Attenuation of virulence of Lawsonia intracellularis after in vitro passages and its effects on the experimental reproduction of porcine proliferative enteropathy

In Silico Evaluation of Lawsonia intracellularis Genes Orthologous to Genes Associated with Pathogenesis in Other Intracellular Bacteria

Proliferative enteropathy is an enteric disease caused by the bacterium Lawsonia intracellularis, which affects several species of domestic and wild animals. The mechanisms underlying the mechanisms employed by L. intracellularis to cause host cell proliferation are poorly understood, mostly because this bacterium is extremely difficult to isolate and propagate in vitro. Comparative genomics methods for searching for genes orthologous to genes known to be associated with pathogenesis allow identification of genes potentially involved in pathogenesis by the pathogen of interest. The goal of this study was to carry out in silico research on L. intracellularis genes orthologous to genes required for intracellular invasion and survival present in other pathogenic bacteria, particularly Brucella abortus, B. melitensis, B. suis, Listeria monocytogenes, Mycobacterium tuberculosis, Mycobacterium avium subspecies paratuberculosis, Salmonella enterica, Yersinia pestis, Y. enterocolitica, and Y. pseudotuberculosis. A total of 127 genes associated with invasion and intracellular survival from five known intracellular bacteria were mapped against the predicted proteomes of all L. intracellularis strains publicly available on GenBank, using the OrthoFinder program. A total of 45 L. intracellularis genes were orthologous to genes associated with pathogenesis of other intracellular bacteria. Genes putatively associated with signal the transduction of chemotaxis and cell motility were identified. Genes related to DNA binding and repair were also identified, with some of them supporting a possible association of bacteria with macrophages or inducing pro-inflammatory responses. The homology-based identification of these genes suggests their potential involvement in the virulence and pathogenicity of L. intracellularis, opening avenues for future research and insights into the molecular mechanisms of Lawsonia-elicited proliferative enteropathy.

Probiotics mediated gut microbiota diversity shifts are associated with reduction in histopathology and shedding of Lawsonia intracellularis

BACKGROUND

Clinical intervention during bacterial infections in farm animals such as pigs commonly includes the use of antimicrobials. With the rise of antimicrobial resistance and the attempts to reduce the use of antibiotics in food animals, effective alternatives are urgently needed to reduce or even remove pathogens and disease risks. Improving clinical outcomes and overall pig health by using probiotics appears attractive. However, reliable data sets on the efficacy of probiotics are scarce. The obligate intracellular bacterium Lawsonia intracellularis is widespread in pigs and associated with severe enteropathy, mainly in the ileum, commonly resulting in substantial reduction in weight gain. The impact of three in-feed probiotics and a commercial live L. intracellularis vaccine was compared in a pig challenge model. Probiotic treatment was associated with reduced L. intracellularis fecal shedding and reduced gut lesions. Here, the bacterial microbiota of the ileum of these pigs was characterized with 16S rRNA gene sequencing and was subsequently analyzed with bioinformatics tools.

RESULTS

The greatest microbial richness was observed in the probiotic treated group T03-LAW, which accounted for 87% of richness observed in the study. Treatment had a significant impact on both the microbiota structure and taxonomic profile in the ileum, explaining between 26 and 36% of the structural variation, with the strongest association in the T03-LAW group. Overall, the largest changes were observed for the pigs treated with in-feed Bacillus pumilus; the microbiota of these pigs had the greatest diversity and highest richness. We also observed depleted and enriched core microbiota amongst the groups; however, there was no correlation with clinical characteristics. The results suggest that an increased diversity of the ileal microbiota is associated with a reduction in shedding, i.e. a unit increase in Shannon diversity index resulted in 2.8 log reduction in shedding.

CONCLUSIONS

Probiotic supplementation of a base feed ration increased ileum microbiota diversity leading to a mitigation of the effects of a pathogenic L. intracellularis challenge. An even and diverse microbiota community benefits pigs infected with L. intracellularis, however, investigations are needed to determine if this is also true for other pathogens. The study unambiguously demonstrates the usefulness of probiotic supplementation in reducing the impact of enteric pathogens and pathogen shedding rates in food animals without the use of antimicrobials.

