The adherent abilities of Clostridium perfringens strains are critical for the pathogenesis of avian necrotic enteritis

Alternative to antimicrobial growth promoters in the diets of broilers challenged with subclinical necrotic enteritis

Necrotic enteritis (NE) is a disease of worldwide distribution, which affects young broilers and causes economic losses on a scale of 6 billion dollars per year. For decades, NE was controlled in poultry flocks by dietary administration of low doses of antimicrobial growth promoters (AGPs). However, an increase in NE incidence was noted after the AGP ban. This study aimed to compare the effect of an antibiotic (Enramycin) diet to a combination of sodium butyrate, hydrolyzed yeast, and zinc proteinate (ViligenTM) on broiler diets regarding performance, blood parameters, intestinal permeability, morphology and lesions, and carcass yield of broilers challenged with Eimeria spp. and Clostridium perfringens to simulate subclinical necrotic enteritis. A total of 1,150 one-day-old male broiler chickens with an initial average weight of 43.9 ± 0.65 g were allocated to 50 experimental pens. Animals were divided into 5 groups: Negative control (NC) without additives; Positive control (PC) with 0.12 g/ton of Enramycin (8%); V500, V1000, and V1500 with the addition of 500, 1.000, and 1.500 g/ton of Viligen, respectively. All animals were challenged by Eimeria spp. at 7 d of age and by C. perfringens at 17, 18, and 19 d for induction of subclinical NE. The broilers fed with all concentrations of Viligen showed similar performance, blood parameters, intestinal permeability, and carcass yield compared to PC broilers. However, NC broilers showed higher FCR compared to PC broilers from 1 to 33 d (1.42 vs. 1.39) (P = 0.048) and from 1 to 42 d (1.51 vs. 1.49) (P < 0.001). V1500 broilers had fewer intestinal lesions at 28 d when compared to the PC treatment (P < 0.05) and showed that higher Viligen inclusion resulted in lower intestinal damage. At 21 d, the V500 group showed higher intestinal morphology characteristics (VH:VD 4.9 vs. 3.5) compared to the PC treatment (P < 0.001). Thus, in this study, the dietary addition of Viligen to broilers challenged by an experimental model of subclinical NE resulted in lower intestinal damage and similar performance to that obtained by the addition of Enramycin.

Research Note: Clostridium perfringens NetB and CnaA neutralizing nanobodies in feed reduce the incidence of poultry necrotic enteritis

Necrotic enteritis is a devastating disease to poultry caused by the bacterium Clostridium perfringens. As a novel approach to combating poultry necrotic enteritis, we identified and characterized several hundred single domain antibody fragments (or nanobodies) capable of binding either the NetB toxin or the collagen-binding adhesin (CnaA) of C. perfringens. Many of the nanobodies could neutralize the in vitro functions of NetB or CnaA with inhibitory concentrations in the nanomolar range. The nanobodies were also screened for proteolytic stability in an extract derived from gastrointestinal tract fluids of chickens. A collection of 6 nanobodies (4 targeting NetB and 2 targeting CnaA) with high neutralizing activity and high gastrointestinal tract extract stability were expressed and secreted by Pichia pastoris or Bacillus subtilis. Chickens were given a feed with 1 of the 2 nanobody-containing groups: 1) nanobody-containing P. pastoris supernatants that were semi-purified, lyophilized, and enterically coated, or 2) B. subtilis spores from strains containing the nanobody genes. Compared to untreated chickens (23.75% mortality), mortality of chickens receiving feed modified with the P. pastoris and B. subtilis products decreased to 11.25 and 7.5%, respectively. These results offer a new opportunity to improve the control of poultry necrotic enteritis by incorporating highly specific nanobodies or bacteria expressing these nanobodies directly into chicken feed.

Determination of the virulence status of Clostridium perfringens strains using a chicken intestinal ligated loop model is important for understanding the pathogenesis of necrotic

Necrotic enteritis (NE) is a poultry intestinal disease caused by virulent strains of the bacterium Clostridium perfringens (C. perfringens). This anaerobic bacterium produces a wide range of enzymes and toxins in the gut which leads to NE development. It is generally accepted by the poultry veterinarians that netB-positive C. perfringens strains are virulent and netB-negative strains do not cause NE. However, NE pathogenesis remains unclear as contradictory results have been reported. The use of experimental in vivo models is a valuable tool to understand the pathogenesis of a disease. In this study, a chicken ligated loop model was used to determine the virulence status of 79 C. perfringens strains from various geographical locations, sources, and genotype profiles. According to our model and based on histologic lesion scoring, 9 C. perfringens strains were classified as commensal, 35 as virulent, and 34 as highly virulent. The virulence of only 1 C. perfringens strain could not be classified as its lesion score was variable (from <10 to >15). In general, NE lesions were more severe in intestinal loops inoculated with netB-positive C. perfringens strains than those inoculated with netB-negative strains. The prevalence of netB among strains classified as commensal, virulent, and highly virulent was 56% (5/9), 54%, (19/35), and 59% (20/34). These results suggest that NetB is not required to cause NE lesions and that other factors are also involved. The classification of the virulence status of C. perfringens strains should not be based solely on the presence or absence of this toxin. Therefore, the use of an in vivo model is essential to distinguish commensal from virulent strains of C. perfringens.

Necrotic enteritis and antibiotic-free production of broiler chickens: Challenges in testing and using alternative products

The global trend towards raising broiler chickens without the use of in-feed antibiotics (IFAs) means that there is an ongoing need to develop alternative treatments capable of delivering the benefits that IFAs previously provided. IFAs supported the productivity performance of chickens and played a key role in maintaining their health. Necrotic enteritis (NE) is an important disease of broilers that affects health, productivity, and welfare, and was previously well controlled by IFAs. However, with the reduction in IFA use, NE is resurgent in some countries. Vaccines and various feed additives, including pre-, pro-, and postbiotics, phytobiotics, fatty acids, and phage therapies have been introduced as alternative methods of NE control. While some of these feed additives have specific activity against the NE pathogen, Clostridium perfringens, most have the more general goal of reinforcing gut health. Extensive reviews of the effects of many of these feed additives on gut health have been published recently. Hence, rather than cover previously well reviewed areas of research this review focuses on the challenges and pitfalls in undertaking experimental assessment of alternative NE treatments and translating laboratory research to real world commercial production settings. The review is based on the author's particular experience, reading, thoughts, and analysis of the available information and inevitably presents a particular understanding that is likely to be at odds with others thinking on these issues. It is put forward to stimulate thinking and discussion on the issues covered.

Digital PCR: a tool in clostridial mutant selection and detection

The ClosTron mutagenesis system has enabled researchers to efficiently edit the clostridial genome. Since site-specific insertion of the mobile ClosTron insert may cause errors, validation is key. In this paper we describe the use of digital PCR (dPCR) as an alternative tool in selecting clostridial mutant strains. Clostridium perfringens chitinase mutant strains were constructed in which the mobile ClosTron intron was inserted into one of the chitinase genes. On-target insertion of the mobile intron was validated through conventional PCR. In order to confirm the absence of off-target insertions, dPCR was used to determine the amount of the ClosTron intron as well as the amount of a reference gene, located in close proximity to the interrupted gene. Subsequently, mutant strains containing an equivalent amount of both genes were selected as these do not contain additional off-target mobile ClosTron inserts. The outcome of this selection procedure was confirmed through a validated PCR-based approach. In addition to its application in mutant selection, dPCR can be used in other aspects of clostridial research, such as the distinction and easy quantification of different types of strains (wildtype vs. mutant) in complex matrices, such as faecal samples, a process in which other techniques are hampered by bacterial overgrowth (plating) or inhibition by matrix contaminants (qPCR). This research demonstrates that dPCR is indeed a high-throughput method in the selection of clostridial insertion mutants as well as a robust and accurate tool in distinguishing between wildtype and mutant C. perfringens strains, even in a complex matrix such as faeces. KEY POINTS: • Digital PCR as an alternative in ClosTron mutant selection • Digital PCR is an accurate tool in bacterial quantification in a complex matrix • Digital PCR is an alternative tool with great potential to microbiological research.

