Microbial interactions impact stress tolerance in a model oral community.

Unlabelled: Understanding the molecular mechanisms governing microbial interactions is crucial for unraveling the complexities of microbial communities and their ecological impacts. Here, we employed a two-species model system comprising the oral bacteria and to investigate how synergistic and antagonistic interactions between microbes impact their resilience to environmental change and invasion by other microbes. We used an colony biofilm model and focused on two . -produced extracellular molecules, L-lactate and HO, which are known to impact fitness of this dual-species community. While the ability of to cross-feed on -produced L-lactate enhanced its fitness during co-culture, this function showed little impact on the ability of co-cultures to resist environmental change. In fact, the ability of to catabolize L-lactate may be detrimental in the presence of tetracycline, highlighting the complexity of interactions under antimicrobial stress. Furthermore, HO, known for its antimicrobial properties, had negative impacts on both species in our model system. However, HO production by enhanced tolerance to tetracycline, suggesting a protective role under antibiotic pressure. Finally, significantly inhibited the bacterium from invading biofilms, but this inhibition was lost during co-culture with and in a murine abscess model, where actually promoted invasion. These data indicate that microbial interactions can impact fitness of a bacterial community upon exposure to stresses, but these impacts are highly environment dependent.

Importance: Microbial interactions are critical modulators of the emergence of microbial communities and their functions. However, how these interactions impact the fitness of microbes in established communities upon exposure to environmental stresses is poorly understood. Here, we utilized a two-species community consisting of and to examine the impact of synergistic and antagonistic interactions on microbial resilience to environmental fluctuations and susceptibility to microbial invasion. We focused on the -produced extracellular molecules, L-lactate and HO, which have been shown to mediate interactions between these two microbes. We discovered that seemingly beneficial functions, such as cross-feeding on -produced L-Lactate, can paradoxically exacerbate vulnerabilities, such as susceptibility to antibiotics. Moreover, our data highlight the context-dependent nature of microbial interactions, emphasizing that a seemingly potent antimicrobial, such as HO, can have both synergistic and antagonistic effects on a microbial community dependent on the environment.