biostudies-literatureOscarsson JUmeå UniversitetTokyo University of AgricultureKarolinska InstitutetSwedish Research Council109650https://www.ebi.ac.uk/biostudies/studies/S-EPMC11033201biostudies-literatureUnknown27(5)Microbial ecosystems experience spatial and nutrient restrictions leading to the coevolution of cooperation and competition among cohabiting species. To increase their fitness for survival, bacteria exploit machinery to antagonizing rival species upon close contact. As such, the bacterial type VI secretion system (T6SS) nanomachinery, typically expressed by pathobionts, can transport proteins directly into eukaryotic or prokaryotic cells, consequently killing cohabiting competitors. Here, we demonstrate for the first time that oral symbiont <i>Aggregatibacter aphrophilus</i> possesses a T6SS and can eliminate its close relative oral pathobiont <i>Aggregatibacter actinomycetemcomitans</i> using its T6SS. These findings bring nearer the anti-bacterial prospects of symbionts against cohabitiniScienceBacterial symbionts in oral niche use type VI secretion nanomachinery for fitness increase against pathobionts.PMC110332012017-011982021-035287002667Wai SNBao KJohansson AShiratsuchi ABostanci NWolski WMowsumi FRAung KMGrossmann JLindholm MOscarsson JBelibasakis GNfalseBacterial symbionts in oral niche use type VI secretion nanomachinery for fitness increase against pathobionts.Microbial ecosystems experience spatial and nutrient restrictions leading to the coevolution of cooperation and competition among cohabiting species. To increase their fitness for survival, bacteria exploit machinery to antagonizing rival species upon close contact. As such, the bacterial type VI secretion system (T6SS) nanomachinery, typically expressed by pathobionts, can transport proteins directly into eukaryotic or prokaryotic cells, consequently killing cohabiting competitors. Here, we demonstrate for the first time that oral symbiont <i>Aggregatibacter aphrophilus</i> possesses a T6SS and can eliminate its close relative oral pathobiont <i>Aggregatibacter actinomycetemcomitans</i> using its T6SS. These findings bring nearer the anti-bacterial prospects of symbionts against cohabitin2024-01-01T00:00:00Z2024 May2026-05-26T17:03:40.321Z2026-05-25T03:06:58.097ZS-EPMC110332013865098910.1016/j.isci.2024.109650