Identification of chemotaxis operon cheYZA and cheA gene expression under stressful conditions in Piscirickettsia salmonis

In vitro effects of phytogenic feed additive on Piscirickettsia salmonis growth and biofilm formation

Piscirickettsiosis is the main cause of mortality in salmonids of commercial importance in Chile, which is caused by Piscirickettsia salmonis, a Gram-negative, γ-proteobacteria that can produce biofilm as one of its virulence factors. The Chilean salmon industry uses large amounts of antibiotics to control piscirickettsiosis outbreaks, which has raised concern about its environmental impact and the potential to induce antibiotic resistance. Thus, the use of phytogenic feed additives (PFA) with antibacterial activity emerges as an interesting alternative to antimicrobials. Our study describes the antimicrobial action of an Andrographis paniculate-extracted PFA on P. salmonis planktonic growth and biofilm formation. We observed complete inhibition of planktonic and biofilm growth with 500 and 400 μg/mL of PFA for P. salmonis LF-89 and EM-90-like strains, respectively. Furthermore, 500 μg/mL of PFA was bactericidal for both evaluated bacterial strains. Sub-inhibitory doses of PFA increase the transcript levels of stress (groEL), biofilm (pslD), and efflux pump (acrB) genes for both P. salmonis strains in planktonic and sessile conditions. In conclusion, our results demonstrate the antibacterial effect of PFA against P. salmonis in vitro, highlighting the potential of PFA as an alternative to control Piscirickettsiosis.

Response of phosphorus fractions transformation and microbial community to carbon-to-phosphorus ratios during sludge composting

Phosphorus (P) is one of the essential nutrient elements for plant growth and development. Sludge compost products can be used as an important source of soil P to solve the shortage of soil P. The difference in the initial carbon-to-phosphorus ratio (C/P) will lead to difference in the bacterial community, which would affect the biological pathway of P conversion in composting. However, few studies have been reported on adjusting the initial C/P of composting to explore P conversion. Therefore, this study investigated the response of P component transformations, bacterial community and P availability to C/P during sludge composting by adjusting initial C/P. The results showed that increasing C/P promoted the mineralization of organic P and significantly increased the content of the labile P. High C/P also increased the relative content of available P, especially when the C/P was at 45 and 60, it reached 60.51% and 60.47%. High C/P caused differences in the community structure, and improved the binding ability of microbial network modules and the competitiveness of microbial communities. Additionally, high C/P strengthened the effect of microbial communities on the transformation of P components. Finally, the study showed that C/P was the main contributor to P content variation (64.7%) and indirectly affected P component conversion by affecting the microbial community. Therefore, adjusting the C/P is crucial to improve the P utilization rate of composting products.

Flagella-mediated adhesion of Escherichia coli O157:H7 to surface of stainless steel, glass and fresh produces during sublethal injury and recovery

E. coli O157:H7 can be induced into sublethally injured (SI) state by lactic acid (LA) and regain activity in nutrient environments. This research clarified the role of flagella-related genes (fliD, fliS, cheA and motA) in adhesion of E. coli O157:H7 onto stainless steel, glass, lettuce, spinach, red cabbage and cucumber during LA-induced SI and recovery by plate counting. Results of adhesion showed improper flagellar rotation caused by the deletion of motA resulting in the decreased adhesion. Motility of wildtype determined by diameter of motility halo decreased in SI state and repaired with recovery time increasing, lagging behind changes in expression of flagella-related genes. Flagellar function-impaired strains all exhibited non-motile property. Thus, we speculated that flagella-mediated motility is critical in early stage of adhesion. We also found the effects of Fe2+, Ca2+ and Mn2+ on adhesion or motility of wildtype was independent of bacterial states. However, the addition of Ca2+ and Mn2+ did not affect motility of flagellar function-impaired strains as they did on wildtype. This research provides new insights to understand the role of flagella and cations in bacterial adhesion, which will aid in development of anti-adhesion agents to reduce bio-contamination in food processing.

