Safety and immunogenicity of a live-attenuated auxotrophic candidate vaccine against the intracellular pathogen Rhodococcus equi

Role of riboflavin biosynthesis gene duplication and transporter in Aeromonas salmonicida virulence in marine teleost fish

Active flavins derived from riboflavin (vitamin B₂) are essential for life. Bacteria biosynthesize riboflavin or scavenge it through uptake systems, and both mechanisms may be present. Because of riboflavin's critical importance, the redundancy of riboflavin biosynthetic pathway (RBP) genes might be present. Aeromonas salmonicida, the aetiological agent of furunculosis, is a pathogen of freshwater and marine fish, and its riboflavin pathways have not been studied. This study characterized the A. salmonicida riboflavin provision pathways. Homology search and transcriptional orchestration analysis showed that A. salmonicida has a main riboflavin biosynthetic operon that includes ribD, ribE1, ribBA, and ribH genes. Outside the main operon, putative duplicated genes ribA, ribB and ribE, and a ribN riboflavin importer encoding gene, were found. Monocistronic mRNA ribA, ribB and ribE2 encode for their corresponding functional riboflavin biosynthetic enzyme. While the product of ribBA conserved the RibB function, it lacked the RibA function. Likewise, ribN encodes a functional riboflavin importer. Transcriptomics analysis indicated that external riboflavin affected the expression of a relatively small number of genes, including a few involved in iron metabolism. ribB was downregulated in response to external riboflavin, suggesting negative feedback. Deletion of ribA, ribB and ribE1 showed that these genes are required for A. salmonicida riboflavin biosynthesis and virulence in Atlantic lumpfish (Cyclopterus lumpus). A. salmonicida riboflavin auxotrophic attenuated mutants conferred low protection to lumpfish against virulent A. salmonicida. Overall, A. salmonicida has multiple riboflavin endowment forms, and duplicated riboflavin provision genes are critical for A. salmonicida infection.

Rhodococcus equi Foal Pneumonia: Update on Epidemiology, Immunity, Treatment, and Prevention

Pneumonia in foals caused by the bacterium Rhodococcus equi has a worldwide distribution and is a common cause of disease and death for foals. The purpose of this narrative review is to summarise recent developments pertaining to the epidemiology, immune responses, treatment, and prevention of rhodococcal pneumonia of foals. Screening tests have been used to implement earlier detection and treatment of foals with presumed subclinical R. equi pneumonia to reduce mortality and severity of disease. Unfortunately, this practice has been linked to the emergence of antimicrobial resistant R. equi in North America. Correlates of protective immunity for R. equi infections of foals remain elusive, but recent evidence indicates that innate immune responses are important both for mediating killing and orchestrating adaptive immune responses. A macrolide antimicrobial in combination with rifampin remains the recommended treatment for foals with R. equi pneumonia. Great need exists to identify which antimicrobial combination is most effective for treating foals with R. equi pneumonia and to limit emergence of antimicrobial-resistant strains. In the absence of an effective vaccine against R. equi, passive immunisation remains the only commercially-available method for effectively reducing the incidence of R. equi pneumonia. Because passive immunisation is expensive, labour-intensive, and carries risks for foals, great need exists to develop alternative approaches for passive and active immunisation.

Randomized, controlled trial comparing Rhodococcus equi and poly-N-acetyl glucosamine hyperimmune plasma to prevent R equi pneumonia in foals

Background

Hyperimmune plasma raised against β‐1→6‐poly‐N‐acetyl glucosamine (PNAG HIP) mediates more opsonophagocytic killing of Rhodococcus equi (R equi) than does R equi hyperimmune plasma (RE HIP) in vitro. The relative efficacy of PNAG HIP and RE HIP to protect foals against R equi pneumonia, however, has not been evaluated.

Hypothesis

Transfusion with PNAG HIP will be superior to RE HIP in foals for protection against R equi pneumonia in a randomized, controlled, blinded clinical trial.

Animals

Four hundred sixty Quarter Horse and Thoroughbred foals at 5 large breeding farms in the United States.

