Demonstration of the efficacy of a Salmonella Enteritidis live vaccine for chickens according to the current European Pharmacopoeia Monograph

Age of challenge is important in Salmonella Enteritidis studies in pullets and hens: a systematic review

Nontyphoidal serovars of Salmonella enterica subsp enterica frequently colonize the intestinal tracts of chickens, creating risks of contamination of meat and egg food products. These serovars seldom cause disease in chickens over 3 weeks of age. Colonization is generally transient but can continue to circulate in a flock for many months. Vaccination of breeders and layers is the most effective method of control of infections with serovars Enteritidis and Typhimurium, and the development of these vaccines or other preventative treatments requires challenge studies to demonstrate efficacy. However, establishing a successful challenge model where the control birds are colonized to a sufficient extent to be able to demonstrate a statistically significant reduction from the vaccine or treatment is problematic. A meta-analysis of published S. Enteritidis challenge studies was performed to pursue the best challenge model conditions that provide consistent control colonization outcomes. Challenge at sexual maturity was significantly more effective in achieving at least 80% colonization of control hens.RESEARCH HIGHLIGHTSSalmonella challenge chicken models do not always achieve high colonization levels in controls.The age of hen is important in achieving good caecal colonization.Challenge around sexual maturity provides the best control colonization outcome.A challenge dose rate of 105 CFU/ bird is adequate in birds under 30 weeks of age.

Pre-Harvest Non-Typhoidal Salmonella Control Strategies in Commercial Layer Chickens

Non-typhoidal Salmonella (NTS) infections in poultry, particularly in commercial-layer chickens, pose a critical risk to food safety and public health worldwide. NTS bacteria can remain undetected in poultry flocks, contaminating products and potentially leading to gastroenteritis in humans. This review examines pre-harvest control strategies for NTS in layer chickens, including biosecurity protocols, vaccinations, feed additives, genetic selection, and environmental management. These strategies have substantially reduced Salmonella colonization and product contamination rates in the commercial layer industry. By evaluating these strategies, this review highlights the importance of integrated control measures to limit NTS colonization, reduce antimicrobial resistance, and improve poultry health. This review aims to provide producers, researchers, and policymakers with insights into effective practices to minimize Salmonella contamination and enhance both animal and human health outcomes.

Multireceptor phage cocktail against Salmonella enterica to circumvent phage resistance

Non-Typhoidal Salmonella (NTS) is one of the most common food-borne pathogens worldwide, with poultry products being the major vehicle for pathogenesis in humans. The use of bacteriophage (phage) cocktails has recently emerged as a novel approach to enhancing food safety. Here, a multireceptor Salmonella phage cocktail of five phages was developed and characterized. The cocktail targets four receptors: O-antigen, BtuB, OmpC, and rough Salmonella strains. Structural analysis indicated that all five phages belong to unique families or subfamilies. Genome analysis of four of the phages showed they were devoid of known virulence or antimicrobial resistance factors, indicating enhanced safety. The phage cocktail broad antimicrobial spectrum against Salmonella, significantly inhibiting the growth of all 66 strains from 20 serovars tested in vitro. The average bacteriophage insensitive mutant (BIM) frequency against the cocktail was 6.22 × 10-6 in S. Enteritidis, significantly lower than that of each of the individual phages. The phage cocktail reduced the load of Salmonella in inoculated chicken skin by 3.5 log10 CFU/cm2 after 48 h at 25°C and 15°C, and 2.5 log10 CFU/cm2 at 4°C. A genome-wide transduction assay was used to investigate the transduction efficiency of the selected phage in the cocktail. Only one of the four phages tested could transduce the kanamycin resistance cassette at a low frequency comparable to that of phage P22. Overall, the results support the potential of cocktails of phage that each target different host receptors to achieve complementary infection and reduce the emergence of phage resistance during biocontrol applications.

The Immunological Basis for Vaccination

Vaccination is crucial for health protection of poultry and therefore important to maintaining high production standards. Proper vaccination requires knowledge of the key players of the well-orchestrated immune system of birds, their interdependence and delicate regulation, and, subsequently, possible modes of stimulation through vaccine antigens and adjuvants. The knowledge about the innate and acquired immune systems of birds has increased significantly during the recent years but open questions remain and have to be elucidated further. Despite similarities between avian and mammalian species in their composition of immune cells and modes of activation, important differences exist, including differences in the innate, but also humoral and cell-mediated immunity with respect to, for example, signaling transduction pathways, antigen presentation, and cell repertoires. For a successful vaccination strategy in birds it always has to be considered that genotype and age of the birds at the time point of immunization as well as their microbiota composition may have an impact and may drive the immune reactions into different directions. Recent achievements in the understanding of the concept of trained immunity will contribute to the advancement of current vaccine types helping to improve protection beyond the specificity of an antigen-driven immune response. The fast developments in new omics technologies will provide insights into protective B- and T-cell epitopes involved in cross-protection, which subsequently will lead to the improvement of vaccine efficacy in poultry.