Cohabitation of Piscirickettsia salmonis genogroups (LF-89 and EM-90): synergistic effect on growth dynamics

Piscirickettsia salmonis, the biological agent of Salmonid Rickettsial Septicemia (SRS), is a facultative intracellular bacterium that can be divided into two genogroups (LF-89 and EM-90) with different virulence levels and patterns. Studies have found co-infection of these genogroups in salmonid farms in Chile, but it is essential to assess whether this interaction within the host is related to virulence and changes in pathogen dynamics. In this study, we studied four isolates from EM-90 and one LF-89 isolate chosen based on their genomic differences. The aim was to evaluate how co-cultivation affects bacterial growth performance and virulence factor expression in Atlantic salmon (Salmo salar) in vitro and in vivo. In vitro results using FN2 medium, showed a similar growth curve between co-cultures of LF-89 and EM-90 compared to EM-90 monocultures. This was explained by the higher ratio of EM-90 to LF-89 in all co-cultures. When evaluating the expression of virulence factors, it was discovered that the luxR gene was expressed only in EM-90-like isolates and that there were significant differences between mono- and co-cultures for flaA and cheA, suggesting a response to cohabitation. Moreover, during in vivo co-cultures, transcriptomic analysis revealed an upregulation of transposases, flagellum-related genes (fliI and flgK), transporters, and permeases that could unveil novel virulence effectors used in the early infection process of P. salmonis. Thus, our work has shown that cohabitation of P. salmonis genogroups can modulate their behavior and virulence effector expression. These data can contribute to new strategies and approaches to improve the current health treatments against this salmonid pathogen.

Functional network analysis identifies multiple virulence and antibiotic resistance targets in Stenotrophomonas maltophilia

Stenotrophomonas maltophilia, an emerging multidrug-resistant opportunistic bacterium in humans is of major concern for immunocompromised individuals for causing pneumonia and bloodborne infections. This bacterial pathogen is associated with a considerable fatality/case ratio, with up to 100%, when presented as hemorrhagic fever. It is resistant to commonly used drugs as well as to antibiotic combinations. In-silico based functional network analysis is a key approach to get novel insights into virulence and resistance in pathogenic organisms. This study included the protein-protein interaction (PPI) network analysis of 150 specific genes identified for antibiotic resistance mechanism and virulence pathways. Eight proteins, namely, PilL, FliA, Smlt2260, Smlt2267, CheW, Smlt2318, CheZ, and FliM were identified as hub proteins. Further docking studies of 58 selected phytochemicals were performed against the identified hub proteins. Deoxytubulosine and corosolic acid were found to be potent inhibitors of hub proteins of pathogenic S. maltophilia based on protein-ligand interactive study. Further pharmacophore studies are warranted with these molecules to develop them as novel antibiotics against S. maltophilia.

Biofilm formation of pathogenic bacteria isolated from aquatic animals

Bacterial biofilm formation is one of the dynamic processes, which facilitates bacteria cells to attach to a surface and accumulate as a colony. With the help of biofilm formation, pathogenic bacteria can survive by adapting to their external environment. These bacterial colonies have several resistance properties with a higher survival rate in the environment. Especially, pathogenic bacteria can grow as biofilms and can be protected from antimicrobial compounds and other substances. In aquaculture, biofilm formation by pathogenic bacteria has emerged with an increased infection rate in aquatic animals. Studies show that Vibrio anguillarum, V. parahaemolyticus, V. alginolyticus, V. harveyi, V. campbellii, V. fischeri, Aeromonas hydrophila, A. salmonicida, Yersinia ruckeri, Flavobacterium columnare, F. psychrophilum, Piscirickettsia salmonis, Edwardsiella tarda, E. ictaluri, E. piscicida, Streptococcus parauberis, and S. iniae can survive in the environment by transforming their planktonic form to biofilm form. Therefore, the present review was intended to highlight the principles behind biofilm formation, major biofilm-forming pathogenic bacteria found in aquaculture systems, gene expression of those bacterial biofilms and possible controlling methods. In addition, the possibility of these pathogenic bacteria can be a serious threat to aquaculture systems.

