Bacterial second messenger 3',5'-cyclic diguanylate attracts Caenorhabditis elegans and suppresses its immunity

Cyclic dinucleotides mediate bacterial immunity by dinucleotide cyclase in Vibrio

The cyclic GMP-AMP (cGAMP) synthase (cGAS) recognizes cytosolic DNA and synthesizes the second messenger, cGAMP, thus activating the adaptor protein stimulator of interferon genes (STING) and initiating the innate immune responses against microbial infections. cGAS-STING pathway has been crucially implicated in autoimmune diseases, cellular senescence, and cancer immunotherapy, while the cGAS-like receptors in bacteria can protect it against viral infections. Dinucleotide cyclase in Vibrio (DncV) is a dinucleotide cyclase originally identified in Vibrio cholerae. The synthesis of cyclic nucleotides by DncV, including c-di-GMP, c-di-AMP, and cGAMP mediates bacterial colonization, cell membrane formation, and virulence. DncV is a structural and functional homolog of the mammalian cytoplasmic DNA sensor, cGAS, implicating cGAS-STING signaling cascades may have originated in the bacterial immune system. Herein, we summarize the roles of DncV in bacterial immunity, which are expected to provide insights into the evolution of cGAS-STING signaling.

Cronobacter sakazakii Cue for the Attraction and Its Impact on the Immunity of Caenorhabditis elegans

Cronobacter sakazakii, an opportunistic foodborne pathogen prevalently detected in contaminated powdered infant formula, is associated with different diseases, including meningitis. It can cross the blood-brain barrier and affects the CNS. The impact of C. sakazakii on host neuronal cells and behavior is largely unknown. Hence, detailed molecular data are required to understand its severity. Caenorhabditis elegans is a unique model for studying chemical communication, as it relies on chemosensation for searching nutritional supplements. Although, C. sakazakii is pathogenic to C. elegans, our analysis indicated that C. elegans was highly attracted toward C. sakazakii compared to its food source, E. coli OP50. To study the cue for the attraction, bioactive components (RNA/Protein/Lipopolysaccharides/Metabolites) of C. sakazakii were isolated and used for observing the chemotaxis behavior of C. elegans. The results signified that C. elegans was more attracted toward acid extracted metabolites than those of the other extraction methods. The combined action of acid extracted metabolites of C. sakazakii and a candidate pathogen drastically reduced the survival of C. elegans. In addition, qPCR analysis suggested that the exposure of isolated metabolites through acid extraction to C. elegans for 24 h modified the candidate immune regulatory genes involved in pathogen recognition and kinase activity such as clec-60, clec-87, lys-7, akt-2, pkc-1, and jnk-1.

Identification of the Genes of the Plant Pathogen Pseudomonas syringae MB03 Required for the Nematicidal Activity Against Caenorhabditis elegans Through an Integrated Approach

Nematicidal potential of the common plant pathogen Pseudomonas syringae has been recently identified against Caenorhabditis elegans. The current study was designed to investigate the detailed genetic mechanism of the bacterial pathogenicity by applying comparative genomics, transcriptomics, mutant library screening, and protein expression. Results showed that P. syringae strain MB03 could kill C. elegans in the liquid assay by gut colonization. The genome of P. syringae MB03 was sequenced and comparative analysis including multi locus sequence typing, and genome-to-genome distance placed MB03 in phylogroup II of P. syringae. Furthermore, comparative genomics of MB03 with nematicidal strains of Pseudomonas aeruginosa (PAO1 and PA14) predicted 115 potential virulence factors in MB03. However, genes for previously reported nematicidal metabolites, such as phenazine, pyochelin, and pyrrolnitrin, were found absent in the MB03 genome. Transcriptomics analysis showed that the growth phase of the pathogen considerably affected the expression of virulence factors, as genes for the flagellum, glutamate ABC transporter, phoP/phoQ, fleS/fleR, type VI secretion system, and serralysin were highly up-regulated when stationary phase MB03 cells interacted with C. elegans. Additionally, screening of a transposon insertion mutant library led to the identification of other nematicidal genes such as acnA, gltP, oprD, and zapE. Finally, the nematicidal activity of selected proteins was confirmed by heterologous expression in Escherichia coli.