The nematode worm Caenorhabditis elegans as an animal experiment replacement for assessing the virulence of different Salmonella enterica strains

Caenorhabditis (C.) elegans has become a popular toxicological and biological test organism in the last two decades. Furthermore, the role of C. elegans as an alternative for replacing or reducing animal experiments is continuously discussed and investigated. In the current study, we investigated whether C. elegans survival assays can help in determining differences in the virulence of Salmonella enterica strains and to what extent C. elegans assays could replace animal experiments for this purpose. We focused on three currently discussed examples where we compared the longevity of C. elegans when fed (i) with S. enterica serovar Enteritidis vaccination or wild-type strains, (ii) with lipopolysaccharide (LPS) deficient rough or LPS forming smooth S. enterica serovar Enteritidis, and (iii) with an S. enterica subsp. diarizonae strain in the presence or absence of the typical pSASd plasmid encoding a bundle of putative virulence factors. We found that the C. elegans survival assay could indicate differences in the longevity of C. elegans when fed with the compared strain pairs to a certain extent. Putatively higher virulent S. enterica strains reduced the lifespan of C. elegans to a greater extent than putatively less virulent strains. The C. elegans survival assay is an effective and relatively easy method for classifying the virulence of different bacterial isolates in vivo, but it has some limitations. The assay cannot replace animal experiments designed to determine differences in the virulence of Salmonella enterica strains. Instead, we recommend using the described method for pre-screening bacterial strains of interest to select the most promising candidates for further animal experiments. The C. elegans assay possesses the potential to reduce the number of animal experiments. Further development of the C. elegans assay in conjunction with omics technologies, such as transcriptomics, could refine results relating to the estimation of the virulent potential of test organisms.

Safety of bivalent live attenuated Salmonella vaccine and its protection against bacterial shedding and tissue invasion in layers challenged with Salmonella

Nontyphoidal Salmonella infection was one of the predominant foodborne illnesses in humans. The medical burden and antimicrobial resistance of salmonellosis gained importance in public health and requested the poultry industry to seek effective measures to control the disease. The objective of this study was to evaluate the safety and effectiveness of a commercial bivalent live attenuated vaccine (AviPro Salmonella DUO) in specific-pathogen-free (SPF) chickens and field layers. It explored its safety and efficacy against medically important strains, Salmonella Enteritidis (SE) and S. Typhimurium (ST). The results demonstrated that ten vaccine doses in SPF chickens and regular doses in commercial layers showed desirable safety without affecting chicken health. Vaccinated layers demonstrated lower flock mortality and higher egg production performance than the unvaccinated layers during the raising and egg production periods. Additionally, no visceral colonization and egg contaminations were detected. Cloacal shedding of vaccine strains was noted, but the colonization of Salmonella disappeared within four weeks of the last vaccination. Regarding vaccine efficacy, one dose significantly reduced Salmonella cloacal shedding (P = 0.037 for SE and P = 0.027 for ST) and viable cell counts (P = 0.003 for SE and ST) on day 7 post the challenges. Significantly low Salmonella loads of cloacal samples on day 14 after the challenges were also determined in the vaccinated group (P = 0.006 for SE; P = 0.041 for ST). Triple immunizations effectively prevented layers from the cloacal shedding on either day 7 or day 14 post Salmonella challenges. Total viable counts of SE and ST in tissues of vaccinated layers were also reduced on day 14 after the challenges (P = 0.026 for SE; P = 0.002 for ST). To conclude, one dose of vaccine exhibited inhibitory effects on Salmonella shedding and tissue invasions in young layers. Following the regimen of triple vaccinations, Salmonella shedding was completely inhibited, and tissue invasions were significantly reduced. Incorporating this vaccine into a comprehensive Salmonella control program is promising to protect layers from the risks of contaminating the flocks and egg products.

