Evaluation of the immunoprotection efficacy of Riemerella anatipestifer fur-deficient mutant as an attenuated vaccine

Functional characterization of RhuB as a second TonB2-dependent hemin receptor in Riemerella anatipestifer CH-1

In the previous study, it was shown that Riemerella anatipestifer (R. anatipestifer, RA), a pathogen in ducks and some other birds, encodes a hemin uptake system. The R. anatipestifer hemin uptake receptor RhuR is a TonB2-dependent hemin transporter. However, it remains unclear whether R. anatipestifer encodes additional TonB-dependent hemin transporters. Herein, we demonstrated that R. anatipestifer hemin uptake receptor B (RhuB) of R. anatipestifer CH-1 (RA CH-1) was negatively regulated by iron and mediated by the Fur protein, and knocking out rhuB damaged the ability of RA CH-1 to utilize iron from duck hemoglobin (Hb) but not that from duck serum. Moreover, the ability to use iron from Hb was restored by the expression rhuB in trans. Furthermore, the RhuB of RA CH-1 is a membrane protein, and recombinant RhuB could bind hemin at a 1:1 molar ratio in vitro. Compared to that of ΔtonB1ΔrhuR, the ability of ΔtonB1ΔrhuRΔrhuB to utilize hemin was impaired; meanwhile, compared to that of ΔtonB2ΔrhuR, the hemin utilization ability of ΔtonB2ΔrhuRΔrhuB was not affected, indicating that RhuB is a TonB2-dependent receptor. Compared to ΔrhuB, ΔrhuBΔrhuA did not affect hemin utilization. However, compared to ΔrhuA, ΔrhuBΔrhuA had reduced ability to utilize hemin, suggesting that RhuA relies on RhuB for its activity. Finally, the deletion of rhuB did not affect the virulence of RA CH-1. These results suggested that RhuB encodes a TonB2-dependent hemin receptor. The characterization of the second TonB-dependent receptor in R. anatipestifer enriches our understanding of the hemin uptake system of this bacterium.IMPORTANCEIron is essential for the survival of most bacteria, and hemin of hemoglobin can serve as an important iron source. In our previous studies, we showed that R. anatipestifer CH-1 encodes a TonB2-dependent hemin receptor RhuR, which is involved in hemin uptake. The deletion of rhuR did not abolish hemin utilization by RA CH-1. We hypothesized that additional hemin uptake systems exist in this bacterium. In this study, we identified the second TonB2-dependent hemin receptor RhuB in RA CH-1 through hemin utilization, protein localization, and hemin-binding experiments. The duck infection model showed that the deletion of rhuB did not affect the virulence of RA CH-1. This study is not only important for further understanding the hemin utilization mechanism of R. anatipestifer, but also for enriching the hemin uptake transporters of gram-negative bacteria.

A novel bivalent inactivated vaccine for ducks against Riemerella anatipestifer based on serotype distribution in southern China

Riemerella anatipestifer (RA) causes epizootic infectious polyserositis in ducks with high mortality and leads to huge economic losses worldwide. Bacterial resistance poses a challenge for the control of the disease, vaccines failed to provide ideal cross-protection. Thus, the preparation of vaccines based on popular serotypes is important. In this study, we collected 700 brain and liver tissues of dead ducks from 8 provinces in southern China from 2016 to 2022 and obtained 195 RA isolates with serotypes 1, 2, 7, and 10. Serotypes 1 and 2 were the most prevalent (82%). A novel bivalent inactivated vaccine WZX-XT5 containing propolis adjuvant was prepared, we chose XT5 (serotype 1) and WZX (serotype 2) as vaccine strains and evaluated WZX-XT5-induced immune response and protective efficacy in ducks. Results showed that the XT5 (LD50, 3.5 × 103 CFU) exhibited high virulence and provided better protection against RA compared with ZXP, DCR and LCF1 (LD50, 108 CFU). Notably, the dose of 109 CFU provided ideal protection compared with 108 CFU, propolis and oil emulsion adjuvants induced stronger protective efficacy compared with aluminum hydroxide adjuvant. Importantly, WZX-XT5 immunization induced high levels of RA-specific IgY, IFN-γ, IL-2, and IL-4 in serum and offered over 90% protection against RA with ultra-high lethal dose in ducks. Additionally, no clinical signs of RA infection or obvious pathological damage in tissues were observed in protected ducks. Overall, this study first reports the identification, serotyping and virulence of RA in ducks of southern China and the preparation of a novel bivalent inactivated vaccine, providing useful scientific information to prevent and control RA infection.