Hyperimmune egg yolk antibodies developed against Clostridium perfringens antigens protect against necrotic enteritis

Necrotic enteritis (NE) is a widespread infectious disease caused by Clostridium perfringens that inflicts major economic losses on the global poultry industry. Due to regulations on antibiotic use in poultry production, there is an urgent need for alternative strategies to mitigate the negative effects of NE. This paper presents a passive immunization technology that utilizes hyperimmune egg yolk immunoglobulin Y (IgY) specific to the major immunodominant antigens of C. perfringens. Egg yolk IgYs were generated by immunizing hens with 4 different recombinant C. perfringens antigens, and their protective effects against NE were evaluated in commercial broilers. Six different spray-dried egg powders were produced using recombinant C. perfringens antigens: α-toxin, NE B-like toxin (NetB; EB), elongation factor-Tu (ET), pyruvate:ferredoxin oxidoreductase, a mixture of 4 antigens (EM-1), and a nonimmunized control (EC). The challenged groups were either provided with different egg powders at a 1% level or no egg powders (EN). The NE challenge model based on Eimeria maxima and C. perfringens dual infection was used. In Experiments 1 and 2, the EB and ET groups exhibited increased body weight gain (BWG; P < 0.01), decreased NE lesion scores (P < 0.001), and reduced serum NetB levels (P < 0.01) compared to the EN and EC groups. IgY against NetB significantly reduced Leghorn male hepatocellular cytotoxicity in an in vitro test (P < 0.01). In Experiment 3, the protective effect of the IgYs mixture (EM-2) against C. perfringens antigens (NetB and EFTu) and Eimeria antigens (elongation factor-1-alpha: EF1α and Eimeria profilin: 3-1E) was tested. The EM-2 group showed similar body weight, BWG, and feed intake from d 7 to 22 compared to the NC group (P < 0.05). On d 20, the EM-2 group showed comparable intestinal permeability, NE lesion scores, and jejunal NetB and collagen adhesion protein levels to the NC group (P < 0.05). In conclusion, dietary mixture containing antibodies to NetB and EFTu provides protection against experimental NE in chickens through passive immunization.

Collagen adhesin protein and necrotic enteritis B-like toxin as biomarkers for early diagnosis of necrotic enteritis in commercial broiler chickens

Mouse monoclonal antibodies (mAbs) reactive with Clostridium perfringens collagen adhesin protein (CNA) and necrotic enteritis B-like toxin (NetB) were developed. The best capture/detection mAb pairs for CNA and NetB were selected based on their affinity and specificity to develop sandwich enzyme-linked immunosorbent assays (ELISAs) to detect CNA and NetB proteins, respectively, in jejunal digesta samples from commercial broiler farms in the United States. Prior to the analysis of samples from commercial broiler flocks, the specificity and sensitivity of the CNA and NetB ELISAs were validated using sera, jejunal digesta, and fecal samples from chickens coinfected with Eimeria maxima and CNA+/NetB+C. perfringens in an animal model of necrotic enteritis (NE). Subsequently, a total of 251 field samples were collected from 74 commercial poultry farms. Among these, 18 samples were from 6 broiler farms that used certified organics (CO), and 155 samples were from 42 farms with nonantibiotics (NA). In jejunal digesta samples, CNA levels ranged from 0.02 to 0.59 ng/mL and NetB levels ranged from 0.09 to 1.91 ng/mL. CNA and NetB levels showed a positive correlation with each other (Pearson correlation coefficient r = 0.772, P < 0.001). CNA and NetB levels in jejunal digesta were significantly decreased in CO farms compared with those from NA farms (P < 0.001). In conclusion, these new C. perfringens antigen-specific sandwich ELISAs offer a sensitive and specific means to detect C. perfringens CNA and NetB proteins as biomarkers of early NE occurrence in field samples from commercial broiler chickens.

Cellular Immune Responses in Lymphoid Tissues of Broiler Chickens Experimentally Infected with Necrotic Enteritis-Producing Clostridium perfringens Strains

Host cellular responses against Clostridium perfringens (CP), the causative agent of necrotic enteritis (NE) in chickens, are poorly understood. In the present study, we first tested the NE-producing ability of seven netB+ CP strains (CP5, CP18, CP26, CP64, CP67, CP68, and NCNE-1), using an experimental infection model of broiler chickens. Evaluation of intestinal gross lesions showed that all the strains, except CP5, were able to produce NE, while CP26 and CP64 strains produced relatively more severe lesions when compared with other groups. Next, cellular responses in the cecal tonsil (CT), bursa of Fabricius, and spleen were evaluated in chickens infected with strains representing variation in the level of virulence, namely, avirulent CP5, virulent CP18, and a relatively more virulent CP26 strain. Immunophenotyping analysis showed that CT or splenic macrophage frequencies were significantly higher in CP18- and CP26-infected chickens compared with uninfected controls, while the frequencies of γδ T-cells and B-cells in the CT of CP26-infected chickens were significantly higher than those in the uninfected, CP5- or CP18-infected groups. The T-cell analysis showed that chickens infected with CP18 and CP26 had a significantly higher number of splenic CD4+ and CD8+ T-cells expressing CD44 and CD28 activation molecules, while CP26-infected chickens also had significantly increased CT frequency of these activated CD4+ and CD8+ T-cells when compared with uninfected or CP5-infected groups. Collectively, our findings suggested that cellular responses, including activation of T-cells, are selectively induced against virulent CP strains and that the NE-producing characteristics of this pathogen may influence the outcome of immunity to NE.

Characterization of Collagen Binding Activity of Clostridium perfringens Strains Isolated from Broiler Chickens

Clostridium perfringens is the etiological agent for necrotic enteritis (NE) in broiler chickens, which causes a substantial economic loss of an estimated USD 6 billion annually in the global poultry industry. Collagen adhesion is involved in the NE pathogenesis in poultry. In this study, the binding capabilities of chicken C. perfringens isolates of various genetic backgrounds (netB^-tpeL^-, netB⁺tpeL^-, netB⁺tpeL⁺) to collagen types I-V and gelatin were examined, and the putative adhesin protein cnaA gene was investigated at the genomic level. In total, 28 C. perfringens strains from healthy and NE-inflicted sick chickens were examined. The results on collagen adhesin-encoding gene cnaA by the quantitative-PCR results indicated that netB^-tpeL^- isolates had much lower copies of the detectable cnaA gene than netB⁺ isolates (10 netB⁺tpeL^- isolates, 5 netB⁺tpeL⁺ isolates). Most of the virulent C. perfringens isolates demonstrated collagen-binding abilities to types I-II and IV-V, while some strains showed weak or no binding to collagen type III and gelatin. However, the netB⁺tpeL⁺ isolates showed significantly higher binding capabilities to collagen III than netB^-tpeL^- and netB⁺tpeL^- isolates. The data in this study suggest that the collagen-binding capability of clinical C. perfringens isolates correlates well with their NE pathogenicity levels, especially for C. perfringens isolates carrying genes encoding crucial virulence factors and virulence-associated factors such as netB, cnaA, and tpeL. These results indicate that the presence of the cnaA gene may be correlated with C. perfringens virulence (particularly for netB⁺ isolates).

