Aeromonas salmonicida is a common pathogenic bacterial species found in both freshwater and marine fish, leading to significant economic losses in the aquaculture industry. YidC is an accessory to SecYEG and is essential for the SecYEG transporter to insert into the bacterial membrane. However, the roles of the yidC gene on the host immune response remain unclear. Here, we compared the pathogenicity of yidC gene-deleted (ΔyidC) strain and wild-type (SRW-OG1) strain of mesophilic A. salmonicida to Orange-spotted grouper (Epinephelus coioides), and explored the impacts of yidC gene on the immune response of E. coioides to mesophilic A. salmonicida infection by using Red/ET recombineering. In this study, the E. coioides in the Secondary infected group had a 53.9% higher survival rate than those in the Primary infected group. In addition, the adhesion ability of ΔyidC strain decreased by about 83.36% compared with that of the wild-type (SRW-OG1) strain. Further comparison of the biological phenotype of SRW-OG1 and ΔyidC revealed that this yidC gene could regulate the expression of genes related to iron metabolism and have no effect on bacterial growth under the limited iron concentration. In the low concentration of Fe3+ and Fe2+ environment, SRW-OG1 can obtain iron ions by regulating yidC. Based on the above results, yidC gene contributed to the pathogenicity of mesophilic A. salmonicida to E. coioides, deletion of yidC gene promoted the inflammation and immune response of E. coioides to mesophilic A. salmonicida infection.

Salmonella enterica serovar Typhimurium, which is a common foodborne pathogen, causes both intestinal and systemic infections in hosts. Salmonella has a complex pathogenic mechanism that involves invasive capacity and intracellular survivability, which hampers research on virulence of Salmonella. The virulence of Salmonella is primarily studied through Salmonella pathogenicity islands (SPIs). However, there are also genes outside these SPIs that significantly impact virulence. Macrophage survival gene msgA is positioned at a region independent of the SPIs and conserved in Salmonella. However, there has been limited research on msgA to date. This study aims to investigate the virulent function of msgA to deepen our understanding of Salmonella virulence. Proteomic and RT-qPCR analyses reveal that MsgA influences multiple metabolic pathways and the expression of SPIs. The depletion of msgA led to the significantly reduced invasive capacity and intracellular survivability, and thus the decreased virulence of Salmonella. In conclusion, our study suggests that MsgA is an important regulator that mainly regulates virulence. Further research into the function of MsgA will enhance the understanding of Salmonella pathogenesis and promote the application of Salmonella for medical treatment.
OBJECTIVE: Salmonella enterica serovar Typhimurium is a common foodborne pathogen, it has a complex pathogenic mechanism that involves invasive capacity and intracellular survivability. The virulence of Salmonella is primarily studied through its pathogenicity islands. In contrast, virulence genes located outside the Salmonella pathogenicity islands (SPIs) have received less attention. Macrophage survival gene (MsgA) is positioned at a region independent of the SPIs and conserved in Salmonella. Our research indicates that MsgA is a novel global regulator influencing the metabolic pathways and SPIs. Further research into the function of MsgA will enhance the understanding of Salmonella pathogenesis and promote the application of Salmonella for medical treatment.

Fimbriae are essential virulence factors for many bacterial pathogens. Fimbriae are extracellular structures that attach bacteria to surfaces. Thus, fimbriae mediate a critical step required for any pathogen to establish infection by anchoring a bacterium to host tissue. The human pathogen enterohemorrhagic Escherichia coli (EHEC) O157:H7encodes 16 fimbriae that may be important for EHEC to initiate infection and allow for productive expression of virulence traits important in later stages of infection, including a type III secretion system (T3SS) and Shiga toxin; however, the roles of most EHEC fimbriae are largely uncharacterized. Here, we provide evidence that two EHEC fimbriae, Yad and Yeh, modulate expression of diverse genes including genes encoding T3SS and Shiga toxin and that these fimbriae are required for robust colonization of the gastrointestinal tract. These findings reveal a significant and previously unappreciated role for fimbriae in bacterial pathogenesis as important determinants of virulence gene expression.IMPORTANCEFimbriae are extracellular proteinaceous structures whose defining role is to anchor bacteria to surfaces. This is a fundamental step for bacterial pathogens to establish infection in a host. Here, we show that the contributions of fimbriae to pathogenesis are more complex. Specifically, we demonstrate that fimbriae influence expression of virulence traits essential for disease progression in the intestinal pathogen enterohemorrhagic Escherichia coli. Gram-positive and Gram-negative bacteria express multiple fimbriae; therefore, these findings may have broad implications for understanding how pathogens use fimbriae, beyond adhesion, to initiate infection and coordinate gene expression, which ultimately results in disease.

