A team of scientists from the Center for Neuroscience and Cell Biology of the University of Coimbra (CNC-UC), integrated in the Centre for Innovative Biomedicine and Biotechnology (CiBB), has identified a new set of factors that help explain the high virulence of the multi-resistant bacterium Staphylococcus aureus, responsible for more than one million deaths per year and one of the main causes of hospital and community-acquired disease.
The study now published provides new clues as to how the bacterium hides, survives, and multiplies within non-professional phagocytes, human cells whose primary function is not immune defense and in which some antibiotics are less effective.
“Understanding the mechanisms of infection and intracellular adaptation of the Staphylococcus aureus bacterium has allowed us to uncover how this microorganism manages to escape the immune system and resist antibiotics, thus expanding recent knowledge about its biology and virulence,” says CNC-UC researcher and study leader Ana Eulálio.
Among the factors discovered in the study, 73 genes were identified that strongly influence the ability of Staphylococcus aureus to invade, persist, and multiply inside human cells, and even cause the death of these cells. The study included a total of 1920 mutants of the bacterium (which had bacterial genetic mutations in all non-essential genes).
Also noteworthy is the discovery of a new function of the nicotinamidase enzyme (PncA), which regulates the virulence system of Staphylococcus aureus—the agr (Accessory Gene Regulator) system, which determines the expression of many virulence factors. This work demonstrated that PncA controls the activity of the agr system by regulating the metabolism of the bacterium.
The systematic analysis of this study—the most comprehensive ever conducted on the intracellular life of Staphylococcus aureus—paves the way for the development of innovative therapies capable of eliminating these “hidden” populations of bacteria, helping to combat chronic and recurrent infections and reduce antimicrobial resistance.
The characterization of the role of the PncA protein in virulence also opens new perspectives for investigating the role of bacterial metabolism in infection. These findings offer multiple starting points for exploring new therapeutic targets and better understanding the interactions between bacteria and their host.
“In recent years, evidence has accumulated that the bacterium Staphylococcus aureus is not only an extracellular pathogen [living outside host cells], but that, on the contrary, it can establish itself inside human cells, contributing to persistent infections,” explains researcher Ana Eulálio.
Conducted using large-scale automated fluorescence microscopy infection assays, the study identifies a set of factors never before associated with the intracellular life and virulence of the bacterium.
Staphylococcus aureus is a pathogen that is one of the main causes of hospital and community-acquired disease. Infections caused by methicillin-resistant Staphylococcus aureus (or MRSA) strains are the second most common cause of death associated with bacterial resistance to antibiotics.
The research by Ana Eulálio's group on the intracellular survival and antibiotic resistance of Staphylococcus aureus bacteria is funded by the 2025 Health Research call of the “la Caixa” Foundation with approximately half a million euros.
The study Systematic identification of bacterial factors driving Staphylococcus aureus intracellular lifestyle in non-professional phagocytes, published in the journal Nature Communications in December 2025, was carried out through a collaboration between CNC-UC and researchers from the Biotechnology Center, Spanish National Research Council (CNB-CSIC) and Imperial College London, and is available here.
Inês Amado da Silva with Catarina Ribeiro (UC)
