ABSTRACT Acinetobacter baumannii has emerged as a major global pathogen due to extensive resistance to last-resort antimicrobials, a high burden of nosocomial infections, and increasing community-acquired cases. Its adaptability is driven by diverse resistance mechanisms, including β-lactamase production, aminoglycoside-modifying enzymes, efflux pump overexpression, target-site mutations, and lipid A remodeling, all of which limit treatment options and worsen clinical outcomes. Pathogenicity is further enhanced by quorum-sensing systems, particularly AbaI/AbaR, which regulate biofilm formation, virulence, and antimicrobial tolerance. Despite extensive research, resistance, quorum sensing, and therapeutic strategies are often examined separately, limiting mechanistic understanding. This review integrates current evidence on the interplay between resistance evolution, quorum sensing, and biofilm persistence, linking these to therapeutic vulnerabilities. It further evaluates emerging interventions, including optimized antibiotic combinations, immunomodulation, drug repurposing, bacteriophage therapy, and alternative approaches, such as antimicrobial peptides, phytochemicals, nanotechnology, and photodynamic therapy to inform improved treatment strategies.

ABSTRACT The treatment of carbapenem-resistant A. baumannii (CRAB) infections has become a major global medical challenge; therefore, there is an urgent need for new antimicrobial development. Antimicrobial peptides hold great promising potential due to their rapid bactericidal activity and low induction of drug resistance. In this study, we screened and synthesized the antimicrobial peptide lachnospirin-1, which exhibited significant bactericidal activity against CRAB and can also effectively eliminate its biofilms and persister cells. Mechanism studies, including fluorescent probe detection, transmission electron microscopy observations, and molecular dynamics simulations, et al. indicated that lachnospirin-1 exerts its effects through multiple mechanisms, such as disrupting bacterial cell membranes, neutralizing lipopolysaccharide, as well as inducing oxidative stress, which also demonstrates its certain potential. In addition, the favorable safety profile and effective in vivo bactericidal activity of lachnospirin-1 were determined by mouse models. Overall, our study found that lachnospirin-1 has considerable research potential as a lead compound and possesses latent value as a clinical therapy for refractory infections caused by CRAB.

ABSTRACT In Salmonella, the type III and type VI secretion systems (T3SS and T6SS) play critical roles in pathogenicity. The bacterium harbors two distinct T3SSs: T3SS-1, encoded by SPI-1, promotes host cell invasion, whereas T3SS-2 (located within SPI-2) facilitates systemic infection. Both systems translocate effector proteins that disrupt normal host cell functions. Meanwhile, the T6SS enhances competitive fitness in the gut by targeting rival bacteria and supports evasion of the host immune response. Recent evidence indicates that these secretion systems share common regulatory components, including the PhoP-PhoQ and PmrA-PmrB two-component systems, as well as signaling molecules such as cyclic di-GMP (c-di-GMP), allowing coordinated activation and functional adaptation during infection. However, the precise mechanisms governing their crosstalk remain poorly understood. This review summarizes the current knowledge of the composition, function, and regulation of T3SS and T6SS in Salmonella, examines their interplay, and provides perspectives for future research into pathogenesis.

ABSTRACT Fusobacterium nucleatum was long regarded as a single species and later subdivided into four subspecies (nucleatum, polymorphum, animalis, vincentii/fusiforme). In 2022, these subspecies were validated as separate species and further members of the F. nucleatum complex have been proposed (F. watanabei, F. paranimalis sp. nov.). Given the increasing evidence linking F. nucleatum to various diseases, identifying (sub-)species-specific virulence factors has become essential. Infection in mammalian hosts depends on virulence factors that can be surface-exposed, released into the extracellular environment, or injected directly into host cells. This narrative review aims to address the different pathogenic potentials of each former subspecies. These differences range from adhesin diversity and metabolic adaptations, the repertoire of ABC transporters, lyases and type IV conjugative pili to the capability to invade tissues, evade the immune system and form biofilms and outer membrane vesicles.

ABSTRACT Bacterial metabolism is important for antibiotic resistance and tolerance. However, the impact of indole on bacterial metabolism and antibiotic efficacy has not been fully elucidated. In this study, we investigated the effect and specific mechanism of exogenous indole on the antibiotic susceptibility of Edwardsiella tarda, a common pathogen in freshwater and marine fish farming. We found that exogenous indole promoted E. tarda tolerance to the antibiotic florfenicol, and reprogrammed the E. tarda metabolome. A total of 108 metabolites were detected, including 66 differential metabolites that regulate various metabolic pathways, such as the tricarboxylic acid (TCA) cycle and nucleotide metabolism. Exogenous indole disrupted the TCA cycle in E. tarda by increasing the intracellular NADH contents and activating the respiratory chain to increase the reactive oxygen species levels, thereby increasing the intracellular Fe2+ content to activate the Fenton reaction, which in turn promotes the oxidative stress response. Furthermore, indole inhibited antibiotic entry into the cell and activated efflux pumps to reduce the intracellular antibiotic content, ultimately promoting antibiotic tolerance. In vivo, exogenous indole compromised the ability of florfenicol to protect fish survival and eliminate pathogenic bacteria. These results shed light on the metabolic changes induced by indole and suggest future directions for addressing antibiotic tolerance and clinical infections of E. tarda in aquaculture. This study serves as a reminder of the adverse effects of combining antibiotics with metabolites in aquaculture.

