According to WHO, approximately 6% of the worldwide population who contract Covid-19 – some 400 million people – go on to develop a long-lasting form of the disease . These figures demonstrate that the persistent form of the disease remains a problem and is a major public health challenge .
In 2021, in the midst of the Covid-19 pandemic, University of Louvain (UCLouvain, Belgium) and its hospital, the Cliniques universitaires Saint-Luc, launched a large-scale study to determine whether it is possible to predict , during the acute phase of infection, the development of long-term symptoms . The ultimate goal is to better understand the underlying mechanisms and, potentially, develop a preventive therapeutic solution .
After five years of research, scientists have discovered the key role played by a bacterium , Dolosigranulum pigrum , that is naturally present in the respiratory microbiome. When present in greater numbers, these bacteria are associated with a reduction in the persistence of symptoms characteristic of long Covid .
Jean Cyr Yombi, Leïla Belkir, and Julien De Greef – UCLouvain professors who are infectious disease specialists at the Cliniques universitaires Saint-Luc – analysed the severity of long Covid symptoms in 156 patients , focusing primarily on severe fatigue, cognitive impairments, and respiratory issues (shortness of breath). Next, UCLouvain professors Laure Elens and Patrice Cani and UCLouvain Louvain Drug Research Institute postdoctoral researcher Bradley Ward analysed in blood samples and nasopharyngeal swabs the molecular signatures associated with this severe form of the disease – a combination of invisible factors that help explain why these symptoms persist in some patients but not in others.
UCLouvain and Cliniques universitaires Saint-Luc researchers stated, “This study suggests that certain so-called protective bacteria in the respiratory microbiome may be associated with improved recovery following viral respiratory infections (such as long Covid or influenza), and that their alteration (particularly in the context of severe infection or non-targeted antibiotic therapy) may influence longer-term clinical outcomes.” In other words, when the bacterium is abundant in the body, it appears to protect against long Covid or severe influenza (through a mechanism that has yet to be elucidated). Conversely, when it is scarce , researchers observe a predisposition to developing a persistent form of the disease.
The good news is that the protective effect of this bacterium was already known in relation to infectious influenza. This new discovery, published in the research journal Microbiology Spectrum , attests to the potentially beneficial role of Dolosigranulum pigrum and will, scientists hope, accelerate research in the field and drive exploration of new therapeutic strategies with a view to developing a probiotic – in the form of a nasal spray , for example – to be taken prior to the winter season, in order to protect the population from severe infectious diseases such as Covid-19 or influenza.
Another key finding of the study is the impact of non-targeted antibiotics on the respiratory microbiome’s ability to defend itself against severe infections . Hence the importance of regulating the use of antibiotics.
This research involved the University of Louvain HYGIEIA consortium and the Cliniques universitaires Saint-Luc, and was made possible thanks to the support of the Sofina Covid Solidarity Fund and partnerships with the Fondation Saint-Luc, the FNRS (via an emergency research grant) and the WEL Research Institute of the Walloon Region.
Microbiology Spectrum
Randomized controlled/clinical trial
Human tissue samples
Association of nasopharyngeal Dolosigranulum pigrum and Corynebacterium species with post-acute sequelae of SARS-CoV-2 in a longitudinal cohort
17-Mar-2026