A Polyphasic Approach for Phenotypic and Genetic Characterization of the Fastidious Aquatic Pathogen Francisella noatunensis subsp. orientalis

Identification of a dCache-type chemoreceptor in Campylobacter jejuni that specifically mediates chemotaxis towards methyl pyruvate

The foodborne pathogenic bacterium Campylobacter jejuni utilizes chemotaxis to assist in the colonization of host niches. A key to revealing the relationship among chemotaxis and pathogenicity is the discovery of signaling molecules perceived by the chemoreceptors. The C. jejuni chemoreceptor Tlp11 is encoded by the highly infective C. jejuni strains. In the present study, we report that the dCache-type ligand-binding domain (LBD) of C. jejuni ATCC 33560 Tlp11 binds directly to novel ligands methyl pyruvate, toluene, and quinoline using the same pocket. Methyl pyruvate elicits a strong chemoattractant response, while toluene and quinoline function as the antagonists without triggering chemotaxis. The sensory LBD was used to control heterologous proteins by constructing chimeras, indicating that the signal induced by methyl pyruvate is transmitted across the membrane. In addition, bioinformatics and experiments revealed that the dCache domains with methyl pyruvate-binding sites and ability are widely distributed in the order Campylobacterales. This is the first report to identify the class of dCache chemoreceptors that bind to attractant methyl pyruvate and antagonists toluene and quinoline. Our research provides a foundation for understanding the chemotaxis and virulence of C. jejuni and lays a basis for the control of this foodborne pathogen.

Pathogenic bacteria prevalence in cultured Nile tilapia in Southwest Mexico: A real-time PCR analysis

The present study investigates molecular-based PCR techniques to estimate the prevalence of fish pathogens in southwest Mexico where recurrent mortality in the tilapia cultures has been observed. Sample of internal organs and lesions of Nile tilapia were taken and analysed in 2018, 2019, 2020 and 2022 to detect bacterial pathogens using PCR. No samples were taken in 2021 due to the COVID-19 pandemic. The real-time PCR conditions were optimized to allow a qualitative reliable detection of the bacteria from fixed fish tissue. A total of 599 pond- and cage-cultured tilapia from the southwestern Mexican Pacific (Guerrero, Oaxaca and Chiapas states) were analysed. In this tropical region, during 2018 and 2019 water temperatures of the tilapia cultures were generally with the optimal range to grow Nile tilapia, although extreme values were recorded on some farms. Most of the tilapia sampled were apparently healthy. No Francisella sp. was detected in any sample, and Staphylococcus sp. was the most prevalent (from 0% to 64%) bacteria from the three states over time. Low prevalence of Aeromonas sp. was found, from 0% to 4.3%, although the fish pathogen Aeromonas dhakensis was not detected. Sterptococcus iniae was only detected in Chiapas in 2019 at a low prevalence (1.4%), while the major tilapia pathogen S. agalactiae was detected at a high prevalence (from 0% to 59%) in the three Mexican states. This is the first detection of these pathogenic bacteria in rural farms using real-time PCR and constitutes a great risk for tilapia aquaculture in Mexico, as well as a potential dispersion of these pathogens to other aquaculture areas.

Therapeutic efficacy of enrofloxacin in treatment of Francisella orientalis infections in juvenile Nile tilapia (Oreochromis niloticus L.)

Outbreaks of infections by Francisella orientalis represent one of the main obstacles to Nile tilapia (Oreochromis niloticus L.) farming. It is responsible for acute mortality in fingerlings and juveniles. The main control measure available is oral antibiotic therapy. This study compared the therapeutic efficacy of the antibiotics enrofloxacin and oxytetracycline, the most commonly used antimicrobial, against francisellosis in juvenile Nile tilapia (O. niloticus). Fish were challenged with a virulent isolate of F. orientalis and treated with medicated feed containing one of two doses of oxytetracycline (100 or 300 mg/kg of live weight (LW)) or 10 mg/kg of LW of enrofloxacin. The positive and negative control groups received feed without antibiotics; the negative control group was unchallenged. The results showed that enrofloxacin at a dose of 10 mg/kg of LW is effective against francisellosis in juvenile Nile tilapia (O. niloticus). Treatment with oxytetracycline did not eliminate the pathogen from the infected host, and the surviving fish became carriers. Enrofloxacin was able to cure the fish of infection with F. orientalis. This study suggests that enrofloxacin is a better option for treating francisellosis in Nile tilapia (O. niloticus L.). It controls mortality and avoids the carrier state in the fish, thus reducing the possibility of recurrence in the affected batches.

