern France: common beech, pedunculate oak, glutinous alder, wild cherry, sycamore maple, common ash, Canadian poplar (Robusta), European larch, common spruce, and aspen.
The bark was used for each tree species to produce a powder or “extract” through various chemical methods. Each extract was tested on a set of microorganisms, including bacteria and microscopic fungi. For this, a culture of the microorganism is mixed with the extract in different concentrations, and then, after 24 hours of incubation, the growth of the microorganisms is evaluated. Thus, it was observed that most microorganisms’ growth was inhibited for three of the ten extracts. These were pedunculate oak ( Quercus robur ), black alder ( Alnus glutinosa ), and wild cherry ( Prunus avium ).
The rest of the study focused on these three most promising species, aiming to assess the nature of the extracts’ antimicrobial effect. In other words: do bark extracts inhibit the growth of microorganisms (we will then speak of a bacteriostatic effect for bacteria or fungistatic for fungi), or do they go so far as to destroy these microorganisms (we will then speak of bactericidal or fungicidal effect)?
The tests carried out showed that the three extracts actually exhibited bactericidal and fungicidal activities on certain microorganisms. The extract, which then turned out to be the most interesting, is obtained from the cherry tree because it exhibited a lethal activity on nine of the strains of microorganisms tested. In particular, the cherry extract has shown a bactericidal action on pathogens belonging to the genera Enterococcus (urinary tract infections, endocarditis, etc.) and Listeria (listeriosis), but also, and especially on strains of Staphylococcus aureus, the infamous Staphylococcus aureus responsible for more than 14% of nosocomial infections. The antibacterial effect of cherry was observed even for relatively low extract concentrations.
Natural molecules preventing the formation of biofilm.
However, the problem with antibacterial agents stems from their deleterious effects when not used correctly, particularly by leading to the formation of biofilm. Therefore, it appeared essential to verify whether the extract of wild cherry, at low doses, did not promote the formation of biofilm by staphylococci aureus. For this, Staphylococcus aureus cultures were mixed with the extract, and then the amount of biofilm formed on the plastic walls was analyzed. It was thus observed that, even at low concentrations, the extract of cherry tree did not cause the appearance of biofilm, but that, on the contrary, it made it possible to prevent its formation appreciably.
On the strength of these engaging results, the investigations then focused on identifying the molecule at the origin of these antibacterial and antibiofilm effects. Nuclear magnetic resonance analyses made it possible to identify around fifteen molecules present in the wild cherry extract, and the extract was then broken down into fractions containing the various molecules. Additional tests showed that the fractions with the strongest antibacterial and antibiofilm effects contained a particular chemical species: dihydro wogonin, a molecule belonging to the class of flavonoids, a group of molecules known for its antimicrobial effects.
The discovery of new molecules of interest is an essential asset when infections are more and more complex to treat. The plant and renewable origin of dihydro wogonin is an additional argument that encourages research around these resources. Other tree species potentially harboring effective molecules whose role to play in the fight against multidrug-resistant pathogens could prove to be crucial in the years to come.