Functional characterization of two TolC in the resistance to drugs and metals and in the virulence of Riemerella anatipestifer

IMPORTANCE

Riemerella anatipestifer (RA) is a notorious duck pathogen, characterized by a multitude of serotypes that exhibit no cross-reaction with one another. Moreover, RA is resistant to various antibacterial agents. Consequently, understanding the mechanisms behind resistance and identifying potential targets for drug development have become pressing needs. In this study, we show that the two TolC proteins play a role in the resistance to different drugs and metals and in the virulence. The results suggest that TolCA has a wider range of efflux substrates than TolCB. Except for gentamicin, neither TolCA nor TolCB was involved in the efflux of the other tested antibiotics. Strikingly, TolCA but not TolCB enhanced the frequency of resistance-conferring mutations. Moreover, TolCA was involved in RA virulence. Given its conservation in RA, TolCA has potential as a drug target for the development of therapeutics against RA infections.

Effects of dietary nano-selenium supplementation on Riemerella anatipestifer vaccinated and challenged Pekin ducklings (Anas platyrhynchos)

Riemerella anatipestifer (RA) is a common disease causing economic losses to duck farms worldwide. Novel supplements are crucially needed to control this bacterium, enhance poultry performance, and produce synergistic effects with vaccines in stimulating the immune system. This study investigated the effect of nano-selenium (Nano-Se) on the vaccinated (VAC) and challenged (Ch) Pekin ducklings (Anas platyrhynchos) with RA. Five experimental groups (G1-G5) were included in this study: G1 was the control group, G2 was the RA-challenged group, G3 was the Nano-Se+Ch group, G4 was the VAC+Ch group, and G5 was the Nano-Se+VAC+Ch group. The Nano-Se (0.3 mg/kg diet) was supplemented for 5 weeks post-vaccination (PV). The ducklings were vaccinated subcutaneously with the RA vaccine at 7 days of age and challenged with RA at the 3rd week PV. Blood, pharyngeal swabs and tissue samples were collected at the 3rd week PV and at different times post-challenge (PC). The growth performance (weight gain and feed conversion ratio), clinical signs, gross lesions, mortality, bacterial shedding, haematological, immunological, and biochemical parameters, cytokines production, and histopathological lesion scores showed significant differences (P < 0.05) between the challenged (G2) group and the supplemented (G3 & G5) groups. G5 showed the highest (P < 0.05) growth performance, phagocytic activity, IgM and IgG, splenic interleukin-2 (IL-2), IL-10, and interferon-gamma (IFN-γ) gene expressions, and the lowest mortality, bacterial shedding, hepatic and renal damage, heterophil/lymphocyte ratio and lesion scores compared to the other groups. In conclusion, the supplementation of nano-selenium for five weeks in the diet can improve the growth performance, immune status, and cytokines production in ducklings vaccinated and challenged with RA.

Functional Characterization of FeoAB in Iron Acquisition and Pathogenicity in Riemerella anatipestifer