Evaluation of swine enteroids as in vitro models for Lawsonia intracellularis infection1,2

The enteric pathogen Lawsonia intracellularis is one of the main causes of diarrhea and compromised weight gain in pigs worldwide. Traditional cell-line cultures have been used to study L. intracellularis pathogenesis. However, these systems fail to reproduce the epithelial changes observed in the intestines of L. intracellularis-infected pigs, specifically, the changes in intestinal cell constitution and gene expression. A more physiologically accurate and state-of-the-art model is provided by swine enteroids derived from stem cell-containing crypts from healthy pigs. The objective of this study was to verify the feasibility of two-dimensional swine enteroids as in vitro models for L. intracellularis infection. We established both three- and two-dimensional swine enteroid cultures derived from intestinal crypts. The two-dimensional swine enteroids were infected by L. intracellularis in four independent experiments. Enteroid-infected samples were collected 3 and 7 d postinfection for analysis using real-time quantitative PCR and L. intracellularis immunohistochemistry. In this study, we show that L. intracellularis is capable of infecting and replicating intracellularly in two-dimensional swine enteroids derived from ileum.

Immune response and protection against Lawsonia intracellularis infections in pigs

Lawsonia intracellularis are Gram-negative, obligate intracellular bacteria that cause proliferative enteropathy (PE), an economically important disease for the pig industry. Numerous reviews have been published on the characteristics and pathogenesis of this bacterium since its isolation and taxonomic characterization, with most reviews only partially covering how the host immune response develops during infection and the immune correlates of protection. With the development of increasingly more sophisticated immunological assays and tools for the pig, the immune response against L. intracellularis at distinct stages of pathogenesis has been published. In this review, we discuss current knowledge of the pig immune response against L. intracellularis and strategies to achieve immune protection. The immune response is presented in relation to chronological progression of pathological lesions and clinical symptoms, with emphasis on innate immunity and the adaptive humoral and cell-mediated immune response. The aim is to achieve a comprehensive understanding of the host immune response with respect to the stage-dependent cellular and biochemical processes important during PE development. Also, strategies for development of immune protection and new vaccination technologies are discussed in the light of new discoveries in the field.

Evaluation of mouse enteroids as a model for Lawsonia intracellularis infection

Lawsonia intracellularis, an obligate intracellular bacterium, is an important enteric pathogen in pig herds and horse farms worldwide. The hallmark feature of L. intracellularis infection is the proliferation of epithelial cells in intestinal crypts. A major limitation to the study of L. intracellularis infection is the lack of an in vitro model that reproduces the changes observed in proliferative enteropathy. Here we investigated the suitability of mouse enteroids as a model to study L. intracellularis infection. Mouse enteroids were microinjected with L. intracellularis, filter-sterilized L. intracellularis culture supernatant, or sterile cell culture media (DMEM). L. intracellularis antigen was detected in mouse enteroids by immunohistochemistry and was located mostly in the basal region of the epithelium. There was no differential growth of enteroids among treatment groups, and cellular proliferation was not increased in L. intracellularis-infected enteroids in relation to non-infected enteroids based on immunofluorescence staining. L. intracellularis infection did not induce changes in gene expression of Ki-67 (proliferation marker), Sox9 (marker for transit amplifying cells) and Muc2 (marker for goblet cells). These results indicate that although L. intracellularis antigen is detectable in mouse enteroids, indicating susceptibility to infection, mouse enteroids fail to replicate the cellular proliferation and gene expression changes observed in proliferative enteropathy. Nevertheless, we have successfully demonstrated that mouse enteroids can be used to model days-long intracellular pathogen infection, serving as potential models for the study of other pathogens of interest in veterinary medicine.

