Bacteria have evolved multiple secretion systems for delivering effector proteins into the cytosol of neighboring cells, but the roles of many of these effectors remain unknown. Here, we show that Yersinia pseudotuberculosis secretes an effector, CccR, that can act both as a toxin and as a transcriptional factor. The effector is secreted by a type VI secretion system (T6SS) and can enter nearby cells of the same species and other species (such as Escherichia coli) via cell-cell contact and in a contact-independent manner. CccR contains an N-terminal FIC domain and a C-terminal DNA-binding domain. In Y. pseudotuberculosis cells, CccR inhibits its own expression by binding through its DNA-binding domain to the cccR promoter, and affects the expression of other genes through unclear mechanisms. In E. coli cells, the FIC domain of CccR AMPylates the cell division protein FtsZ, inducing cell filamentation and growth arrest. Thus, our results indicate that CccR has a dual role, modulating gene expression in neighboring cells of the same species, and inhibiting the growth of competitors.

Microbial species often occur in complex communities and exhibit intricate synergistic and antagonistic interactions. To avoid predation and compete for favorable niches, bacteria have evolved specialized protein secretion systems. The type VI secretion system (T6SS) is a versatile secretion system widely distributed among Gram-negative bacteria that translocates effectors into target cells or the extracellular milieu via various physiological processes. Pseudomonas aeruginosa is an opportunistic pathogen responsible for many diseases, and it has three independent T6SSs (H1-, H2-, and H3-T6SS). In this study, we found that the H3-T6SS of highly virulent P. aeruginosa PA14 is negatively regulated by OxyR and OmpR, which are global regulatory proteins of bacterial oxidative and acid stress. In addition, we identified a H3-T6SS effector PA14_33970, which is located upstream of VgrG3. PA14_33970 interacted directly with VgrG3 and translocated into host cells. Moreover, we found that H3-T6SS and PA14_33970 play crucial roles in oxidative, acid, and osmotic stress resistance, as well as in motility and biofilm formation. PA14_33970 was identified as a new T6SS effector promoting biofilm formation and thus named TepB. Furthermore, we found that TepB contributes to the virulence of P. aeruginosa PA14 toward Caenorhabditis elegans. Overall, our study indicates that H3-T6SS and its biofilm-promoting effector TepB are regulated by OxyR and OmpR, both of which are important for adaptation of P. aeruginosa PA14 to multiple stressors, providing insights into the regulatory mechanisms and roles of T6SSs in P. aeruginosa.

Drought, intensified by climate change, poses a mounting threat to global food security by severely constraining crop productivity. While microbial inocula

The formation of reactive oxygen species is harmful and can destroy intracellular macromolecules such as lipids, proteins, and DNA, even leading to bacterial death. To cope with this situation, microbes have evolved a variety of sophisticated mechanisms, including antioxidant enzymes, siderophores, and the type VI secretion system (T6SS). However, the mechanism of oxidative stress resistance in Cupriavidus pinatubonensis is unclear. In this study, we identified Reut_A2805 as an OxyR ortholog in C . pinatubonensis , which positively regulated the expression of T6SS1 by directly binding to its operon promoter region. The study revealed that OxyR-regulated T6SS1 combats oxidative stress by importing iron into bacterial cells. Moreover, the T6SS1-mediated outer membrane vesicles-dependent iron acquisition pathway played a crucial role in the oxidative stress resistance process. Finally, our study demonstrated that the T6SS1 and siderophore systems in C. pinatubonensis exhibit different responses in combating oxidative stress under low-iron conditions, providing a comprehensive understanding of how bacterial iron acquisition systems function in diverse conditions. IMPORTANCE: The ability to eliminate reactive oxygen species is crucial for bacterial survival. Continuous formation of hydroperoxides can damage metalloenzymes, disrupt DNA integrity, and even result in cell death. While various mechanisms have been identified in other bacterial species to combat oxidative stress, the specific mechanism of oxidative stress resistance in C. pinatubonensis remains unclear. The importance of this study is that we elucidate the mechanism that OxyR-regulated T6SS1 combats oxidative stress by importing iron with the help of bacterial outer membrane vesicle. Moreover, the study highlights the contrasting responses of T6SS1- and siderophore-mediated iron acquisition systems to oxidative stress. This study provides a comprehensive understanding of bacterial iron acquisition and its role in oxidative stress resistance in C. pinatubonensis under low-iron conditions.

Urease converts urea into ammonia and carbon dioxide, providing a nitrogen and carbon source for microbial growth and serving as an important mechanism for human bacterial pathogens to survive in acidic conditions, which can be regulated by many factors. As a global regulator, the ferric uptake regulator (Fur) regulates a series of genes and pathways involved in many different cellular processes and the virulence of the enteric bacterium Yersinia pseudotuberculosis ( Yptb ). However, whether Fur regulates the urease activity in Yptb was still unknown. In this study, we found that urease is positively regulated by Fur in response to manganese ions (Mn ²⁺ ), and this regulation by Fur is mediated by specific recognition of the promoter region of urease in Yptb . Furthermore, urease is induced by Mn ²⁺ via Fur under low nutrient conditions. Moreover, we provided evidence that urease plays an important role in acid and osmotic stress resistance, biofilm formation, and virulence of Yptb . Our findings provide insights into understanding the regulatory mechanism and multiple functions of urease in Yptb . IMPORTANCE: Urease catalyzes the breakdown of urea into ammonia and carbamate, which are widely distributed among bacterial species and play an important role as an important acid resistance system and virulence factor. In most bacterial species, urease expression is tightly regulated in response to environmental cues such as nitrogen status, pH, growth phase, substrate availability, or transcriptional regulators. In this study, we found that urease from Yptb is positively regulated by Fur in response to Mn ²⁺ under low nutrient conditions, which functions to combat acid and osmotic stress and enhance biofilm formation, and plays a crucial role in virulence. Importantly, this is the first demonstration of a direct role for Fur and Mn ²⁺ in regulating urease expression in Yptb . This study provides a comprehensive understanding of the regulatory mechanisms and functions of urease from Yptb .

Microbial interactions, particularly bacteria–fungus interactions, are research hotspots within microbial ecology and pathogenic biology. However, their underlying molecular mechanisms remain poorly understood, especially how bacterial pathogens recognize and exploit fungal signaling molecules for fungal predation. Here, we demonstrate that Pseudomonas aeruginosa employs an integrated tripartite farnesol-sensing system to detect and eliminate Candida albicans hyphae: The chemoreceptor PctA mediates directional migration toward hyphae; the type IV pilus sensor PilJ activates antifungal type III secretion system (T3SS) expression; and the quorum regulator PqsR monitors farnesol levels to coordinate virulence-metabolic switching. This mechanism enables bacteria to convert farnesol into their own signaling language ( Pseudomonas quinolone signal) according to fungi status, thereby adaptively modulating their virulence expression and metabolism to cope with complex competitive microbial environments. Furthermore, bioinformatics analysis and functional validation confirm that the PctA–PilJ–PqsR triad is conserved across P . aeruginosa , suggesting that this interkingdom communication is widespread. In conclusion, this study reveals that P . aeruginosa orchestrates a targeted predation strategy against filamentous fungi by coordinating three interkingdom receptors, providing a theoretical foundation and potential molecular targets for understanding of interkingdom communication strategies among microorganisms and the development of signal molecule–based microbial prevention and control technologies.