The regulator of capsule synthesis (Rcs) system, an atypical two-component system prevalent in numerous gram-negative bacteria, serves as a sophisticated regulatory phosphorylation cascade mechanism. It plays a pivotal role in perceiving environmental stress and regulating the expression of downstream genes to ensure host survival. During the signaling transduction process, various proteins participate in phosphorylation to further modulate signal inputs and outputs. Although the structure of core proteins related to the Rcs system has been partially well-defined, and two models have been proposed to elucidate the intricate molecular mechanisms underlying signal sensing, a systematic characterization of the signal transduction process of the Rcs system remains challenging. Furthermore, exploring its corresponding regulator outputs is also unremitting. This review aimed to shed light on the regulation of bacterial virulence by the Rcs system. Moreover, with the assistance of the Rcs system, biosynthesis technology has developed high-value target production. Additionally, via this review, we propose designing chimeric Rcs biosensor systems to expand their application as synthesis tools. Finally, unsolved challenges are highlighted to provide the basic direction for future development of the Rcs system.

Survival of Mycobacterium tuberculosis within the host macrophages requires the bacterial virulence regulator PhoP, but the underlying reason remains unknown. 3\',5\'-Cyclic adenosine monophosphate (cAMP) is one of the most widely used second messengers, which impacts a wide range of cellular responses in microbial pathogens including M. tuberculosis. Herein, we hypothesized that intra-bacterial cAMP level could be controlled by PhoP since this major regulator plays a key role in bacterial responses against numerous stress conditions. A transcriptomic analysis reveals that PhoP functions as a repressor of cAMP-specific phosphodiesterase (PDE) Rv0805, which hydrolyzes cAMP. In keeping with these results, we find specific recruitment of the regulator within the promoter region of rv0805 PDE, and absence of phoP or ectopic expression of rv0805 independently accounts for elevated PDE synthesis, leading to the depletion of intra-bacterial cAMP level. Thus, genetic manipulation to inactivate PhoP-rv0805-cAMP pathway decreases cAMP level, stress tolerance, and intracellular survival of the bacillus.

bb0616 of Borrelia burgdorferi, the Lyme disease pathogen, encodes a hypothetical protein of unknown function. In this study, we showed that BB0616 was not surface-exposed or associated with the membrane through localization analyses using proteinase K digestion and cell partitioning assays. The expression of bb0616 was influenced by a reduced pH but not by growth phases, elevated temperatures, or carbon sources during in vitro cultivation. A transcriptional start site for bb0616 was identified by using 5\' rapid amplification of cDNA ends, which led to the identification of a functional promoter in the 5\' regulatory region upstream of bb0616. By analyzing a bb0616-deficient mutant and its isogenic complemented counterparts, we found that the infectivity potential of the mutant was significantly attenuated. The inactivation of bb0616 displayed no effect on borrelial growth in the medium or resistance to oxidative stress, but the mutant was significantly more susceptible to osmotic stress. In addition, the production of global virulence regulators such as BosR and RpoS as well as virulence-associated outer surface lipoproteins OspC and DbpA was reduced in the mutant. These phenotypes were fully restored when gene mutation was complemented with a wild-type copy of bb0616. Based on these findings, we concluded that the hypothetical protein BB0616 is required for the optimal infectivity of B. burgdorferi, potentially by impacting B. burgdorferi virulence gene expression as well as survival of the spirochete under stressful conditions.

tRNA modifications play important roles in maintaining translation accuracy in all domains of life. Disruptions in the tRNA modification machinery, especially of the anticodon stem loop, can be lethal for many bacteria and lead to a broad range of phenotypes in baker\'s yeast. Very little is known about the function of tRNA modifications in host-pathogen interactions, where rapidly changing environments and stresses require fast adaptations. We found that two closely related fungal pathogens of humans, the highly pathogenic Candida albicans and its much less pathogenic sister species, Candida dubliniensis, differ in the function of a tRNA-modifying enzyme. This enzyme, Hma1, exhibits species-specific effects on the ability of the two fungi to grow in the hypha morphology, which is central to their virulence potential. We show that Hma1 has tRNA-threonylcarbamoyladenosine dehydratase activity, and its deletion alters ribosome occupancy, especially at 37°C-the body temperature of the human host. A C. albicans HMA1 deletion mutant also shows defects in adhesion to and invasion into human epithelial cells and shows reduced virulence in a fungal infection model. This links tRNA modifications to host-induced filamentation and virulence of one of the most important fungal pathogens of humans.IMPORTANCEFungal infections are on the rise worldwide, and their global burden on human life and health is frequently underestimated. Among them, the human commensal and opportunistic pathogen, Candida albicans, is one of the major causative agents of severe infections. Its virulence is closely linked to its ability to change morphologies from yeasts to hyphae. Here, this ability is linked-to our knowledge for the first time-to modifications of tRNA and translational efficiency. One tRNA-modifying enzyme, Hma1, plays a specific role in C. albicans and its ability to invade the host. This adds a so-far unknown layer of regulation to the fungal virulence program and offers new potential therapeutic targets to fight fungal infections.