Collective behavior and virulence arsenal of the fish pathogen Piscirickettsia salmonis in the biofilm realm

Piscirickettsiosis is a fish disease caused by the Gram-negative bacterium Piscirickettsia salmonis. This disease has a high socio-economic impact on the Chilean salmonid aquaculture industry. The bacterium has a cryptic character in the environment and their main reservoirs are yet unknown. Bacterial biofilms represent a ubiquitous mechanism of cell persistence in diverse natural environments and a risk factor for the pathogenesis of several infectious diseases, but their microbiological significance for waterborne veterinary diseases, including piscirickettsiosis, have seldom been evaluated. This study analyzed the in vitro biofilm behavior of P. salmonis LF-89^T (genogroup LF-89) and CA5 (genogroup EM-90) using a multi-method approach and elucidated the potential arsenal of virulence of the P. salmonis LF-89^T type strain in its biofilm state. P. salmonis exhibited a quick kinetics of biofilm formation that followed a multi-step and highly strain-dependent process. There were no major differences in enzymatic profiles or significant differences in cytotoxicity (as tested on the Chinook salmon embryo cell line) between biofilm-derived bacteria and planktonic equivalents. The potential arsenal of virulence of P. salmonis LF-89^T in biofilms, as determined by whole-transcriptome sequencing and differential gene expression analysis, consisted of genes involved in cell adhesion, polysaccharide biosynthesis, transcriptional regulation, and gene mobility, among others. Importantly, the global gene expression profiles of P. salmonis LF-89^T were not enriched with virulence-related genes upregulated in biofilm development stages at 24 and 48 h. An enrichment in virulence-related genes exclusively expressed in biofilms was also undetected. These results indicate that early and mature biofilm development stages of P. salmonis LF-89^T were transcriptionally no more virulent than their planktonic counterparts, which was supported by cytotoxic trials, which, in turn, revealed that both modes of growth induced important and very similar levels of cytotoxicity on the salmon cell line. Our results suggest that the aforementioned biofilm development stages do not represent hot spots of virulence compared with planktonic counterparts. This study provides the first transcriptomic catalogue to select specific genes that could be useful to prevent or control the (in vitro and/or in vivo) adherence and/or biofilm formation by P. salmonis and gain further insights into piscirickettsiosis pathogenesis.

Complete Lipopolysaccharide of Piscirickettsia salmonis Is Required for Full Virulence in the Intraperitoneally Challenged Atlantic Salmon, Salmo salar, Model

Bacterial cell envelopes play a critical role in host-pathogen interactions. Macromolecular components of these structures have been closely linked to the virulence of pathogens. Piscirickettsia salmonis is a relevant salmonid pathogen with a worldwide distribution. This bacterium is the etiological agent of piscirickettsiosis, a septicemic disease that causes a high economic burden, especially for the Chilean salmon farming industry. Although P. salmonis has been discovered long ago, its pathogenicity and virulence mechanisms are not completely understood. In this work, we present a genetic approach for producing in-frame deletion mutants on genes related to the biosynthesis of membrane-associated polysaccharides. We provide a detailed in vitro phenotype description of knock-out mutants on wzx and wcaJ genes, which encode predicted lipopolysaccharide (LPS) flippase and undecaprenyl-phosphate glucose phosphotransferase enzymes, respectively. We exhibit evidence that the wzx mutant strain carries a defect in the probably most external LPS moiety, while the wcaJ mutant proved to be highly susceptible to the bactericidal action of serum but retained the ability of biofilm production. Beyond that, we demonstrate that the deletion of wzx, but not wcaJ, impairs the virulence of P. salmonis in an intraperitoneally infected Atlantic salmon, Salmo salar, model of piscirickettsiosis. Our findings support a role for LPS in the virulence of P. salmonis during the onset of piscirickettsiosis.