Osteoimmunology: A Link between Gastrointestinal Diseases and Skeletal Health in Chickens

Bone serves as a multifunctional organ in avian species, giving structural integrity to the body, aiding locomotion and flight, regulating mineral homeostasis, and supplementing calcium for eggshell formation. Furthermore, immune cells originate and reside in the bone marrow, sharing a milieu with bone cells, indicating a potential interaction in functions. In avian species, the prevalence of gastrointestinal diseases can alter the growth and the immune response, which costs a great fortune to the poultry industry. Previous studies have shown that coccidiosis and necrotic enteritis can dramatically reduce bone quality as well. However, possible mechanisms on how bone quality is influenced by these disease conditions have not yet been completely understood, other than the reduced feed intake. On the other hand, several mediators of the immune response, such as chemokines and cytokines, play a vital role in the differentiation and activation of osteoclasts responsible for bone resorption and osteoblasts for bone formation. In the case of Eimeria spp./Clostridium perfringens coinfection, these mediators are upregulated. One possible mechanism for accelerated bone loss after gastrointestinal illnesses might be immune-mediated osteoclastogenesis via cytokines-RANKL-mediated pathways. This review article thus focuses on osteoimmunological pathways and the interaction between host immune responses and bone biology in gastrointestinal diseases like coccidiosis and necrotic enteritis affecting skeletal health.

Mucosal and systemic lymphoid immune responses against Clostridium perfringens strains with variable virulence in the production of necrotic enteritis in broiler chickens

Necrotic enteritis (NE), caused by Clostridium perfringens, is an economically important disease of chickens. Although NE pathogenesis is moderately well studied, the host immune responses against C. perfringens are poorly understood. The present study used an experimental NE model to characterize lymphoid immune responses in the caecal tonsils (CT), bursa of Fabricius, Harderian gland (HG) and spleen tissues of broiler chickens infected with four netB+ C. perfringens strains (CP1, CP5, CP18, and CP26), of which CP18 and CP26 strains also carried the tpeL gene. The gross and histopathological lesions in chickens revealed CP5 to be avirulent, while CP1, CP18, and CP26 strains were virulent with CP26 being "very virulent". Gene expression analysis showed that, while the virulent strains induced a significantly upregulated expression of pro-inflammatory IL-1β gene in CT, the CP26-infected birds had significantly higher CT transcription of IFNγ and IL-6 pro-inflammatory genes compared to CP5-infected or uninfected chickens. Furthermore, CP26 infection also led to significantly increased bursal and HG expression of the anti-inflammatory/regulatory genes, IL-10 or TGFβ, compared to control, CP5 and CP1 groups. Additionally, the splenic pro- and anti-inflammatory transcriptional changes were observed only in the CP26-infected chickens. An antibody-mediated response, as characterized by increased IL-4 and/or IL-13 transcription and elevated IgM levels in birds infected with virulent strains, particularly in the CP26-infected group compared to uninfected controls, was also evident. Collectively, our findings suggest that lymphoid immune responses during NE in chickens are spatially regulated such that the inflammatory responses against C. perfringens depend on the virulence of the strain.

Animal Model Studies, Antibiotic Resistance and Toxin Gene Profile of NE Reproducing Clostridium perfringens Type A and Type G Strains Isolated from Commercial Poultry Farms in China

Poultry necrotic enteritis (NE) is a complex and multifactorial disease caused by Clostridium perfringens types. Earlier, the disease was prevented and/or controlled through the addition of in-feed antibiotics and antimicrobial growth promoters (AGPs). The ban on the use of these agents as feed additives has been a major reason for re-emergence of this disease leading to huge economic losses to the world poultry industry. Understanding the pathogenesis of NE by developing an effective experimental model remains challenging and lacks consistency owing to the involvement of several critical factors involved in causing lesions of disease in the field. In this study, locally characterized C. perfringens types, i.e., ACP (toxinotype A), and GCP (toxinotype G), obtained from NE outbreaks on commercial farms in China (2020–2022), were used to experimentally induce NE in Specific-Pathogen-Free (SPF) chicks. The lesion scores observed on day 20 were 1.9 ± 1.10 (GCP strain) and 1.5 ± 1.08 (ACP strain), and both had significant difference as compared to the control group. The inclusion of fishmeal in addition to oral clostridial dose, i.e., fishmeal (day 7 onward) + Clostridia (7.5 × 108 cfu/mL consecutively for 04 days) induced a lesion score of 2.0 ± 1.15 in respective groups. Use of coccidia (Eimeria necatrix) on day 9 followed by clostridia challenge enhanced the lesion scores to 2.5 ± 1.08 and 2.2 ± 1.23 for type G and type A strains, respectively. When both predisposing factors (coccidia + fish meal) were given together, i.e., fishmeal (day 7 onward) and coccidia (day 9) along with clostridia, the lesion scores were 3.2 ± 1.22 (GCP + coccidia + fish meal) and 3.0 ± 1.15 (ACP + coccidia + fish meal). These results were significantly different from group 1 (ACP) and 2 (GCP), in which only C. perfringens was used to induce NE. The clinical signs as well as histopathological lesions in experimentally induced groups were found similar as reported in the literature. The two type G strains identified in this study were also used for susceptibility testing against various drugs. Both strains were found to be resistant to amikacin, doxycycline, metronidazole, neomycin, nystatin, polymyxin B, streptomycin, and tetracycline. Variable susceptibility was seen against ceftriaxone, florfenicol, gentamicin, and kanamycin drugs. Amoxicillin, ampicillin, cefotaxime, ciprofloxacin, enrofloxacin, ofloxacin, and penicillin were effective drugs based upon their low level of resistance and therefore they might be preferred over other antimicrobial agents for proper treatment/prophylaxis of NE infections. Further studies are needed to study the pathogenesis of NE in detail in experimentally induced models along with continuous monitoring of the resistance pattern of C. perfringens strains in the field.

Immunization with Pooled Antigens for Clostridium perfringens Conferred Partial Protection against Experimental Necrotic Enteritis in Broiler Chickens

Necrotic enteritis (NE) is a multifactorial and important enteric infectious disease etiologically caused by pathogenic C. perfringens infection, accounting for the estimated loss of around USD 6 billion in the global poultry industry. The increasing incidence of NE was found to be associated with the voluntary reduction or withdrawal of antibiotic growth promoters from animal feed during recent years. Therefore, the development of effective vaccines specific to NE assumes a priority for the poultry industry. This study aimed to identify the potential C. perfringens proteins as vaccine targets for NE. Three recombinant C. perfringens proteins targeting five antigens were prepared: two chimeric proteins (alpha-toxin and NetB, fructose-1,6-bisphosphate aldolase (FBA) and a zinc metalloprotease (Zm)), and one single collagen adhesion protein (Cna). Their protection efficacies were evaluated with a potent challenge model of Eimeria maxima/C. perfringens dual infections using a netB+tpeL+ C. perfringens strain. Young chicks were immunized twice subcutaneously with adjuvanted C. perfringens proteins on Days 4 and 15. At six days after the second immunization, the chickens immunized with Cna, FBA, and Zm antigens, and alpha-toxin had much higher serum antibody titers than unvaccinated controls prior to the challenge. Following the challenge, the pooled antigen-immunized group demonstrated no mortality and the least lesion scores against virulent challenge. The results indicate that the immunization with multicomponent antigens, including C. perfringens housekeeping protein Cna, may confer partial protection.