Pseudomonas aeruginosa, a bacterium causing infections in immunocompromised individuals, regulates several of its virulence functions using three interlinked quorum sensing (QS) systems (las, rhl, and pqs). Despite its presumed importance in regulating virulence, dysfunction of the las system regulator LasR occurs frequently in strains isolated from various environments, including clinical infections. This newfound abundance of LasR-defective strains calls into question existing hypotheses regarding their selection. Indeed, current assumptions concerning factors driving the emergence of LasR-deficient isolates and the role of LasR in the QS hierarchy must be reconsidered. Here, we propose that LasR is not the primary master regulator of QS in all P. aeruginosa genetic backgrounds, even though it remains ecologically significant. We also revisit and complement current knowledge on the ecology of LasR-dependent QS in P. aeruginosa, discuss the hypotheses explaining the putative adaptive benefits of selecting against LasR function, and consider the implications of this renewed understanding.

Siderophores are secreted ferric ion chelators used to obtain iron in nutrient-limited environmental niches, including human hosts. While all Escherichia coli express the enterobactin (Ent) siderophore system, isolates from patients with urinary tract infections additionally express the genetically distinct yersiniabactin (Ybt) siderophore system. To determine whether the Ent and Ybt systems are functionally redundant for iron uptake, we compared the growth of different isogenic siderophore biosynthetic mutants in the presence of transferrin, a human iron-binding protein. We observed that Ybt expression does not compensate for deficient Ent expression following low-density inoculation. Using transcriptional and product analysis, we found this non-redundancy to be attributable to a density-dependent transcriptional stimulation cycle in which Ybt functions as an autoinducer. These results distinguish the Ybt system as a combined quorum-sensing and siderophore system. These functions may reflect Ybt as a public good within bacterial communities or as an adaptation to confined, subcellular compartments in infected hosts. This combined functionality may contribute to the extraintestinal pathogenic potential of E. coli and related Enterobacterales.IMPORTANCEPatients with urinary tract infections are often infected with Escherichia coli strains carrying adaptations that increase their pathogenic potential. One of these adaptations is the accumulation of multiple siderophore systems, which scavenge iron for nutritional use. While iron uptake is important for bacterial growth, the increased metabolic costs of siderophore production could diminish bacterial fitness during infections. In a siderophore-dependent growth condition, we show that the virulence-associated yersiniabactin siderophore system in uropathogenic E. coli is not redundant with the ubiquitous E. coli enterobactin system. This arises not from differences in iron-scavenging activity but because yersiniabactin is preferentially expressed during bacterial crowding, leaving bacteria dependent upon enterobactin for growth at low cell density. Notably, this regulatory mode arises because yersiniabactin stimulates its own expression, acting as an autoinducer in a previously unappreciated quorum-sensing system. This unexpected result connects quorum-sensing with pathogenic potential in E. coli and related Enterobacterales.

Several bacterial pathogens, including Salmonella enterica, can cause persistent infections in humans by mechanisms that are poorly understood. By comparing genomes of isolates longitudinally collected from 256 prolonged salmonellosis patients, we identified repeated mutations in global regulators, including the barA/sirA two-component regulatory system, across multiple patients and Salmonella serovars. Comparative RNA-seq analysis revealed that distinct mutations in barA/sirA led to diminished expression of Salmonella pathogenicity islands 1 and 4 genes, which are required for Salmonella invasion and enteritis. Moreover, barA/sirA mutants were attenuated in an acute salmonellosis mouse model and induced weaker transcription of host immune responses. In contrast, in a persistent infection mouse model, these mutants exhibited long-term colonization and prolonged shedding. Taken together, these findings suggest that selection of mutations in global virulence regulators facilitates persistent Salmonella infection in humans, by attenuating Salmonella virulence and inducing a weaker host inflammatory response.