Streptococcus pneumoniae is a main cause of bacterial pneumonia worldwide. It can also cause ear and sinus infections and, in more severe cases, meningitis or sepsis. It shares the human respiratory tract with many closely related bacterial species. Among them is Streptococcus mitis , a common member of the microbiome that can remain with us for a long time without causing harm.
The new study , published in the mBio journal of the American Society for Microbiology, shows that these bacteria do not simply coexist, they actively compete. The project is led by João Borralho, researcher at Instituto de Tecnologia Química e Biológica António Xavier of NOVA University of Lisbon ( ITQB NOVA ).
The study shows that S. mitis can kill S. pneumoniae during a physiological state known as competence, when bacteria become able to take up DNA from their surroundings. During this state, S. mitis produces antimicrobial molecules called bacteriocins that damage neighboring S. pneumoniae cells, causing them to die and release their genetic material. The released DNA can then be taken up by S. mitis , allowing for evolution of this species.
“We often think of commensal bacteria as passive inhabitants of the microbiome. Our work shows that they actively compete with neighboring bacteria to acquire new genetic information”, explains João Borralho.
According to Raquel Sá-Leão , Principal Investigator of the lab, “understanding how bacteria exchange DNA is essential because these processes contribute to bacterial adaptation, including the spread of antibiotic resistance and virulence traits”. She adds, “the bacteriocins identified in this study selectively target human pathogens such as S. pneumoniae , highlighting their potential as future antimicrobial strategies designed to specifically target pathogens while preserving beneficial members of the microbiome.”
The findings may also be relevant for epidemiological studies that rely on DNA markers to detect bacteria in respiratory samples. Because streptococci can exchange DNA, markers used to detect S. pneumoniae may be found in other species such as S. mitis , leading to false positive results. Recognizing the movement of DNA across bacteria can therefore help researchers interpret surveillance results more accurately.
The study was developed as part of João Borralho PhD studies, with João Lança, Joana Bryton, and Raquel Sá-Leão at ITQB NOVA, in collaboration with Wilson Antunes from the Unidade Militar Laboratorial de Defesa Biológica e Química. Future research will explore the therapeutic potential of the bacteriocins produced by S. mitis and further investigate how DNA exchanges between species shapes bacterial evolution in environments similar to the human respiratory tract.
mBio
Streptococcus mitis bacteriocins drive contact-dependent lysis of S. pneumoniae facilitating transformation in multispecies environments
3-Mar-2026