Necrotic enteritis in chickens: a review of pathogenesis, immune responses and prevention, focusing on probiotics and vaccination

Necrotic enteritis (NE), caused by Clostridium perfringens (CP), is one of the most common of poultry diseases, causing huge economic losses to the poultry industry. This review provides an overview of the pathogenesis of NE in chickens and of the interaction of CP with the host immune system. The roles of management, nutrition, probiotics, and vaccination in reducing the incidence and severity of NE in poultry flocks are also discussed.

Short and Medium Chain Fatty Acids and Their Derivatives as a Natural Strategy in the Control of Necrotic Enteritis and Microbial Homeostasis in Broiler Chickens

The use of antibiotic growth promoters (AGPs) has historically been the most important prophylactic strategy for the control of Necrotic Enteritis (NE) caused by some Clostridium perfringens toxin types in poultry. During the last five decades, AGPs have also been supplemented in feed to improve body weight gain and feed efficiency as well as to modulate the microbiome (consisting of microbes and their genes both beneficial and potentially harmful) and reduce enteric pathogens, among other benefits. New regulatory requirements and consumer preferences have led to strong interest in natural alternatives to the AGPs for the prevention and control of illnesses caused by enteric pathogens. This interest is not just focused on the direct removal or inhibition of the causative microorganisms but also the improvement of intestinal health and homeostasis using a range of feed additives. A group of promising feed additives is short- and medium-chain fatty acids (SCFA and MCFA, respectively) and their derivatives. The use of SCFA and MCFA, including butyric, caproic, caprylic, capric, and lauric acids, has shown strong effects against NE in broilers both at experimental and commercial levels. These fatty acids also benefit intestinal health integrity and homeostasis. Other effects have also been documented, including increases in intestinal angiogenesis and gene expression of tight junctions. Chemical modifications to improve stability and point of release in the intestine have been shown to improve the efficacy of SCFA and MCFA and their derivatives. The aim of this review is to give an overview of SCFA, MCFA and their derivatives, as an alternative to replace AGPs to control the incidence and severity of NE in poultry.

Role of Clostridium perfringens Necrotic Enteritis B-like Toxin in Disease Pathogenesis

Necrotic enteritis (NE) is a devastating enteric disease caused by Clostridium perfringens type A/G that impacts the global poultry industry by compromising the performance, health, and welfare of chickens. Coccidiosis is a major contributing factor to NE. Although NE pathogenesis was believed to be facilitated by α-toxin, a chromosome-encoded phospholipase C enzyme, recent studies have indicated that NE B-like (NetB) toxin, a plasmid-encoded pore-forming heptameric protein, is the primary virulence factor. Since the discovery of NetB toxin, the occurrence of NetB⁺ C. perfringens strains has been increasingly reported in NE-afflicted poultry flocks globally. It is generally accepted that NetB toxin is the primary virulent factor in NE pathogenesis although scientific evidence is emerging that suggests other toxins contribute to NE. Because of the complex nature of the host-pathogen interaction in NE pathogenesis, the interaction of NetB with other potential virulent factors of C. perfringens needs better characterization. This short review will summarize the primary virulence factors involved in NE pathogenesis with an emphasis on NetB toxin, and a new detection method for large-scale field screening of NetB toxin in biological samples from NE-afflicted commercial broiler flocks.

Putative antigenic proteins identified by comparative and subtractive reverse vaccinology in necrotic enteritis-causing Clostridium perfringens isolated from broiler chickens

BACKGROUND

Avian necrotic enteritis (NE) caused by Clostridium perfringens is a disease with a major economic impact, generating losses estimated to 6 billion of dollars annually for the poultry industry worldwide. The incidence of the disease is particularly on the rise in broiler chicken flocks eliminating the preventive use of antibiotics. To date, no alternative allows for the efficient prevention of NE and a control of the disease using a vaccinal strategy would be mostly prized. For this purpose, comparative and subtractive reverse vaccinology identifying putative immunogenic bacterial surface proteins is one of the most promising approaches.

RESULTS

A comparative genomic study was performed on 16 C. perfringens strains isolated from healthy broiler chickens and from broilers affected with necrotic enteritis. Results showed that the analyzed genomes were composed of 155,700 distinct proteins from which 13% were identified as extracellular, 65% as cytoplasmic and 22% as part of the bacterial membrane. The evaluation of the immunogenicity of these proteins was determined using the prediction software VaxiJen®.

CONCLUSIONS

For the most part, proteins with the highest scores were associated with an extracellular localisation. For all the proteins analyzed, the combination of both the immunogenicity score and the localisation prediction led to the selection of 12 candidate proteins that were mostly annotated as hypothetical proteins. We describe 6 potential candidates of higher interest due to their antigenic potential, their extracellular localisation, and their possible role in virulence of C. perfringens.

A Novel PilR/PilS Two-Component System Regulates Necrotic Enteritis Pilus Production in Clostridium perfringens

Clostridium perfringens causes necrotic enteritis (NE) in poultry. A chromosomal locus (VR-10B) was previously identified in NE-causing C. perfringens strains that encodes an adhesive pilus (NE pilus), along with a two-component system (TCS) designated here as PilRS. While the NE pilus is important in pathogenesis, the role of PilRS remains to be determined. The current study investigated the function of PilRS, as well as the Agr-like quorum-sensing (QS) system and VirSR TCS in the regulation of pilin production. Isogenic pilR, agrB, and virR null mutants were generated from the parent strain CP1 by insertional inactivation using the ClosTron system, along with the respective complemented strains. Immunoblotting analyses showed no detectable pilus production in the CP1pilR mutant, while production in its complement (CP1pilR+) was greater than wild-type levels. In contrast, pilus production in the agrB and virR mutants was comparable or higher than the wild type but reduced in their respective complemented strains. When examined for collagen-binding activity, the pilR mutant showed significantly lower binding to most collagen types (types I to V) than parental CP1 (P ≤ 0.05), whereas this activity was restored in the complemented strain (P > 0.05). In contrast, binding of agrB and virR mutants to collagen showed no significant differences in collagen-binding activity compared to CP1 (P > 0.05), whereas the complemented strains exhibited significantly reduced binding (P ≤ 0.05). These data suggest the PilRS TCS positively regulates pilus production in C. perfringens, while the Agr-like QS system may serve as a negative regulator of this operon. IMPORTANCE Clostridium perfringens type G isolates cause necrotic enteritis (NE) in poultry, presenting a major challenge for poultry production in the postantibiotic era. Multiple factors in C. perfringens, including both virulent and nonvirulent, are involved in the development of the disease. We previously discovered a cluster of C. perfringens genes that encode a pilus involved in adherence and NE development, along with a predicted two-component regulatory system (TCS), designated PilRS. In the present study, we have demonstrated the role of PilRS in regulating pilus production and collagen binding of C. perfringens. In addition, the Agr-like quorum sensing signaling pathway was found to be involved in the regulation. These findings have identified additional targets for developing nonantibiotic strategies to control NE disease.

Persistence of a Clostridium perfringens Strain in a Broiler Chicken Farm over a Three-Year Period

Clostridium perfringens, a commensal of the intestinal tract of many animal species, has been associated with necrotic enteritis (NE), an economically significant poultry disease. Clostridium perfringens is known to survive in the environment for extended periods of time through the formation of spores. These spores have the potential to be transmitted to subsequent flocks. Persistence of a single C. perfringens strain in a broiler chicken farm environment has, however, been poorly documented. The aim of this study was to compare multiple isolates of C. perfringens collected over time in a single farm with recurrent episodes of NE. Isolates were recovered from the intestines of chickens affected with NE (2014 and 2016 outbreaks) and from healthy chickens (2017), as well as from environmental samples (2016). PCR characterization of those isolates showed that all sampling groups contained netB-positive isolates except for the environmental samples. Moreover, results showed that all environmental isolates were positive for the cna adhesin whereas other groups had lower numbers of cna-positive isolates. Biofilm formation assays showed that most of the isolates were able to form biofilm. Pulsed-field gel electrophoresis analysis showed that one clone was present in every sampling group, with the exception of the 2014 outbreak. However, one clone found in the latter group was highly similar, having 94% similarity with the persistent C. perfringens clone. This study describes for the first time the persistence of a C. perfringens strain on a broiler chicken house over a 3-yr period.