Development of an inactivated whole cell vaccine through immersion for immunoprophylaxis of Francisella orientalis infections in Nile tilapia (Oreochromis niloticus L.) fingerlings and juveniles

Francisella orientalis infections, known as francisellosis, are one of the most important diseases affecting the production of Nile tilapia, causing high mortality rates in the most susceptible fish stages: fingerlings and juveniles. Antibiotic therapy is the method of choice for treating the disease, as there are no commercially available vaccines. In this study, we developed an inactivated whole-cell vaccine using an isolate of F. orientalis in combination with the aqueous adjuvant Montanide IMS 1312 VG, which was administered to Nile tilapia through immersion. Two immunization trials (1 and 2) were conducted with fish at the fingerling and juvenile stages. For each trial, five different experimental groups were established: a complete vaccine (bacterin in combination with aqueous adjuvant), bacterin, aqueous adjuvant, and positive and negative controls. Thirty days after vaccination, an experimental challenge was performed through intraperitoneal injection of the same F. orientalis isolate. As a result, the vaccinated fingerlings were the only group in which mortality and progression of clinical signs of francisellosis were statistically significantly reduced, although relative percentage of survival (RPS) was low at 50%. In the juvenile group, RPS was higher at 63%, but not statistically significant. Nevertheless, an RPS of only 50% is acceptable for using vaccines in the field. The bacterin and adjuvant treatments alone were not effective, showing an RPS of 37% and 0%, respectively. Post-vaccination mortality was observed in the group exposed only to the adjuvant, which may indicate excessive immune stimulation at this stage. Interestingly, the immune response elicited by the vaccine was unable to eliminate the pathogen from the host; therefore, the surviving animals became carriers. Although the immune response elicited by the vaccine was unable to eliminate the pathogen from the host, this vaccine formulation could be a viable alternative for use in the field and serve as another means of controlling the mortality caused by the pathogen. Our study provides the first report of vaccination, using immersion, against francisellosis at the most susceptible stages of farmed Nile tilapia. Future studies should address the efficiency of immersion vaccines under field conditions.

Transcriptome analysis of immune- and iron-related genes after Francisella noatunensis subsp. orientalis infection in Nile tilapia (Oreochromis niloticus)

Francisella noatunensis subsp. orientalis (Fno) is a gram-negative intracellular bacterium identified in many fish species worldwide, including cultured Nile tilapia (Oreochromis niloticus) in Taiwan. To investigate the gene expression responses to Fno infection, we performed transcriptome analysis of the head kidney and spleen in Nile tilapia using RNA-seq. Total RNA was extracted from the head kidney and spleen of infected (Fno-injected) and uninfected (control) tilapia at 1-day and 2-days post-infection, and RNA-seq was performed using the Illumina HiSeq™ 4000 platform. After de novo assembly, a total of 106,534 transcripts were detected. These transcripts were annotated and categorized into a total of 7171 genes based on the KEGG pathway database. Differentially expressed genes (DEGs) were significantly (2-fold difference comparing Fno and PBS groups at each time point) enriched in the immune-related pathways, including the following: complement and coagulation cascades, cytokine-cytokine receptor interaction, hematopoietic cell lineage, lysosome, phagosome. We identified the upregulation of inflammatory cytokine-, apoptosis-, and neutrophil-related genes, and downregulation of complement- and lymphocyte-related genes. Additionally, we found the induction of natural resistance-associated macrophage protein 1 (NRAMP1) and heme responsive gene-1 (HRG1). Anemia of inflammation, caused by intracellular iron storage in spleen after Fno infection, was also observed. This study provides natural disease control strategies against Fno infection in tilapia. It is suggested that intercellular iron storage is a host protection strategy.