Transcriptional analysis of metabolic and virulence genes associated with biofilm formation in Piscirickettsia salmonis strains

Piscirickettsia salmonis is a facultative intracellular bacterium that generates piscirickettsiosis affecting salmonids in Chile. The bacterium has the adaptability to survive in the marine environment under multiple stressful conditions. In this sense, this work focused on the analysis of a gene battery associated with biofilm formation under different culture conditions and on the adaptability of this biofilm to different media. The results indicated that the strains LF-89, IBM-034 and IBM-040 were strong biofilm producers, evidencing adaptability to the media by increasing the amount of biofilm through successive growths. Transcript levels of six genes described in various bacteria and P. salmonis, considered to have metabolic functions, and playing a relevant role in biofilm formation, were analyzed to evaluate bacterial functionality in the biofilm. The genes mazE-mazF, implicated in biofilm and stress, were markedly overexpressed in the biofilm condition in the three strains. For its part, gene gltA, an indicator of metabolic activity and related to virulence inhibition in Salmonella typhimurium, also seems to restrain the pathogenesis process in P. salmonis by inhibiting the expression of the virulence-associated genes liso and tcf. Finally, the expression of the glnA gene suggests the use of glutamine as an essential element for the growth of the biofilm.

Biofilm Produced In Vitro by Piscirickettsia salmonis Generates Differential Cytotoxicity Levels and Expression Patterns of Immune Genes in the Atlantic Salmon Cell Line SHK-1

Piscirickettsia salmonis is the causative agent of Piscirickettsiosis, an infectious disease with a high economic impact on the Chilean salmonid aquaculture industry. This bacterium produces biofilm as a potential resistance and persistence strategy against stressful environmental stimuli. However, the in vitro culture conditions that modulate biofilm formation as well as the effect of sessile bacteria on virulence and immune gene expression in host cells have not been described for P. salmonis. Therefore, this study aimed to analyze the biofilm formation by P. salmonis isolates under several NaCl and iron concentrations and to evaluate the virulence of planktonic and sessile bacteria, together with the immune gene expression induced by these bacterial conditions in an Atlantic salmon macrophage cell line. Our results showed that NaCl and Fe significantly increased biofilm production in the LF-89 type strain and EM-90-like isolates. Additionally, the planktonic EM-90 isolate and sessile LF-89 generated the highest virulence levels, associated with differential expression of il-1β, il-8, nf-κb, and iκb-α genes in SHK-1 cells. These results suggest that there is no single virulence pattern or gene expression profile induced by the planktonic or sessile condition of P. salmonis, which are dependent on each strain and bacterial condition used.

Improved understanding of biofilm development by Piscirickettsia salmonis reveals potential risks for the persistence and dissemination of piscirickettsiosis

Piscirickettsia salmonis is the causative agent of piscirickettsiosis, a disease with high socio-economic impacts for Chilean salmonid aquaculture. The identification of major environmental reservoirs for P. salmonis has long been ignored. Most microbial life occurs in biofilms, with possible implications in disease outbreaks as pathogen seed banks. Herein, we report on an in vitro analysis of biofilm formation by P. salmonis Psal-103 (LF-89-like genotype) and Psal-104 (EM-90-like genotype), the aim of which was to gain new insights into the ecological role of biofilms using multiple approaches. The cytotoxic response of the salmon head kidney cell line to P. salmonis showed interisolate differences, depending on the source of the bacterial inoculum (biofilm or planktonic). Biofilm formation showed a variable-length lag-phase, which was associated with wider fluctuations in biofilm viability. Interisolate differences in the lag phase emerged regardless of the nutritional content of the medium, but both isolates formed mature biofilms from 288 h onwards. Psal-103 biofilms were sensitive to Atlantic salmon skin mucus during early formation, whereas Psal-104 biofilms were more tolerant. The ability of P. salmonis to form viable and mucus-tolerant biofilms on plastic surfaces in seawater represents a potentially important environmental risk for the persistence and dissemination of piscirickettsiosis.