Centennial Review: Recent developments in host-pathogen interactions during necrotic enteritis in poultry

Necrotic enteritis (NE) is a significant enteric disease in commercial poultry with considerable economic effect on profitability manifested by an estimated $6 billion in annual losses to the global industry. NE presents a unique challenge, being a complex enteric disease that often leads to either clinical (acute) or subclinical (chronic) form. The latter typically results in poor performance (reduced feed intake, weight gain and eventually higher feed conversion ratio [FCR]) with low mortality rates, and represents the greatest economic impact on poultry production. The use of antibiotic growth promoters (AGPs) has been an effective tool in protecting birds from enteric diseases by maintaining enteric health and modifying gut microbiota, thus improving broilers’ production efficiency and overall health. The removal of AGPs presented the poultry industry with several challenges, including reduced bird health and immunity as well as questioning the safety of poultry products. Consequently, research on antibiotic alternatives that can support gut health was intensified. Probiotics, prebiotics, essential oils, and organic acids were among various additives that have been tested for their efficacy against NE with some being effective but not to the level of AGPs. The focus of this review is on the relationship between NE pathogenesis, microbiome, and host immune responses, along with references to recent reviews addressing production aspects of NE. With a comprehensive understanding of these dynamic changes, new and programmed strategies could be developed to make use of the current products more effectively or build a stepping stone toward the development of a new generation of supplements.

Comparative in silico genome analysis of Clostridium perfringens unravels stable phylogroups with different genome characteristics and pathogenic potential

Clostridium perfringens causes a plethora of devastating infections, with toxin production being the underlying mechanism of pathogenicity in various hosts. Genomic analyses of 206 public-available C. perfringens strains´ sequence data identified a substantial degree of genomic variability in respect to episome content, chromosome size and mobile elements. However, the position and order of the local collinear blocks on the chromosome showed a considerable degree of preservation. The strains were divided into five stable phylogroups (I–V). Phylogroup I contained human food poisoning strains with chromosomal enterotoxin (cpe) and a Darmbrand strain characterized by a high frequency of mobile elements, a relatively small genome size and a marked loss of chromosomal genes, including loss of genes encoding virulence traits. These features might correspond to the adaptation of these strains to a particular habitat, causing human foodborne illnesses. This contrasts strains that belong to phylogroup II where the genome size points to the acquisition of genetic material. Most strains of phylogroup II have been isolated from enteric lesions in horses and dogs. Phylogroups III, IV and V are heterogeneous groups containing a variety of different strains, with phylogroup III being the most abundant (65.5%). In conclusion, C. perfringens displays five stable phylogroups reflecting different disease involvements, prompting further studies on the evolution of this highly important pathogen.

Clostridium perfringens Produces an Adhesive Pilus Required for the Pathogenesis of Necrotic Enteritis in Poultry

Clostridium perfringens type G strains cause necrotic enteritis (NE) in poultry, an economically important disease that is a major target of in-feed antibiotics. NE is a multifactorial disease, involving not only the critically important NetB toxin but also additional virulence and virulence-associated factors. We previously identified a C. perfringens chromosomal locus (VR-10B) associated with disease-causing strains that is predicted to encode a sortase-dependent pilus. In the current study, we sought to provide direct evidence for the production of a pilus by C. perfringens and establish its role in NE pathogenesis. Pilus structures in virulent C. perfringens strain CP1 were visualized by transmission electron microscopy (TEM) of immunogold-labeled cells. Filamentous structures were observed extending from the cell surface in wild-type CP1 but not from isogenic pilin-null mutant strains. In addition, immunoblotting of cell surface proteins demonstrated that CP1, but not the null mutant strains, produced a high molecular weight ladder-like pattern characteristic of a pilus polymer. Binding to collagen types I, II, and IV was significantly reduced (Tukey's test, P < 0.01) in all three pilin mutants compared to CP1 and could be specifically blocked by CnaA and FimA antisera, indicating that these pilins participate in adherence. Furthermore, fimA and fimB null mutants were both severely attenuated in their ability to cause disease in an in vivo chicken NE challenge model. Together, these results provide the first direct evidence for the production of a sortase-dependent pilus by C. perfringens and confirm its critical role in NE pathogenesis and collagen binding.IMPORTANCE In necrotic enteritis (NE), an intestinal disease of chickens, Clostridium perfringens cells adhere tightly to damaged intestinal tissue, but the factors involved are not known. We previously discovered a cluster of C. perfringens genes predicted to encode a pilus, a hair-like bacterial surface structure commonly involved in adherence. In the current study, we have directly imaged this pilus using transmission electron microscopy (TEM). We also show that inactivation of the pilus genes stops pilus production, significantly reducing the bacterium's ability to bind collagen and cause disease. Importantly, this is the first direct evidence for the production of a sortase-dependent pilus by C. perfringens, revealing a promising new target for developing therapeutics to combat this economically important disease.

Incidence and associated risk factors of necrotic enteritis in Belgian layer pullet flocks

Necrotic enteritis (NE) caused by Clostridium perfringens is commonly reported in broilers. Recently, increased NE prevalence in layer breeds was reported in the Indian subcontinent. NE is also frequently observed by veterinary practitioners in Europe, mainly during the pullet rearing phase. In this study, data from layer pullet flocks in Belgium over a 5-year period (2013-2017) were used to assess the incidence of NE and identify potential risk factors for NE in layer pullets. NE was observed in 26% of the layer pullet flocks receiving veterinary intervention. This accounts for an overall estimated NE incidence of 12.3% in Belgian layer pullet flocks. Occurrence of NE was significantly associated with coccidiosis, with flocks being diagnosed with coccidiosis being two-fold more likely to develop NE. Additionally, birds kept in aviary houses were less prone to NE than flocks reared in floor systems or enriched cages. At necropsy, necrotic lesions in the small intestine were comparable to NE in broilers. A single strain of C. perfringens was isolated from the necrotic lesions of three different birds from the same flock; however, no NetB could be detected.

The ever-expanding tcp conjugation locus of pCW3 from Clostridium perfringens

The spore-forming, anaerobic Gram positive pathogen Clostridium perfringens encodes many of its disease-causing toxins on closely related conjugative plasmids. Studies of the tetracycline resistance plasmid pCW3 have identified many of the genes involved in conjugative transfer, which are located in the tcp conjugation locus. Upstream of this locus is an uncharacterised region (the cnaC region) that is highly conserved. This study examined the importance in pCW3 conjugation of several highly conserved proteins encoded in the cnaC region. Conjugative mating studies suggested that the SrtD, TcpN and Dam proteins were required for efficient pCW3 transfer between C. perfringens cells from the same strain background. The requirement of these proteins for conjugation was amplified in matings between C. perfringens cells of different strain backgrounds. Additionally, the putative collagen adhesin protein, CnaC, was only required for the optimal transfer of pCW3 between cells of different strain backgrounds. Based on these studies we postulate that CnaC, SrtD, TcpN and Dam are involved in enhancing the transfer frequency of pCW3. These studies have led to a significant expansion of the tcp conjugation locus, which now encompasses a 19 kb region.