Francisellosis of Yesso scallops Mizuhopecten yessoensis in Japan is caused by a novel type of Francisella halioticida

Francisella halioticida, the causative agent of francisellosis of the giant abalone Haliotis gigantea, has also been isolated from Yesso scallops Mizuhopecten yessoensis, which presented with orange/pinkish lesions in the adductor muscle and experienced high mortality. However, it is not clear whether the F. halioticida isolated from the giant abalone and Yesso scallops are phenotypically and genetically identical to each other. The present study revealed that isolates from the giant abalone and Yesso scallops were phenotypically different, with slower growth in modified eugon broth and a lack of prolyl aminopeptidase and phenylalanine aminopeptidase in Yesso scallop isolates. Additionally, we found that 3 of 8 housekeeping genes were different between them. Based on these phenotypic and genetic differences, we propose that F. halioticida isolated from Yesso scallops in Japan be designated as the 'J-scallop type' to distinguish it from strains from abalone ('abalone type'). Whole-genome sequencing analysis of a strain belonging to the J-scallop type showed that the overall similarity between the J-scallop and abalone type strains was estimated to be 99.84%. In accordance with a lack of prolyl aminopeptidase activity, in general, all of the J-scallop type strains examined have a 1 bp deletion in the responsible gene encoding prolyl aminopeptidase. This deletion was confirmed in all F. halioticida in diseased Yesso scallops examined, suggesting that in Japan, francisellosis of Yesso scallops is caused by a novel type of F. halioticida and not by the abalone type.

Hemophagocytic Lymphohistiocytosis in Children with Chronic Granulomatous Disease-Single-Center Experience from North India

BACKGROUND

Chronic granulomatous disease (CGD) is an inherited defect in components of the nicotinamide adenine dinucleotide phosphate oxidase complex that results in potential life-threatening infective and noninfective complications. Hemophagocytic lymphohistiocytosis (HLH) is an unusual but important inflammatory complication of CGD. Optimal management strategies have not yet been identified in children with CGD who develop HLH.

OBJECTIVE

To analyze clinical and laboratory features of HLH in CGD from a tertiary-care center in North India.

METHODS

A retrospective review of medical records of children with CGD diagnosed in the last 20 years was performed. Clinical and laboratory features of children with CGD who developed HLH were analyzed.

RESULTS

Of 80 patients diagnosed with CGD, 5 (6.25%) had evidence of HLH. All 5 were males; 4 had X-linked CGD and 1 had autosomal recessive CGD (NCF2 defect). Two children with CGD had HLH as the predominant presenting manifestation mimicking the clinical presentation of congenital HLH. Infectious triggers identified were bloodstream infections (n = 3) (Candida albicans, Burkholderia cenocepacia, Francisella noatuensis), pneumonia (n = 4), and splenic abscess (n = 1). We document the first human infection with a fish pathogen, F. noatuensis, in a child with X-linked CGD. Although mortality was seen in 3 children who received only intravenous (IV) immunoglobulin therapy, the other 2 who received IV methylprednisolone pulse therapy survived.

CONCLUSION

HLH can be a presenting manifestation of CGD, and workup for CGD must be considered in children with HLH. Early recognition with optimal management of both infectious trigger and HLH is very important to prevent mortality.

Synergistic infection of Ichthyophthirius multifiliis and Francisella noatunensis subsp. orientalis in hybrid red tilapia (Oreochromis sp.)