The effects of Lawsonia intracellularis, Salmonella enterica serovar Typhimurium and co-infection on IL-8 and TNFα expression in IPEC-J2 cells

Lawsonia intracellularis is among the most important enteric pathogens of swine and has been shown to be a risk factor for increased Salmonella enterica shedding. S. enterica serovar Typhimurium, in addition to being a significant pathogen of swine, also remains one of the most common causes of foodborne illness worldwide. Inflammation and the expression of IL8 and TNFα are an important process in the establishment of S. Typhimurium infection. Yet the effect of L. intracellularis on the expression of these cytokines by enterocytes, the niche both pathogens occupy during infection, is poorly understood. In this study we compared cytokine gene expression between singly and dually infected IPEC-J2 cells, a non-transformed porcine enterocyte cell line. Our results show that L. intracellularis leads to increased expression of IL8 and TNFα and has an additive effect on their expression in co-infection. The increase in expression of inflammatory cytokines may be one mechanism by which L. intracellularis favors S. Typhimurium infection.

Effects of Lawsonia intracellularis infection in the proliferation of different mammalian cell lines

Lawsonia intracellularis is an obligate intracellular bacterium that causes proliferative enteropathy in various animal species. While cellular proliferation of intestinal cells is recognized as the hallmark of L. intracellularis infection in vivo, it has not been demonstrated in in vitro models. In order to assay the effect of L. intracellularis, various cell lines were infected with pathogenic and non-pathogenic passages of the bacterium. Because of the high proliferative rate of these cell lines, serum deprivation, which is known to reduce proliferation, was applied to each of the cell lines to allow the observation of proliferation induced by L. intracellularis. Using antibodies for Ki-67 and L. intracellularis in dual immunofluorescence staining, we observed that L. intracellularis was more frequently observed in proliferating cells. Based on wound closure assays and on the amount of eukaryotic DNA content measured over time, we found no indication that cell lines infected with L. intracellularis increased proliferation and migration when compared to non-infected cells (p > 0.05). Cell arrest due to decreased serum in the culture media was cell-line dependent. Taken together, our findings provide data to support and expand previous subjective observations of the absence of in vitro proliferation caused by L. intracellularis in cell cultures and confirm that cell lines infected by L. intracellularis fail to serve as adequate models for understanding the cellular changes observed in proliferative enteropathy-affected intestines.

Evaluation of the involvement of mice (Mus musculus) in the epidemiology of porcine proliferative enteropathy

The aim of this study was to evaluate the fecal-oral transmission of L. intracellularis between mice and pigs. The study was divided into two parts. The first part aimed to determine whether mice could be infected by feces from pigs that are experimentally infected with L. intracellularis. Thirty-four Swiss mice received L. intracellularis PCR-positive feces from experimentally infected pigs (M1) for four consecutive days. Twelve other mice received swine negative feces (M2). Pools of mice feces were collected on alternating days post-exposure (dpe). The second part of the study aimed to test whether pigs could be infected when exposed to L. intracellularis PCR-positive feces from experimentally infected mice. Twelve 5-week-old pigs received feed mixed with L. intracellularis PCR-positive mice feces (P1), while the other two pigs received PCR-negative mice feces (P2) for four consecutive days. In the first study, the amount of L. intracellularis provided to M1 boxes per day was between 10⁶ and 10⁸. Mice shed, an average of 10⁴ bacterial units every collection day. Three mice from M1 were positive for L. intracellularis by immunohistochemistry (IHC) at the end of the study. In the second part of the study, pigs in P1 received an average of 10⁵ bacterial units per day. Ten pigs were infected by L. intracellularis based on positive qPCR and/or immunohistochemistry and serology results. These pigs shed an average of 10⁴L. intracellularis/g of feces. Mice and pigs experimentally infected with L. intracellularis can infect each other, therefore, rodents should be considered players in the epidemiology of this disease in pig farms.

Recent Advances in Understanding the Pathogenesis of Lawsonia intracellularis Infections

Proliferative enteropathy is an infectious disease caused by an obligate intracellular bacterium, Lawsonia intracellularis, and characterized by thickening of the intestinal epithelium due to enterocyte proliferation. The disease is endemic in swine herds and has been occasionally reported in various other species. Furthermore, outbreaks among foals began to be reported on breeding farms worldwide within the past 5 years. Cell proliferation is directly associated with bacterial infection and replication in the intestinal epithelium. As a result, mild to severe diarrhea is the major clinical sign described in infected animals. The dynamics of L. intracellularis infection in vitro and in vivo have been well characterized, but little is known about the genetic basis for the pathogenesis or ecology of this organism. The present review focuses on the recent advances regarding the pathogenesis and host-pathogen interaction of L. intracellularis infections.