Spotlight on avian pathology: untangling contradictory disease descriptions of necrotic enteritis and necro-haemorrhagic enteritis in broilers

Necrotic enteritis (NE) is one of the most detrimental infectious diseases in the modern poultry industry, characterized by necrosis in the small intestine. It is commonly accepted that NetB-producing C. perfringens type G strains are responsible for the disease. However, based on both macroscopic and histopathological observations, two distinct types of NE are observed. To date, both a haemorrhagic form of NE and the type G-associated non-haemorrhagic disease entity are commonly referred to as NE and the results from scientific research are interchangeably used, without distinguishing between the disease entities. Therefore, we propose to rename the haemorrhagic disease entity to necro-haemorrhagic enteritis.

Focal duodenal necrosis in chickens: attempts to reproduce the disease experimentally and diagnostic considerations

Focal duodenal necrosis (FDN) is an intestinal disease of egg-layer chickens characterized by multifocal necrosis of the duodenal loop and proximal jejunum. Affected flocks usually have decreased egg weights and drops in egg production. Previous studies have associated this condition with Clostridium perfringens infection. We tried to reproduce FDN by experimental infection of egg-laying chickens using different netB-positive and netB-negative C. perfringens strains, and duodenal homogenate obtained from FDN lesions. Chickens challenged with C. perfringens and/or duodenal homogenate developed duodenitis after challenge. Gross lesions included mucosal erosions, hyperemia, mucosal hemorrhages, and watery intestinal content. Microscopic lesions included mild enterocyte degeneration and necrosis, and mild-to-moderate hemorrhage and lymphoplasmacytic and heterophilic infiltration of the lamina propria. Two netB-positive C. perfringens strains closely related to necrotic enteritis pathogenic strains, by genomic composition, were re-isolated from lesions. Necrosis of intestinal crypts was observed in chickens challenged with duodenal homogenate with or without C. perfringens coinfection. Characteristic microscopic FDN lesions with significant necrosis and loss of villus enterocytes were not reproduced.

NetF-producing Clostridium perfringens and its associated diseases in dogs and foals

The role of type A Clostridium perfringens in canine acute hemorrhagic diarrhea syndrome and foal necrotizing enteritis is poorly characterized. However, a highly significant association between the presence of novel toxigenic C. perfringens and these specific enteric diseases has been described. These novel toxigenic strains produce 3 novel putative toxins, which have been designated NetE, NetF, and NetG. Although not conclusively demonstrated, current evidence suggests that NetF is likely the major virulence factor in strains responsible for canine acute hemorrhagic diarrhea syndrome and foal necrotizing enteritis. NetF is a beta-pore-forming toxin that belongs to the same toxin superfamily as CPB and NetB toxins produced by C. perfringens. The netF gene is encoded on a conjugative plasmid that, in the case of netF, also carries another putative toxin gene, netE. In addition, these strains consistently also carry a cpe tcp-conjugative plasmid, and a proportion also carry a separate netG tcp-conjugative plasmid. The netF and netG genes form part of a locus with all the features of the pathogenicity loci of tcp-conjugative plasmids. The netF-positive isolates are clonal in origin and fall into 2 clades. Disease in dogs or foals can be associated with either clade. Thus, these are strains with unique virulence-associated characteristics associated with serious and sometimes fatal cases of important enteric diseases in 2 animal species.

Two putative zinc metalloproteases contribute to the virulence of Clostridium perfringens strains that cause avian necrotic enteritis

Two putative zinc metalloproteases encoded by Clostridium perfringens have been implicated in the pathogenesis of necrotic enteritis, an economically significant poultry disease that is caused by this anaerobic bacterium. These proteases have ~64% amino acid identity and are encoded by the zmpA and zmpB genes. We screened 83 C. perfringens isolates by PCR for the presence of these genes. The first gene, zmpB, is chromosomally located and was present in all screened strains of C. perfringens, regardless of their origin and virulence. The second gene, zmpA, is plasmid-borne and was only found in isolates derived from chickens with necrotic enteritis. We describe the generation of insertionally inactivated mutants of both zmpA and zmpB in a virulent C. perfringens isolate. For each mutant, a significant (p < 0.001) reduction in virulence was observed in a chicken necrotic enteritis disease model. Examples of each mutant strain were characterized by whole genome sequencing, which showed that there were a few off-site mutations with the potential to affect the virulence of these strains. To confirm the importance of these genes, independently derived zmpA and zmpB mutants were constructed in different virulent C. perfringens isolates and shown to have reduced virulence in the experimental disease induction model. A zmpA-zmpB double mutant also was generated and shown to have significantly reduced virulence, to the same extent as the respective single mutants. Our results provide evidence that both putative zinc metalloproteases play an important role in disease pathogenesis.

Genomic analysis on broiler-associated Clostridium perfringens strains and exploratory caecal microbiome investigation reveals key factors linked to poultry necrotic enteritis

Background

Clostridium perfringens is a key pathogen in poultry-associated necrotic enteritis (NE). To date there are limited Whole Genome Sequencing based studies describing broiler-associated C. perfringens in healthy and diseased birds. Moreover, changes in the caecal microbiome during NE is currently not well characterised. Thus, the aim of this present study was to investigate C. perfringens virulence factors linked to health and diseased chickens, including identifying putative caecal microbiota signatures associated with NE.

Results

We analysed 88 broiler chicken C. perfringens genomes (representing 66 publicly available genomes and 22 newly sequenced genomes) using different phylogenomics approaches and identified a potential hypervirulent and globally-distributed clone spanning 20-year time-frame (1993-2013). These isolates harbored a greater number of virulence genes (including toxin and collagen adhesin genes) when compared to other isolates. Further genomic analysis indicated exclusive and overabundant presence of important NE-linked toxin genes including netB and tpeL in NE-associated broiler isolates. Secondary virulence genes including pfoA, cpb2, and collagen adhesin genes cna, cnaA and cnaD were also enriched in the NE-linked C. perfringens genomes. Moreover, an environmental isolate obtained from farm animal feeds was found to encode netB, suggesting potential reservoirs of NetB-positive C. perfringens strains (toxinotype G). We also analysed caecal samples from a small sub-set of 11 diseased and healthy broilers for exploratory microbiome investigation using 16S rRNA amplicon sequencing, which indicated a significant and positive correlation in genus Clostridium within the wider microbiota of those broilers diagnosed with NE, alongside reductions in beneficial microbiota members.

Conclusions

These data indicate a positive association of virulence genes including netB, pfoA, cpb2, tpeL and cna variants linked to NE-linked isolates. Potential global dissemination of specific hypervirulent lineage, coupled with distinctive microbiome profiles, highlights the need for further investigations, which will require a large worldwide sample collection from healthy and NE-associated birds.

Adherence of Clostridium perfringens spores to human intestinal epithelial Caco-2 cells

Clostridium perfringens is a gram-positive, spore-forming bacillus, and is a causative agent of foodborne infection, antibiotic-associated diarrhoea and sporadic diarrhoea in humans. In cases of antibiotic-associated and sporadic diarrhoea, C. perfringens colonises the intestine, proliferates and causes disease. However, bacterial colonisation of the intestine is not considered necessary in the pathogenesis of foodborne illness, because such pathogenesis can be explained by anchorage-independent production of diarrhoeic toxin by the bacterium in the intestine. In this study, we used an in vitro adherence assay to examine the adherence of C. perfringens spores to human intestinal Caco-2 cells. Adherence of spores from isolates of foodborne illness and nosocomial infection was observed within 15 min, and plateaued 60 min after inoculation. Electron microscopy revealed a tight association of spores with the surface of Caco-2 cells. The adherence of vegetative cells could not be confirmed by the same method, however. These results suggest that C. perfringens spores may adhere to intestinal epithelial cells in vivo, although its biological significance remains to be determined.