Francisella noatunensis subsp. orientalis (Fno) and Ichthyophthirius multifiliis (Ich) are deadly infectious pathogens in farmed tilapia, particularly during cold season when the water temperature drops to under 25 °C. We hypothesized that infection of the ectoparasite Ich might enhance susceptibility of hybrid red tilapia (Oreochromis sp.) to the facultative intracellular bacterium Fno. To prove the hypothesis, the experiment was designed as follows. Hybrid red tilapia naturally infected by Ich at 9 ± 6 theronts/fish gills and 4 ± 3 theronts/fish skin were distributed into 5 distinct groups exposed to different concentrations of Fno. In parallel, the same number of Ich-free tilapia were challenged to only Fno in the same manner. The results showed that cumulative mortality in the Fno single infection with 2.88 × 10⁶ CFU mL^-1 of water was 25 ± 7%, whereas 100% mortality was found in the coinfection treatment at dose of 1.93 × 10⁵ CFU mL^-1 of water. No mortality was observed in both control groups (Ich-infected and Ich-free fish). The coinfected fish revealed typical clinical signs and histopathological manifestations of francisellosis and ichthyophthiriasis. This study revealed synergistic effect of the Ich and Fno infection in hybrid red tilapia leading to the exacerbated mortality. Thus, farming management of fish to be free from the Ich ectoparasite might reduce risk of francisellosis and probably other bacterial diseases in farmed tilapia.

Reclassification of Francisella noatunensis subsp. orientalis Ottem et al. 2009 as Francisella orientalis sp. nov., Francisella noatunensis subsp. chilensis subsp. nov. and emended description of Francisella noatunensis

Francisella noatunensis is a fastidious facultative intracellular bacterial pathogen that causes 'piscine francisellosis', a serious disease affecting both marine and fresh water farmed and wild fish worldwide. Currently two F. noatunensis subspecies are recognized, i.e. F. noatunensis subsp. noatunensis and F. noatunensis subsp. orientalis. In the present study, the taxonomy of F. noatunensis was revisited using a polyphasic approach, including whole genome derived parameters such as digital DNA-DNA hybridization, whole genome average nucleotide identity (wg-ANIm), whole genome phylogenetic analysis, whole genome G+C content, metabolic fingerprinting and chemotaxonomic analyses. The results indicated that isolates belonging to F. noatunensis subsp. orientalis represent a phenotypically and genetically homogenous taxon, clearly distinguishable from F. noatunensis subsp. noatunensis that fulfils requirements for separate species status. We propose, therefore, elevation of F. noatunensis subsp. orientalis to the species rank as Francisella orientalis sp. nov. with the type strain remaining as Ehime-1^T (DSM 21254^T=LMG 24544^T). Furthermore, we identified sufficient phenotypic and genetic differences between F. noatunensis subsp. noatunensis recovered from diseased farmed Atlantic salmon in Chile and those isolated from wild and farmed Atlantic cod in Northern Europe to warrant proposal of the Chilean as a novel F. noatunensis subspecies, i.e. Francisella noatunensis subsp. chilensis subsp. nov. with strain PQ1106^T (CECT 9798^T=NCTC14375^T) as the type strain. Finally, we emend the description of F. noatunensis by including further metabolic information and the description of atypical strains.

Immune Responses and Protective Efficacy of a Formalin-Killed Francisella Noatunensis Subsp. Orientalis Vaccine Evaluated through Intraperitoneal and Immersion Challenge Methods in Oreochromis Niloticus

Francisella noatunensis subsp. orientalis (Fno), an intracellular bacterium, causes systemic granulomatous diseases, resulting in high mortality and huge economic losses in Taiwanese tilapia farming. In this study, we tested the efficacy of a formalin-killed Fno vaccine in cultured tilapia. Fno was isolated from diseased tilapia, inactivated with formalin, and mixed with the mineral oil base adjuvant (Montanide^TM ISA 763 AVG). A total of 300 tilapia were divided into two groups. The experimental group was intraperitoneally injected with 0.1 mL of vaccine, which was substituted with phosphate-buffered saline (PBS) in the control group. A booster was administered at 2 weeks post-immunization. Tilapia were challenged at 6 weeks post primary immunization by intraperitoneal (IP) injection and immersion methods. Mortality was recorded at 21 and 60 days. The results revealed that the vaccine induced a greater antibody titer and led to 71% and 76% of relative percent survival (RPS) after the IP and immersion challenge. The transcripts of proinflammatory cytokines and immune-related genes, including interleukin-1β (IL-1β), tumor necrosis factor alpha (TNFα), C-X-C motif chemokine ligand 8 (CXCL8), and interleukin-17C (IL-17C), were significantly upregulated after vaccination. Additionally, vaccinated fish had lower bacterial loads in the blood and lower granuloma intensities in the kidney, spleen, liver, and gill than control fish. The results in this study demonstrate that the inactivated Fno vaccine could be an essential resource in Taiwanese tilapia farming.

