Cyclic di-GMP Modulates a Metabolic Flux for Carbon Utilization in Salmonella enterica Serovar Typhimurium

Phylogenetic distribution and characterization of conserved C-di-GMP metabolizing proteins in filamentous cyanobacterium Arthrospira

Cyclic diguanosine monophosphate (c-di-GMP) is a second messenger in bacteria that regulates multiple biological functions, including biofilm formation, virulence, and intercellular communication. However, c-di-GMP signaling is virtually unknown in economically important filamentous cyanobacteria, Arthrospira. In this study, we predicted 31 genes encoding GGDEF-domain proteins from A. platensis NIES39 as potential diguanylate cyclases (DGCs). Phylogenetic distribution analysis showed five genes (RS09460, RS04865, RS26155, M01840, and E02220) with highly conserved distribution across 25 Arthrospira strains. Adc1 encoded by RS09460 was further characterized as a typical DGC. By establishing the genetic transformation system of Arthrospira, we demonstrated that the overexpression of Adc1 promoted the production of extracellular polymeric substances (EPS), which in turn caused the aggregation of filaments. We also confirmed that RS04865 and RS26155 may encode active DGCs, while enzymatic activity assays showed that proteins encoded by M01840 and E02220 have phosphodiesterase (PDE) activity. Meta-analysis revealed that the expression profiles of RS09460 and RS04865 were unaffected under 31 conditions, suggesting that they may function as conserved genes in maintaining the basal level of c-di-GMP in Arthrospira. In summary, this report will provide the basis for further studies of c-di-GMP signal in Arthrospira.

Salmonella typhimurium targeting with monoclonal antibodies prevents infection in mice

Salmonella is a prevalent foodborne and waterborne pathogens threating global public health and food safety. Given the diversity of Salmonella serotypes and the emergence of antibiotic-resistant strains, there is an urgent need for the development of broadly protective therapies. This study aims to prepare monoclonal antibodies (Mabs) with broad reactivity against multi-serotype Salmonella strains, potentially offering cross-protection. We prepared two Mabs F1D4 and B7D4 against protein FliK and BcsZ, two potential vaccine candidates against multi-serotype Salmonella. The two Mabs belonging to IgG1 isotype exhibited high titers of 1:256,000 and 1:512,000 respectively, as well as broad cross-reactivity against 28 different serotypes of Salmonella strains with percentages of 89.29% and 92.86%, correspondingly. Neutralizing effects of the two Mabs on Salmonella growth, adhesion, invasion and motility was evaluated in vitro using bacteriostatic and bactericidal activity with and without complement and bacterial invasion inhibition assay. Additionally, cytotoxicity assays, animal toxicity analyses, and pharmacokinetic evaluations demonstrated the safety and sustained effectiveness of both Mabs. Furthermore, F1D4 or B7D4-therapy in mice challenged with S. Typhimurium LT2 exhibited milder organs damage and lower Salmonella colonization, as well as the higher relative survival of 86.67% and 93.33% respectively. This study produced two broadly reactive and potential cross protective Mabs F1D4 and B7D4, which offered new possibilities for immunotherapy of salmonellosis.

Genomic Diversity, Antimicrobial Resistance, Plasmidome, and Virulence Profiles of Salmonella Isolated from Small Specialty Crop Farms Revealed by Whole-Genome Sequencing

Salmonella is the leading cause of death associated with foodborne illnesses in the USA. Difficulty in treating human salmonellosis is attributed to the development of antimicrobial resistance and the pathogenicity of Salmonella strains. Therefore, it is important to study the genetic landscape of Salmonella, such as the diversity, plasmids, and presence antimicrobial resistance genes (AMRs) and virulence genes. To this end, we isolated Salmonella from environmental samples from small specialty crop farms (SSCFs) in Northeast Ohio from 2016 to 2021; 80 Salmonella isolates from 29 Salmonella-positive samples were subjected to whole-genome sequencing (WGS). In silico serotyping revealed the presence of 15 serotypes. AMR genes were detected in 15% of the samples, with 75% exhibiting phenotypic and genotypic multidrug resistance (MDR). Plasmid analysis demonstrated the presence of nine different types of plasmids, and 75% of AMR genes were located on plasmids. Interestingly, five Salmonella Newport isolates and one Salmonella Dublin isolate carried the ACSSuT gene cassette on a plasmid, which confers resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, and tetracycline. Overall, our results show that SSCFs are a potential reservoir of Salmonella with MDR genes. Thus, regular monitoring is needed to prevent the transmission of MDR Salmonella from SSCFs to humans.