Comparative Genomics Reveals Pathogenicity-Related Loci in Shewanella algae

Shewanella algae is an emerging marine zoonotic pathogen and accounts for considerable mortality and morbidity in compromised hosts. However, there is scarce literature related to the understanding of the genetic background of virulence determinants in S. algae. In this study, we aim to determine the occurrence of common virulence genes in S. algae using whole-genome sequence and comparative genomic analysis. Comparative genomics reveals putative-virulence genes related to bile resistance, chemotaxis, hemolysis, and motility. We detected the existence of hlyA, hlyD, and hlyIII involved in hemolysis. We also found chemotaxis gene cluster cheYZA operon and cheW gene. The results provide insights into the genetic basis underlying pathogenicity in S. algae.

Engineering a defined culture medium to grow Piscirickettsia salmonis for its use in vaccine formulations

Piscirickettsia salmonis is a facultative Gram-negative intracellular bacterium that produces piscirickettsiosis, disease that causes a high negative impact in salmonid cultures. The so-far-unidentified nutritional requirements have hindered its axenic culture at laboratory and industrial scales for the formulation of vaccines. The present study describes the development of a defined culture medium for P. salmonis. The culture medium was formulated through rational design involving auxotrophy test and statistical designs of experiments, considering the genome-scale metabolic reconstruction of P. salmonis reported by our group. The whole optimization process allowed for a twofold increase in biomass and a reduction of about 50% of the amino acids added to the culture medium. The final culture medium contains twelve amino acids, where glutamic acid, threonine and arginine were the main carbon and energy sources, supporting 1.65 g/L of biomass using 6.5 g/L of amino acids in the formulation. These results will contribute significantly to the development of new operational strategies to culture this bacterium for the production of vaccines.

Comparative Pan-Genome Analysis of Piscirickettsia salmonis Reveals Genomic Divergences within Genogroups

Piscirickettsia salmonis is the etiological agent of salmonid rickettsial septicemia, a disease that seriously affects the salmonid industry. Despite efforts to genomically characterize P. salmonis, functional information on the life cycle, pathogenesis mechanisms, diagnosis, treatment, and control of this fish pathogen remain lacking. To address this knowledge gap, the present study conducted an in silico pan-genome analysis of 19 P. salmonis strains from distinct geographic locations and genogroups. Results revealed an expected open pan-genome of 3,463 genes and a core-genome of 1,732 genes. Two marked genogroups were identified, as confirmed by phylogenetic and phylogenomic relationships to the LF-89 and EM-90 reference strains, as well as by assessments of genomic structures. Different structural configurations were found for the six identified copies of the ribosomal operon in the P. salmonis genome, indicating translocation throughout the genetic material. Chromosomal divergences in genomic localization and quantity of genetic cassettes were also found for the Dot/Icm type IVB secretion system. To determine divergences between core-genomes, additional pan-genome descriptions were compiled for the so-termed LF and EM genogroups. Open pan-genomes composed of 2,924 and 2,778 genes and core-genomes composed of 2,170 and 2,228 genes were respectively found for the LF and EM genogroups. The core-genomes were functionally annotated using the Gene Ontology, KEGG, and Virulence Factor databases, revealing the presence of several shared groups of genes related to basic function of intracellular survival and bacterial pathogenesis. Additionally, the specific pan-genomes for the LF and EM genogroups were defined, resulting in the identification of 148 and 273 exclusive proteins, respectively. Notably, specific virulence factors linked to adherence, colonization, invasion factors, and endotoxins were established. The obtained data suggest that these genes could be directly associated with inter-genogroup differences in pathogenesis and host-pathogen interactions, information that could be useful in designing novel strategies for diagnosing and controlling P. salmonis infection.