Whole cell inactivated autogenous vaccine effectively protects red Nile tilapia (Oreochromis niloticus) against francisellosis via intraperitoneal injection

Francisella noatunensis subsp. orientalis is a pathogen of tilapia and other warm-water fish for which no vaccines are commercially available. In this study, a whole cell formalin-inactivated vaccine was developed for the first time using the highly virulent isolate STIR-GUS-F2f7 and the oil-based adjuvant Montanide™ ISA 763A VG. The efficacy of the vaccine was assessed in red Nile tilapia via intraperitoneal (i.p.) injection using homologous experimental infection and correlates of protection such as seral antibody production and bacterial loads in the spleen. For immunization, fish were i.p. injected with 0.1 ml of the vaccine, the adjuvant alone or PBS. At 840 degree days post-vaccination, all fish were i.p. injected with 4.0 × 10³ CFU/fish of pathogenic bacteria. The RPS at the end of the trial was 100% in the vaccinated group with significantly higher survival than in the adjuvant and control groups. The RPS in the adjuvant group was 42%, and no significant difference was seen in survival between this and the PBS group. Moreover, significantly higher antibody titres in the serum and significantly lower bacterial loads in the spleen were detected in the vaccinated fish by ELISA and qPCR, respectively. These findings highlight the potential of autogenous vaccines for controlling francisellosis in tilapia.

Efficacy of an inactivated whole-cell injection vaccine for nile tilapia, Oreochromis niloticus (L), against multiple isolates of Francisella noatunensis subsp. orientalis from diverse geographical regions

Francisellosis, induced by Francisella noatunensis subsp. orientalis (Fno), is an emerging bacterial disease representing a major threat to the global tilapia industry. There are no commercialised vaccines presently available against francisellosis for use in farmed tilapia, and the only available therapeutic practices used in the field are either the prolonged use of antibiotics or increasing water temperature. Recently, an autogenous whole cell-adjuvanted injectable vaccine was developed that gave 100% relative percent survival (RPS) in tilapia challenged with a homologous isolate of Fno. In this study, we evaluated the efficacy of this vaccine against challenge with heterologous Fno isolates. Healthy Nile tilapia, Oreochromis niloticus (∼15 g) were injected intraperitoneally (i.p.) with the vaccine, adjuvant-alone or phosphate buffer saline (PBS) followed by an i.p. challenge with three Fno isolates from geographically distinct locations. The vaccine provided significant protection in all groups of vaccinated tilapia, with a significantly higher RPS of 82.3% obtained against homologous challenge, compared to 69.8% and 65.9% with the heterologous challenges. Protection correlated with significantly higher specific antibody responses, and western blot analysis demonstrated cross-isolate antigenicity with fish sera post-vaccination and post-challenge. Moreover, a significantly lower bacterial burden was detected by qPCR in conjunction with significantly greater expression of IgM, IL-1 β, TNF-α and MHCII, 72 h post-vaccination (hpv) in spleen samples from vaccinated tilapia compared to fish injected with adjuvant-alone and PBS. The Fno vaccine described in this study may provide a starting point for development a broad-spectrum highly protective vaccine against francisellosis in tilapia.

