Hydration conditions as a critical factor in antibiotic-mediated bacterial competition outcomes

Antibiotic secretion plays a pivotal role in bacterial interference competition; yet, the impact of environmental hydration conditions on such competition is not well understood. Here, we investigate how hydration conditions affect interference competition among bacteria, studying the interactions between the antibiotic-producing <i>Bacillus velezensis</i> FZB42 and two bacterial strains susceptible to its antibiotics: <i>Xanthomonas euvesicatoria</i> 85-10 and <i>Pseudomonas syringae</i> DC3000. Our results show that wet-dry cycles significantly modify the response of the susceptible bacteria to both the supernatant and cells of the antibiotic-producing bacteria, compared to constantly wet conditions. Notably<i>, X. euvesicatoria</i> shows increased protection against both the cells and supernatants of <i>B. velezensis</i> under wet-dry cycles, while <i>P. syringae</i> cells become more susceptible under wet-dry cycles. In addition, we observed a reciprocal interaction between <i>P. syringae</i> and <i>B. velezensis</i>, where <i>P. syringae</i> inhibits <i>B. velezensis</i> under wet conditions. Our findings highlight the important role of hydration conditions in shaping bacterial interference competition, providing valuable insights into the microbial ecology of water-unsaturated surfaces, with implications for applications such as biological control of plant pathogens and mitigating antibiotic resistance.IMPORTANCEOur study reveals that hydration conditions, particularly wet-dry cycles, significantly influence antibiotic-mediated competition between bacterial species. We revealed that the effectiveness of antibiotics produced by <i>Bacillus velezensis</i> against two susceptible bacterial species: <i>Xanthomonas</i> and <i>Pseudomonas</i> varies based on these hydration conditions. Unlike traditional laboratory environments, many real-world habitats, such as soil, plant surfaces, and even animal skin, undergo frequent wet-dry cycles. These conditions affect bacterial competition dynamics and outcomes, with wet-dry cycles providing increased protection for some bacteria while making others more susceptible. Our findings highlight the importance of considering environmental hydration when studying microbial interactions and developing biological control strategies. This research has important implications for improving agricultural practices and understanding natural microbial ecosystems.