ABSTRACT The markedly reduced pathogenicity of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Omicron variant in comparison to earlier strains has raised critical questions regarding its underlying mechanisms. To elucidate the host immune responses driving these differences, we performed single-cell transcriptomic profiling of lung and blood samples from human angiotensin-converting enzyme 2 (hACE2) transgenic mice infected with either the SARS-CoV-2 prototype strain or the Omicron BA.1 variant at 5 days post-inoculation. Both strains induced a reduction in lung cell numbers, with capillary endothelial cells showing the highest number of differentially expressed genes (DEGs). Shared transcriptional responses included upregulation of chemokine (e.g. Gnaq, Lyn, Ccl5) and IL-17 signaling pathways. Notably, Omicron BA.1 infection resulted in downregulation of Txnip, a key gene involved in oxidative stress responses. Genes associated with neutrophil granules and pro-inflammatory functions (Mmp8, S100a8, S100a9) were also downregulated, whereas wound healing pathways were upregulated in immature neutrophils. Additionally, Omicron BA.1 altered gene expression associated with neutrophil migration from blood to lung, and decreased the activation of cell chemotaxis, cytokine-mediated signaling, IL-17 and NF-κB pathways in pro-inflammatory monocytes and pulmonary interstitial macrophages. These findings highlight distinct immunological signatures contributing to the attenuated pathogenicity of Omicron BA.1, particularly through the modulation of neutrophil and monocyte-macrophage responses.

ABSTRACT Mitochondrial function is essential for virulence in Candida albicans, yet the mechanism by which mitochondria influence pathogenesis remains largely undefined. Here, we reveal that the mitochondrial-associated factor Mss2 controls invasive growth through the regulation of calcium–reactive oxygen species (ROS) homeostasis. Deletion of MSS2 results in impaired invasive growth on solid media without affecting hyphal formation in liquid media, indicating that Mss2 controls contact-specific responses. We demonstrate that the regulation of these processes by Mss2 is linked to the regulation of cytosolic calcium levels and cellular ROS production. Furthermore, transcriptomic profiling identified MSS2-regulated genes, including UME6, SAC1, RIM8, and ORF19.1841, whose expression is dependent on calcium and ROS levels. Restoration of invasive phenotypes through exogenous ROS confirms the functional significance of this calcium-ROS circuit. In systemic infection models, similar to mss2Δ, the Mss2 downstream genes exhibit severe virulence defects. Together, this work is the first to show that mitochondrial regulation of a coordinated calcium-ROS circuit is required for invasive hyphal growth and virulence in C. albicans. These findings refine our understanding of fungal invasion and virulence and reveal that targeting mitochondrial signaling could be an important area for antifungal therapeutic interventions.

ABSTRACT Foot-and-mouth disease virus (FMDV) can cause a severe infectious disease that primarily affects even-toed ungulates. FMDV is classified into the genus of Aphthovirus in the family Picornaviridae. FMDV’s 3C protein is a nonstructural protein and, moreover, is a protease (3Cpro) that adopts a chymotrypsin-like fold and harbors a Cys-His-Asp catalytic triad. The 3Cpro plays crucial roles not only in cleaving the FMDV polyprotein but also in degrading various host proteins. Cleavage of the polyprotein contributes to generating different viral polypeptides. Degradation of host proteins possibly affects cellular signaling pathways, making FMDV impair innate immune responses. Here, we systematically reviewed FMDV 3Cpro concerning its multiple characteristics, including nucleotide and protein sequences, crystal structures, enzymatic activities, anti-3Cpro inhibitors, and more importantly, its functions in cleaving the viral polyprotein and host proteins. This review aims to provide a comprehensive insight into FMDV 3Cpro as a protease functioning in the course of viral propagation.

ABSTRACT Thrombomodulin (TM) is a membrane protein with significant roles in coagulation hemostasis and immune response. Its soluble form (sTM) has recently emerged as a key biomarker for severe invasive bacterial infections, including Necrotizing Soft Tissue Infections (NSTI). While various mechanical, chemical, and enzymatic mechanisms have been linked to TM shedding, this study investigates the direct impact of bacterial stimuli on soft tissue cells as primary sources of TM release. We stimulated organotypic models, composed of fibroblast and endothelial cells, with NSTI clinical isolates and found that while Group A Streptococcus and Escherichia coli had minimal effect on TM release, Staphylococcus aureus infection triggered a significant increase of sTM levels. We further assessed whether the secreted proteins of S. aureus led to higher TM levels by increased expression, increased cell toxicity, or direct cleavage of TM from the endothelial cell membrane. To investigate these mechanisms, we performed in vitro stimulations of endothelial monolayers with secreted proteins of two S. aureus isolates differing in their agr-system functionality. Our results indicate that S. aureus agr-regulated proteins induce TM shedding by direct cleavage from the cell membrane, an effect that was inhibited by metalloproteinase inhibitors. Stimulation with the pore-forming protein α-toxin showed similar results, suggesting a potential involvement of ADAM10 in TM cleavage. Additionally, we observed that other agr-regulated proteins can cleave TM directly. Altogether, this study reveals a pathogen-specific mechanism for TM release during S. aureus invasive infection, contributing to its elevated plasma levels and providing deeper insights into the pathophysiology of NSTI.