Effects of temperature changes in the transcriptional profile of the emerging fish pathogen Francisella noatunensis subsp. orientalis

One of the major challenges in Nile tilapia (Oreochromis niloticus L.) farming is the occurrence of bacterial infections, and the Francisella noatunensis subsp. orientalis (FNO) is an important pathogen that has emerged in last decades. Francisellosis outbreaks have been reported in the literature as occurring seasonally when water temperature is below 24 °C. The aim of this study was to quantify the median lethal doses (LD₅₀) of FNO in experimental challenges at 28 °C and 22 °C, and to investigate the impact of temperature changes in whole genome expression using microarray technology. The LD₅₀ for Nile tilapia at 28 °C was ∼10^5.7, whereas at 22 °C, the LD₅₀ was ∼10^2.2, showing that the decrease in temperature enhanced disease outcome. Out of 1917 genes screened, a total of 31 and 19 genes were down- and up-regulated at 22 °C, respectively. These genes were grouped by orthology into functional categories of: amino acid, inorganic ion, and carbohydrate transport and metabolism; transcription; and posttranslational modification, protein turnover, and chaperones. Expression of genes related to metabolism, oxidative stress, and thermal shock were regulated by temperature changes, reflecting an ability of FNO to adapt to the environment. Expression of virulence genes usually required for the Francisella genus was not changed between tested temperatures, including that of genes located on the Francisella Pathogenicity Island.

Molecular characterization and pathogenicity of Francisella noatunensis subsp. orientalis isolated from cultured tilapia (Oreochromis sp.) in Taiwan

Francisella noatunensis subsp. orientalis is a causative agent of systemic granulomatous disease in tilapia. The present study was designed to understand the genetic and phenotypic diversities among Taiwanese Fno isolates obtained from tilapia (n = 17) and green Texas cichlid (Herichthys cyanoguttatus) (n = 1). The enzymatic profiles of the isolates were studied using the API ZYM system. Phylogenetic tree analysis of the 16S rRNA and housekeeping gene and pulsed-field gel electrophoresis (PFGE) were carried out to determine the genotypic characters of all isolates. The phylogenetic tree showed similarity of 99%-100% nucleotide sequences of 16S rRNA and housekeeping genes compared to the Fno references genes from GenBank database. Comparatively, the results revealed an identical profile of enzymatic and PFGE pattern which was distincted from that of F. philomiragia. To understand the pathogenicity, the isolates were intraperitoneal injected to tilapia the gross lesions were observed concomitant with natural outbreak. Median lethal dose upon Nile tilapia and red tilapia were 9.06 × 10³ CFU/fish and 2.08 × 10² CFU/fish, respectively. Thus, our data provide understanding the epidemiology of Taiwanese Fno isolates, and help in development of future control and prevention.

Development of a recombinase polymerase amplification assay for rapid detection of Francisella noatunensis subsp. orientalis

Francisella noatunensis subsp. orientalis (Fno) is the causative agent of piscine francisellosis in warm water fish including tilapia. The disease induces chronic granulomatous inflammation with high morbidity and can result in high mortality. Early and accurate detection of Fno is crucial to set appropriate outbreak control measures in tilapia farms. Laboratory detection of Fno mainly depends on bacterial culture and molecular techniques. Recombinase polymerase amplification (RPA) is a novel isothermal technology that has been widely used for the molecular diagnosis of various infectious diseases. In this study, a recombinase polymerase amplification (RPA) assay for rapid detection of Fno was developed and validated. The RPA reaction was performed at a constant temperature of 42°C for 20 min. The RPA assay was performed using a quantitative plasmid standard containing a unique Fno gene sequence. Validation of the assay was performed not only by using DNA from Fno, closely related Francisella species and other common bacterial pathogens in tilapia farms, but also by screening 78 Nile tilapia and 5 water samples. All results were compared with those obtained by previously established real-time qPCR. The developed RPA showed high specificity in detection of Fno with no cross-detection of either the closely related Francisella spp. or the other tested bacteria. The Fno-RPA performance was highly comparable to the published qPCR with detection limits at 15 and 11 DNA molecules detected, respectively. The RPA gave quicker results in approximately 6 min in contrast to the qPCR that needed about 90 min to reach the same detection limit, taking only 2.7–3 min to determine Fno in clinical samples. Moreover, RPA was more tolerant to reaction inhibitors than qPCR when tested with field samples. The fast reaction, simplicity, cost-effectiveness, sensitivity and specificity make the RPA an attractive diagnostic tool that will contribute to controlling the infection through prompt on-site detection of Fno.