Salmonella Genomics in Public Health and Food Safety

The species Salmonella enterica comprises over 2,600 serovars, many of which are known to be intracellular pathogens of mammals, birds, and reptiles. It is now apparent that Salmonella is a highly adapted environmental microbe and can readily persist in a number of environmental niches, including water, soil, and various plant (including produce) species. Much of what is known about the evolution and diversity of nontyphoidal Salmonella serovars (NTS) in the environment is the result of the rise of the genomics era in enteric microbiology. There are over 340,000 Salmonella genomes available in public databases. This extraordinary breadth of genomic diversity now available for the species, coupled with widespread availability and affordability of whole-genome sequencing (WGS) instrumentation, has transformed the way in which we detect, differentiate, and characterize Salmonella enterica strains in a timely way. Not only have WGS data afforded a detailed and global examination of the molecular epidemiological movement of Salmonella from diverse environmental reservoirs into human and animal hosts, but they have also allowed considerable consolidation of the diagnostic effort required to test for various phenotypes important to the characterization of Salmonella. For example, drug resistance, serovar, virulence determinants, and other genome-based attributes can all be discerned using a genome sequence. Finally, genomic analysis, in conjunction with functional and phenotypic approaches, is beginning to provide new insights into the precise adaptive changes that permit persistence of NTS in so many diverse and challenging environmental niches.

Invasion inhibition effects and immunogenicity after vaccination of SPF chicks with a Salmonella Enteritidis live vaccine

Objective Meat and eggs from chickens infected with Salmonella Enteritidis, Salmonella Typhimurium and Salmonella Infantis are considered to be an important source of Salmonella infections for humans. In order to control Salmonella infections in chickens, basic biosecurity measures are taken in combination with inactivated or attenuated live vaccines. Apart from an adaptive immune response, some live vaccines also induce innate immune mechanisms that prevent or inhibit systemic invasion with homologous Salmonella serovars. It is unknown whether these invasion inhibition effects are also directed against heterologous Salmonella serovars. Furthermore, it is unclear whether the adaptive immune response after vaccination with a Salmonella Enteritidis phage type 4 live vaccine is also directed against other phage types of Salmonella Enteritidis and Typhimurium.

Material and methods Specific pathogen-free day-old chicks were vaccinated orally with a commercially available Salmonella Enteritidis live vaccine. To test the invasion inhibition effect, the animals were challenged orally with a labelled Salmonella Typhimurium or Salmonella Infantis strain 1 day after vaccination. To demonstrate the adaptive immune response against non-phage type 4  Salmonella Enteritidis strains and a monophasic Salmonella Typhimurium strain, the chickens were challenged with Salmonella Enteritidis strains of phage types 1, 8 and 21 and a monophasic Salmonella Typhimurium strain (Definitive Type 193). After challenge, the abundance of the challenge strain in liver and cecal tissue was enumerated and compared with a corresponding control group.

Results Findings showed that the live Salmonella Enteritidis vaccine inhibits systemic invasion after early infection with Salmonella Typhimurium and Salmonella Infantis. Furthermore, adaptive immunity against the tested non-phage type 4  Salmonella Enteritidis strains and the monophasic Salmonella Typhimurium strain was demonstrated.

Conclusion and clinical relevance The results of this study demonstrate that vaccination with the Salmonella Enteritidis phage type 4 live vaccine significantly inhibits the invasion of Salmonella Typhimurium and Infantis. Furthermore, an adaptive immune response was also detected against non-phage type 4  Salmonella Enteritidis strains and a monophasic Salmonella Typhimurium strain.

Salmonella Enteritidis ghost vaccine carrying the hemagglutinin globular head (HA1) domain from H1N1 virus protects against salmonellosis and influenza in chickens

This study evaluated the attenuated Salmonella Enteritidis (SE) ghost strain JOL2114 (Δlon ΔcpxR Δasd), which displays on the bacterial surface the H1N1 hemagglutinin globular head portion (HA1; amino acid residues 63-286) on the bacterial surface for protective efficacy against Salmonella and H1N1 challenge in the chicken model, as the birds are the predominant reservoirs for both diseases. The ghost system enhanced the lysis process by converging two lysis processes found in bacteriophages: bacteriophage PhiX174 lysis gene E and holin-endolysin genes found in bacteriophage λ, complemented with accessory lysis-related proteins Rz/Rz1. The present lysis machinery resulted in complete lysis of host-attenuated SE strains in about 24 hrs of incubation under a non-permissible temperature of 42 °C in the absence of L-arabinose, an antisense inducer that blocks lysis gene expression during the growth phase. SE ghost JOL2114 surface display of HA1 was confirmed by Western blot analysis resulting in an immune-reactive band of 31 kDa in size. Chicken immunization via intramuscular and oral routes yielded both SE and HA1 antigen-specific immune responses. Protective humoral and cell-mediated immune responses were effectively elicited against both Salmonella and influenza challenge. This efficient strategy of ghost generation employs a dual system of phage lysis for biological generation of SE ghosts that preserves the surface antigenic architecture, offering a rapid and effective way to generate vaccines that could be deployed in urgent circumstances to protect against both Salmonella and influenza infection.