Virulence Plasmids of the Pathogenic Clostridia

The clostridia cause a spectrum of diseases in humans and animals ranging from life-threatening tetanus and botulism, uterine infections, histotoxic infections and enteric diseases, including antibiotic-associated diarrhea, and food poisoning. The symptoms of all these diseases are the result of potent protein toxins produced by these organisms. These toxins are diverse, ranging from a multitude of pore-forming toxins to phospholipases, metalloproteases, ADP-ribosyltransferases and large glycosyltransferases. The location of the toxin genes is the unifying theme of this review because with one or two exceptions they are all located on plasmids or on bacteriophage that replicate using a plasmid-like intermediate. Some of these plasmids are distantly related whilst others share little or no similarity. Many of these toxin plasmids have been shown to be conjugative. The mobile nature of these toxin genes gives a ready explanation of how clostridial toxin genes have been so widely disseminated both within the clostridial genera as well as in the wider bacterial community.

Salmonella-vectored vaccine delivering three Clostridium perfringens antigens protects poultry against necrotic enteritis

Necrotic enteritis is an economically important poultry disease caused by the bacterium Clostridium perfringens. There are currently no necrotic enteritis vaccines commercially available for use in broiler birds, the most important target population. Salmonella-vectored vaccines represent a convenient and effective option for controlling this disease. We used a single attenuated Salmonella vaccine strain, engineered to lyse within the host, to deliver up to three C. perfringens antigens. Two of the antigens were toxoids, based on C. perfringens α-toxin and NetB toxin. The third antigen was fructose-1,6-bisphosphate aldolase (Fba), a metabolic enzyme with an unknown role in virulence. Oral immunization with a single Salmonella vaccine strain producing either Fba, α-toxoid and NetB toxoid, or all three antigens, was immunogenic, inducing serum, cellular and mucosal responses against Salmonella and the vectored C. perfringens antigens. All three vaccine strains were partially protective against virulent C. perfringens challenge. The strains delivering Fba only or all three antigens provided the best protection. We also demonstrate that both toxins and Fba are present on the C. perfringens cell surface. The presence of Fba on the cell surface suggests that Fba may function as an adhesin.

Immunization with subunits of a novel pilus produced by virulent Clostridium perfringens strains confers partial protection against necrotic enteritis in chickens

Necrotic enteritis (NE) is an economically important disease of broiler chickens that is caused primarily by Clostridium perfringens strains that produce the NetB toxin. It is controlled in North America principally through the application of in-feed antimicrobials, but alternative control methods, such as vaccination, are urgently needed. We previously identified a cluster of C. perfringens genes prevalent in disease-causing strains, denominated VR-10B, that is predicted to encode a pilus. The current study evaluated the ability of three predicted pilin structural subunits (CnaA, FimA, FimB) to protect against NE in two immunization studies. In the first study, young broiler chickens were immunized twice intramuscularly (i.m.) with CnaA or FimA, which resulted in only a weak serum antibody response, and no reduction in the severity of intestinal lesions following experimental challenge with C. perfringens strain CP1. In the second study, chickens were injected subcutaneously (s.c.) with CnaA, FimB, or a combination of all three proteins, on days 7, 14 and 19, which resulted in a marked antibody response specific to each antigen. Chickens immunized with either CnaA or FimB had significantly reduced NE lesion severity, whereas immunization with all three proteins in combination did not provide protection. Western blot experiments using serum from immunized birds were also performed, providing the first experimental evidence to suggest that this locus may in fact encode a functional pilus structure.

Overexpressing ovotransferrin and avian β-defensin-3 improves antimicrobial capacity of chickens and poultry products

Zoonotic and foodborne diseases pose a significant burden, decreasing both human and animal health. Modifying chickens to overexpress antimicrobials has the potential to decrease bacterial growth on poultry products and boost chicken innate immunity. Chickens overexpressing either ovotransferrin or avian β-defensin-3 (AvβD3) were generated using Tol-2 transposons. Transgene expression at the RNA and protein level was seen in egg white, breast muscle, and serum. There were significant differences in the immune cell populations in the blood, bursa, and spleen associated with transgene expression including an increased proportion of CD8+ cells in the blood of ovotransferrin and AvβD3 transgenic birds. Expression of the antimicrobials inhibited the in vitro growth of human and chicken bacterial pathogens and spoilage bacteria. For example, transgene expression significantly reduced growth of aerobic and coliform bacteria in breast muscle and decreased the growth of Salmonella enterica in egg white. Overall these results indicate that overexpression of antimicrobials in the chicken can impact the immune system and increase the antimicrobial capacity of poultry products.

A Ligated Intestinal Loop Model in Anesthetized Specific Pathogen Free Chickens to Study Clostridium Perfringens Virulence

Necrotic enteritis was studied in chickens using various in vivo infection models. Most of these use a combination of predisposing factors, such as coccidiosis and diet, with gavage or administration via the feed using Clostridium perfringens. In these models, the comparison of multiple C. perfringens strains for virulence studies requires a large number of hosts to obtain significant results. Mortality during the course of the study can be high depending on the experimental model, hence raising ethical concerns regarding animal welfare in research. The development of new infection models requiring fewer animals to study pathogenesis, yet providing statistically significant and valid results, is important in reducing animal use in research. Intestinal ligated loop models have been used to study clostridial infections in various species such as mice, rabbits and calves. Following surgical procedures to create ligated loop segments, C. perfringens strains are injected directly into the loops to establish a close contact between the bacteria and the intestinal mucosa. Samples of the small intestine and luminal contents are taken at the termination of the procedures after a few hours. Multiple bacterial strains can be inoculated in each animal, hence reducing the number of required subjects in the experiments. Also, procedures are performed under general anesthesia to reduce animal pain. In chickens, this model would be more appropriate than oral administration to compare C. perfringens strain pathogenicity because fewer animals are needed, no predisposing factors are required to induce the disease, and pain is controlled by analgesics. The intestinal ligated loop model is poorly described in chickens and standardization is essential for its optimal use. This manuscript provides all the necessary steps to create numerous intestinal ligated loops in chickens and brings information on the critical points to obtain valid results.

Vaccines as alternatives to antibiotics for food producing animals. Part 2: new approaches and potential solutions

Vaccines and other alternative products are central to the future success of animal agriculture because they can help minimize the need for antibiotics by preventing and controlling infectious diseases in animal populations. To assess scientific advancements related to alternatives to antibiotics and provide actionable strategies to support their development, the United States Department of Agriculture, with support from the World Organisation for Animal Health, organized the second International Symposium on Alternatives to Antibiotics. It focused on six key areas: vaccines; microbial-derived products; non-nutritive phytochemicals; immune-related products; chemicals, enzymes, and innovative drugs; and regulatory pathways to enable the development and licensure of alternatives to antibiotics. This article, the second part in a two-part series, highlights new approaches and potential solutions for the development of vaccines as alternatives to antibiotics in food producing animals; opportunities, challenges and needs for the development of such vaccines are discussed in the first part of this series. As discussed in part 1 of this manuscript, many current vaccines fall short of ideal vaccines in one or more respects. Promising breakthroughs to overcome these limitations include new biotechnology techniques, new oral vaccine approaches, novel adjuvants, new delivery strategies based on bacterial spores, and live recombinant vectors; they also include new vaccination strategies in-ovo, and strategies that simultaneously protect against multiple pathogens. However, translating this research into commercial vaccines that effectively reduce the need for antibiotics will require close collaboration among stakeholders, for instance through public–private partnerships. Targeted research and development investments and concerted efforts by all affected are needed to realize the potential of vaccines to improve animal health, safeguard agricultural productivity, and reduce antibiotic consumption and resulting resistance risks.