The bacterium Riemerella anatipestifer requires iron for growth, but the mechanism of iron uptake is not fully understood. In this study, we disrupted the Feo system and characterized its function in iron import in R. anatipestifer ATCC 11845. Compared to the parent strain, the growth of the ΔfeoA, ΔfeoB, and ΔfeoAB strains was affected under Fe3+-limited conditions, since the absence of the feo system led to less intracellular iron than in the parent strain. In parallel, the ΔfeoAB strain was shown to be less sensitive to streptonigrin, an antibiotic that requires free iron to function. The sensitivity of the ΔfeoAB strain to hydrogen peroxide was also observed to be diminished compared with that of the parent strain, which could be related to the reduced intracellular iron content in the ΔfeoAB strain. Further research revealed that feoA and feoB were directly regulated by iron through the Fur regulator and that the transcript levels of feoA and feoB were significantly increased in medium supplemented with 1 mM MnCl2, 400 μM ZnSO4, and 200 μM CuCl2. Finally, it was shown that the ΔfeoAB strain of R. anatipestifer ATCC 11845 was significantly impaired in its ability to colonize the blood, liver, and brain of ducklings. Taken together, these results demonstrated that FeoAB supports ferrous iron acquisition in R. anatipestifer and plays an important role in R. anatipestifer colonization. IMPORTANCE In Gram-negative bacteria, the Feo system is an important ferrous iron transport system. R. anatipestifer encodes an Feo system, but its function unknown. As iron uptake may be required for oxidative stress protection and virulence, understanding the contribution of iron transporters to these processes is crucial. This study showed that the ΔfeoAB strain is debilitated in its ability to import iron and that its intracellular iron content was constitutively low, which enhanced the resistance of the deficient strain to H2O2. We were surprised to find that, in addition to responding to iron, the Feo system may play an important role in sensing manganese, zinc, and copper stress. The reduced colonization ability of the ΔfeoAB strain also sheds light on the role of iron transporters in host-pathogen interactions. This study is important for understanding the cross talk between iron and other metal transport pathways, as well as the pathogenic mechanism in R. anatipestifer.

Manganese Efflux Achieved by MetA and MetB Affects Oxidative Stress Resistance and Iron Homeostasis in Riemerella anatipestifer

In bacteria, manganese homeostasis is controlled by import, regulation, and efflux. Here, we identified 2 Mn exporters, MetA and MetB (manganese efflux transporters A and B), in Riemerella anatipestifer CH-1, encoding a putative cation diffusion facilitator (CDF) protein and putative resistance-nodulation-division (RND) efflux pump, respectively. Compared with the wild type (WT), ΔmetA, ΔmetB, and ΔmetAΔmetB exhibited sensitivity to manganese, since they accumulated more intracellular Mn2+ than the WT under excess manganese conditions, while the amount of iron in the mutants was decreased. Moreover, ΔmetA, ΔmetB, and ΔmetAΔmetB were more sensitive to the oxidant NaOCl than the WT. Further study showed that supplementation with iron sources could alleviate manganese toxicity and that excess manganese inhibited bacterial cell division. RNA-Seq showed that manganese stress resulted in the perturbation of iron metabolism genes, further demonstrating that manganese efflux is critical for iron homeostasis. metA transcription was upregulated under excess manganese but was not activated by MetR, a DtxR family protein, although MetR was also involved in manganese detoxification, while metB transcription was downregulated under iron depletion conditions and in fur mutants. Finally, homologues of MetA and MetB were found to be mainly distributed in members of Flavobacteriaceae. Specifically, MetB represents a novel manganese exporter in Gram-negative bacteria. IMPORTANCE Manganese is required for the function of many proteins in bacteria, but in excess, manganese can mediate toxicity. Therefore, the intracellular levels of manganese must be tightly controlled. Manganese efflux transporters have been characterized in some other bacteria; however, their homologues could not be found in the genome of Riemerella anatipestifer through sequence comparison. This indicated that other types of manganese efflux transporters likely exist. In this study, we characterized 2 transporters, MetA and MetB, that mediate manganese efflux in R. anatipestifer in response to manganese overload. MetA encodes a putative cation diffusion facilitator (CDF) protein, which has been characterized as a manganese transporter in other bacteria, while this is the first observation of a putative resistance-nodulation-division (RND) transporter contributing to manganese export in Gram-negative bacteria. In addition, the mechanism of manganese toxicity was studied by observing morphological changes and by transcriptome sequencing. Taken together, these results are important for expanding our understanding of manganese transporters and revealing the mechanism of manganese toxicity.