Lawsonia intracellularis infection and proliferative enteropathy in foals

Equine proliferative enteropathy (EPE) is a disease of foals caused by the obligate intracellular organism Lawsonia intracellularis. This organism is unique in that it causes proliferation of infected enterocytes, resulting in thickening of the intestinal epithelium, most often the small intestine. This disease affects mainly weanling foals and causes fever, lethargy, peripheral edema, diarrhea, colic and weight loss. The diagnosis of EPE may be challenging and relies on the presence of hypoproteinemia, thickening of segments of the small intestinal wall observed on abdominal ultrasonography, positive serology and molecular detection of L. intracellularis in feces. The epidemiology and genetic basis for pathogenesis for this disease is beginning to be elucidated. Phenotypic traits, genomic features, and gene expression profiles during L. intracellularis infection in vitro and in vivo are presented. In addition, this article reviews the epidemiology, pathological and clinicopathological findings, diagnosis, and control of EPE.

Comparative genome sequencing identifies a prophage-associated genomic island linked to host adaptation of Lawsonia intracellularis infections

Lawsonia intracellularis is an obligate intracellular bacterium and the causative agent of proliferative enteropathy (PE). The disease is endemic in pigs, emerging in horses and has also been reported in a variety of other animal species, including nonhuman primates. Comparing the whole genome sequences of a homologous porcine L. intracellularis isolate cultivated for 10 and 60 passages in vitro, we identified a 18-kb prophage-associated genomic island in the passage 10 (pathogenic variant) that was lost in the passage 60 (non-pathogenic variant). This chromosomal island comprises 15 genes downstream from the prophage DLP12 integrase gene. The prevalence of this genetic element was evaluated in 12 other L. intracellularis isolates and in 53 infected animals and was found to be conserved in all porcine isolates cultivated for up to 20 passages and was lost in isolates cultivated for more than 40 passages. Furthermore, the prophage region was also present in 26 fecal samples derived from pigs clinically affected with both acute and chronic forms of the disease. Nevertheless, equine L. intracellularis isolates evaluated did not harbor this genomic island regardless of the passage in vitro. Additionally, fecal samples from 21 clinically affected horses and four wild rabbits trapped in horse farms experiencing PE outbreaks did not show this prophage-associated island. Although the presence of this prophage-associated island was not essential for a virulent L. intracellularis phenotype, this genetic element was porcine isolate-specific and potentially contributed to the ecological specialization of this organism for the swine host.

Laser microdissection coupled with RNA-seq analysis of porcine enterocytes infected with an obligate intracellular pathogen (Lawsonia intracellularis)

BACKGROUND

Lawsonia intracellularis is an obligate intracellular bacterium and the etiologic agent of proliferative enteropathy. The disease is endemic in pigs, emerging in horses and has been described in various other species including nonhuman primates. Cell proliferation is associated with bacterial replication in enterocyte cytoplasm, but the molecular basis of the host-pathogen interaction is unknown. We used laser capture microdissection coupled with RNA-seq technology to characterize the transcriptional responses of infected enterocytes and the host-pathogen interaction.

RESULTS

Proliferative enterocytes was associated with activation of transcription, protein biosynthesis and genes acting on the G1 phase of the host cell cycle (Rho family). The lack of differentiation in infected enterocytes was demonstrated by the repression of membrane transporters related to nutrient acquisition. The activation of the copper uptake transporter by infected enterocytes was associated with high expression of the Zn/Cu superoxide dismutase by L. intracellularis. This suggests that the intracellular bacteria incorporate intracytoplasmic copper and express a sophisticated mechanism to cope with oxidative stress.

CONCLUSIONS

The feasibility of coupling microdissection and RNA-seq was demonstrated by characterizing the host-bacterial interactions from a specific cell type in a heterogeneous tissue. High expression of L. intracellularis genes encoding hypothetical proteins and activation of host Rho genes infers the role of unrecognized bacterial cyclomodulins in the pathogenesis of proliferative enteropathy.