Avi-Lution® supplemented at 1.0 or 2.0 g/kg in feed improves the growth performance of broiler chickens during challenge with bacitracin-resistant Clostridium perfringens

Avi-Lution® is a defined, patented, synbiotic product containing Saccharomyces cerevisiae, Enterococcus faecium, and Bacillus spp. Broiler chickens (n = 1,250) were experimentally treated as uninoculated controls (uCon), inoculated controls (iCon) with Clostridium perfringens, or inoculated and treated with bacitracin methylene disalicylate (BMD) at 55 mg/kg as an infected/treated control or Avi-Lution® at 1.0 (AvL1) or 2.0 (AvL2) g/kg in feed for 42 d. Each treatment was applied to 10 replicate pens of 25 straight-run, newly hatched chicks. Pens treated with AvL1, AvL2, or BMD showed improved growth, feed efficiency, or mortality from necrotic enteritis compared with iCon pens at d 14, 28, and 42. No differences in these measurements, however, were observed between pens treated with AvL1 and AvL2, which suggests that Avi-Lution® was effective at 1.0 g/kg in feed. Despite improved performance, BMD, AvL1, and AvL2 treatments did not decrease the severity of intestinal lesion scores through 42 d of age compared with the infected control. These results demonstrate that Avi-Lution® improved growth performance and feed conversion rates in broilers challenged with Clostridium perfringens despite no difference in severity of intestinal lesion scores.

Whole genome analysis reveals the diversity and evolutionary relationships between necrotic enteritis-causing strains of Clostridium perfringens

Background

Clostridium perfringens causes a range of diseases in animals and humans including necrotic enteritis in chickens and food poisoning and gas gangrene in humans. Necrotic enteritis is of concern in commercial chicken production due to the cost of the implementation of infection control measures and to productivity losses. This study has focused on the genomic analysis of a range of chicken-derived C. perfringens isolates, from around the world and from different years. The genomes were sequenced and compared with 20 genomes available from public databases, which were from a diverse collection of isolates from chickens, other animals, and humans. We used a distance based phylogeny that was constructed based on gene content rather than sequence identity. Similarity between strains was defined as the number of genes that they have in common divided by their total number of genes. In this type of phylogenetic analysis, evolutionary distance can be interpreted in terms of evolutionary events such as acquisition and loss of genes, whereas the underlying properties (the gene content) can be interpreted in terms of function. We also compared these methods to the sequence-based phylogeny of the core genome.

Results

Distinct pathogenic clades of necrotic enteritis-causing C. perfringens were identified. They were characterised by variable regions encoded on the chromosome, with predicted roles in capsule production, adhesion, inhibition of related strains, phage integration, and metabolism. Some strains have almost identical genomes, even though they were isolated from different geographic regions at various times, while other highly distant genomes appear to result in similar outcomes with regard to virulence and pathogenesis.

Conclusions

The high level of diversity in chicken isolates suggests there is no reliable factor that defines a chicken strain of C. perfringens, however, disease-causing strains can be defined by the presence of netB-encoding plasmids. This study reveals that horizontal gene transfer appears to play a significant role in genetic variation of the C. perfringens chromosome as well as the plasmid content within strains.

Two Lactobacillus Species Inhibit the Growth and α-Toxin Production of Clostridium perfringens and Induced Proinflammatory Factors in Chicken Intestinal Epithelial Cells in Vitro

Clostridium perfringens is the causative pathogen of avian necrotic enteritis. Lactobacillus spp. are well-characterized probiotics with anti-microbial and immune-modulatory activities. In the present study, we investigated the effects of L. acidophilus and L. fermentum on the growth, α-toxin production and inflammatory responses of C. perfringens. In in vitro culture experiments, both lactobacilli inhibited the growth of C. perfringens (P < 0.01), accompanied with a decrease in pH (P < 0.01). Supernatants from lactobacilli cultures also suppressed the growth of C. perfringens during 24 h of incubation (P < 0.01), but this inhibitory effect disappeared after 48 h. Both lactobacilli decreased the α-toxin production of C. perfringens (P < 0.01) without influencing its biomass, and even degraded the established α-toxin (P < 0.01). Lower environmental pH reduced the α-toxin production as well (P < 0.01). Preincubation with L. acidophilus decreased the attachment of C. perfringens to cells (P < 0.01) with the cell cytotoxicity being unaffected. Both lactobacilli pretreatment reduced the up-regulation of proinflammatory factors, peptidoglycan (PGN) receptors and nuclear factor kappa B (NF-κB) p65 in C. perfringens-challenged chicken intestinal epithelial cells (P < 0.05). In conclusion, L. acidophilus and L. fermentum inhibited the pathological effects of C. perfringens in vitro conditions.

Necrotic enteritis locus 1 diguanylate cyclase and phosphodiesterase (cyclic-di-GMP) gene mutation attenuates virulence in an avian necrotic enteritis isolate of Clostridium perfringens

Necrotic enteritis (NE) caused by netB-positive strains of Clostridium perfringens is an important disease of intensively-reared broiler chickens. It is widely controlled by antibiotic use, but this practice that has come under increasing scrutiny and alternative approaches are required. As part of the search for alternative approaches over the last decade, advances have been made in understanding its pathogenesis but much remains to be understood and applied to the control of NE. The objective of this work was to assess the effect on virulence of mutation of the cyclic-di-GMP signaling genes present on the large pathogenicity locus (NELoc-1) in the tcp-encoding conjugative virulence plasmid, pNetB. For this purpose, the diguanylate cyclase (dgc) and phosphodiesterase (pde) genes were individually insertionally inactivated and the two mutants were subsequently complemented with their respective genes. Southern blotting showed that a single gene insertion was present. Mutation of either gene resulted in almost total attenuation of the mutants to cause NE in experimentally-infected broiler chickens, which was fully restored in each case by complementation of the respective mutated gene. Production of NetB-associated cytotoxicity for Leghorn male hepatoma (LMH) cells was unaffected in mutants. We conclude that the cyclic-di-GMP signaling system is important in controlling virulence in a NE C. perfringens strain and might be a target for control of the disease.

Clostridium perfringens Sialidases: Potential Contributors to Intestinal Pathogenesis and Therapeutic Targets

Clostridium perfringens is a major cause of histotoxic and intestinal infections of humans and other animals. This Gram-positive anaerobic bacterium can produce up to three sialidases named NanH, NanI, and NanJ. The role of sialidases in histotoxic infections, such as gas gangrene (clostridial myonecrosis), remains equivocal. However, recent in vitro studies suggest that NanI may contribute to intestinal virulence by upregulating production of some toxins associated with intestinal infection, increasing the binding and activity of some of those toxins, and enhancing adherence of C. perfringens to intestinal cells. Possible contributions of NanI to intestinal colonization are further supported by observations that the C. perfringens strains causing acute food poisoning in humans often lack the nanI gene, while other C. perfringens strains causing chronic intestinal infections in humans usually carry a nanI gene. Certain sialidase inhibitors have been shown to block NanI activity and reduce C. perfringens adherence to cultured enterocyte-like cells, opening the possibility that sialidase inhibitors could be useful therapeutics against C. perfringens intestinal infections. These initial in vitro observations should be tested for their in vivo significance using animal models of intestinal infections.