Why doesn’t Staphylococcus aureus respond to antibiotics? And why, when treatment appears to have worked and is stopped, are infection relapses frequent? To escape antibiotics and the body’s defences, the bacterium hibernates within our cells while waiting for the storm to pass! This is what researchers from the UCLouvain Louvain Drug Research Institute (LDRI) have managed to demonstrate.
An initially harmless bacterium
Staphylococcus aureus is a bacterium found naturally in about 30% of individuals, who are called ‘healthy carriers’. This means that you carry the bacteria but have no disease. It generally lodges in the skin or nose and produces a golden pigment, hence its pretty name. ‘The golden pigment is a virulence factor because it allows the bacterium to resist certain means of defence in our body,’ says LDRI researcher Prof. Françoise Van Bambeke. The substance therefore contributes to the bacterium’s pathogenic nature, allowing it to occupy a niche in the host (colonisation), escape the host’s immune system (immunoevasion), and survive in host cells via intracellular infection.
When it becomes harmful
Under certain circumstances, this initially harmless bacteria can become pathogenic. It can for example produce certain toxins which will cause harmful reactions in the individual. Or it can enter blood circulation via lesions in the skin and reach certain organs, causing serious pathologies such as endocarditis or bone or kidney infections. ‘This bacterium becomes especially dangerous in hospital patients because their immune defences are fragile,’ Prof. Van Bambeke explains. ‘Staphylococcus aureus is responsible for 15% of infections contracted in hospital. Of 1,000 patients admitted to hospital, 1% will develop a staph infection, which is significant.’ Such prevalence adds to the urgency; WHO considers Staphylococcus aureus a priority for research because its high resistance to antibiotics requires new therapeutic strategies. The LDRI team has taken up the subject for several years.
Staphylococcus aureus’s lifestyle
To answer the many questions concerning how the bacterium responds (or doesn’t!) to antibiotics, Frédéric Peyrusson, with the help of Tiep Khac Nguyen, both PhD students on Prof. Van Bambeke’s team, observed the behaviour of Staphylococcus aureus, particularly in the context of chronic recurrent infections. These can go unnoticed: the infections seem to disappear thanks to antibiotics, then resurface. How is it possible? Owing to the bacterium’s ability to delve deeply into certain tissues and adopt a particular lifestyle. ‘It was the intracellular way of life that interested us,’ Prof. Van Bambeke says, ‘which we explained in the article we just published in the journal Nature Communications.’ The bacterium enters and remains inside our cells. It therefore becomes less accessible to our body’s defences and antibiotics.
Hibernating bacteria
Prof. Van Bambeke has repeatedly asked the question: ‘Why do persistent forms of infection not respond to antibiotics?’ Given the intracellular form of Staphylococcus aureus, an exciting avenue of study has opened up for her and her team. ‘We’ve already observed that if you use very high concentrations of antibiotics for long periods of time,’ Prof. Van Bambeke says, ‘you can never completely eliminate intracellular bacteria. The why and how remained to be seen.’ For three years, Mr Peyrusson sought to understand the behaviour of these intracellular bacteria under the pressure of an antibiotic. He observed that they became incapable of multiplying, whereas multiplication is characteristic of bacteria. The intracellular bacteria that persisted in the cells no longer divided. Prof. Van Bambeke explained, ‘The bacteria sort of hibernate, like a bear. While the bear reduces its metabolic activities to those necessary for survival during hibernation, so do the bacteria. They slow down a whole series of metabolic activities, which reduces or even prevents their growth. They maintain only those essential to their survival, including those that allow them to respond to stress imposed by the antibiotic. However, for an antibiotic to work well, bacteria must actively multiply. The modification of the metabolism of these bacteria and their non-multiplication make them impervious to antibiotics. This explains why we can never completely eradicate intracellular bacteria.’
Tracking bacteria
This fascinating demonstration leads to two rather disturbing conclusions. First, after it resists one antibiotic, the intracellular bacteria, no matter how small, won’t respond to any other antibiotic, even one of another class, because the stress response (to the antibiotic) it developed is generalised. If the antibiotic is removed, the bacteria return to their original state. They resume their basic metabolism and start to multiply again, potentially creating a new infection site. To reach this conclusion, Mr Peyrusson developed an individual bacterium tracing system by using fluorescence. If the bacterium multiplied, the fluorescent marker faded. If the bacterium didn’t multiply, the marker remained fluorescent. In parallel, he collected intracellular bacteria that had survived antibiotics and looked at how the expression of their genes was different compared to those of normal bacteria. He was thus able to shed light on the metabolic pathways that were going to shut down and those that were going to start up.
Alternatives to antibiotics?
Now that we understand why the infection persists in our body and can re-emerge after seemingly successful treatment, the question is: What are the alternatives for treating Staphylococcus aureus infections? ‘The difficulty,’ Prof. Van Bambeke says, ‘is that the form of bacterium we’re studying is intracellular. It therefore responds little to defences like antibodies. If we wanted to use this therapeutic option, we would therefore have to associate the antibody with a vector which could enter our cells.’ The good news is that Prof. Van Bambeke’s team has shown which alternative pathways Staphylococcus aureus uses for its metabolism. ‘This won’t happen tomorrow, but a therapeutic alternative would be to specifically target these pathways to prevent the bacteria from entering their dormant state.’ One hope is that future treatment will be able to combine a conventional antibiotic and a substance that can prevent this state of persistence. Thanks to LDRI researchers, this work can be done more rationally, by looking for molecules that act specifically on these metabolic pathways associated with dormancy. And the research isn’t about to stop. ‘We think this discovery can be generalised to other bacteria.’
Lauranne Garitte
A glance at Françoise Van Bambeke's bio
Françoise Van Bambeke obtained her master’s degree in pharmacy in 1991 and her PhD in 1995, both from UCLouvain. After a postdoctoral residency at the Institut Pasteur in Paris, she obtained a permanent position at the FNRS. She is now FNRS research director at the UCLouvain Louvain Drug Research Institute and professor of pharmacology in the Faculty of Pharmacy and Biomedical Sciences. Her main research interests are the study of pharmacokinetic and pharmacodynamic parameters that affect the activity of antibiotics against persistent forms of infections (intracellular infection, for example) and the search for innovative strategies against them. She is also studying the resistance caused by the efflux mechanism and is working on optimising the dosage of antibiotics based on pharmacokinetic and pharmacodynamic concepts.