Methods

A randomized, controlled, blinded clinical trial was conducted in which foals were transfused within 24 hours after birth with 2 L of either RE HIP or PNAG HIP. Study foals were monitored through weaning for clinical signs of pneumonia by farm veterinarians. The primary outcome was the proportion of foals that developed pneumonia after receiving each type of plasma.

Results

The proportion of foals that developed pneumonia was the same between foals transfused with RE HIP (14%; 32/228) and PNAG HIP (14%; 30/215).

Conclusions and Clinical Importance

Results indicate that PNAG HIP was not superior to a commercially available, United States Department of Agriculture‐licensed RE HIP product for protecting foals against R equi pneumonia under field conditions.

Antibody activities in hyperimmune plasma against the Rhodococcus equi virulence -associated protein A or poly-N-acetyl glucosamine are associated with protection of foals against rhodococcal pneumonia

The efficacy of transfusion with hyperimmune plasma (HIP) for preventing pneumonia caused by Rhodococcus equi remains ill-defined. Quarter Horse foals at 2 large breeding farms were randomly assigned to be transfused with 2 L of HIP from adult donors hyperimmunized either with R. equi (RE HIP) or a conjugate vaccine eliciting antibody to the surface polysaccharide β-1→6-poly-N-acetyl glucosamine (PNAG HIP) within 24 hours of birth. Antibody activities against PNAG and the rhodococcal virulence-associated protein A (VapA), and to deposition of complement component 1q (C՛1q) onto PNAG were determined by ELISA, and then associated with either clinical pneumonia at Farm A (n = 119) or subclinical pneumonia at Farm B (n = 114). Data were analyzed using multivariable logistic regression. Among RE HIP-transfused foals, the odds of pneumonia were approximately 6-fold higher (P = 0.0005) among foals with VapA antibody activity ≤ the population median. Among PNAG HIP-transfused foals, the odds of pneumonia were approximately 3-fold (P = 0.0347) and 11-fold (P = 0.0034) higher for foals with antibody activities ≤ the population median for PNAG or C՛1q deposition, respectively. Results indicated that levels of activity of antibodies against R. equi antigens are correlates of protection against both subclinical and clinical R. equi pneumonia in field settings. Among PNAG HIP-transfused foals, activity of antibodies with C՛1q deposition (an indicator of functional antibodies) were a stronger predictor of protection than was PNAG antibody activity alone. Collectively, these findings suggest that the amount and activity of antibodies in HIP (i.e., plasma volume and/or antibody activity) is positively associated with protection against R. equi pneumonia in foals.

Transfusion With 2 L of Hyperimmune Plasma is Superior to Transfusion of 1 L or Less for Protecting Foals Against Subclinical Pneumonia Attributed to Rhodococcus equi

Transfusing foals with Rhodococcus equi hyperimmune plasma (REHIP) is a standard practice at many horse-breeding farms to help prevent R. equi pneumonia. At many large breeding farms, pneumonia is most commonly recognized as subclinical based on thoracic ultrasonography findings. The efficacy of REHIP transfusion and the impact of the volume of plasma transfused for reducing the cumulative incidence of subclinical R. equi pneumonia are unknown. A retrospective cohort study was conducted among foals born and residing through weaning at a large breeding farm. Foals were transfused with either 0 L (n = 2 foals), 1 L (n = 85 foals), or 2 L (n = 62 foals) of REHIP within 36 hours of birth. Volume transfused was principally based on intended use of the foals. All foals at the ranch were routinely screened using thoracic ultrasonography at 5, 7, and 9 weeks of age to detect subclinical pneumonia attributed to R. equi based on farm history. The proportion of the foals receiving < 1 L REHIP that developed subclinical pneumonia (32%; 26/82) was significantly (P = .0068; chi-squared test) greater than that among foals transfused with 2 L of REHIP (12%; 8/68). Despite the important limitations of this observational study, it provides evidence supporting the need for well-designed clinical trials to evaluate the impact of the use and dose of REHIP for preventing subclinical pneumonia. Reducing the incidence of subclinical pneumonia is important because reducing antibiotic treatment of subclinical cases will decrease selection pressure for antimicrobial resistance in R. equi.

Guinea pig infection with the intracellular pathogen Rhodococcus equi

Rhodococcus equi is an opportunistic, intracellular pathogen that causes pyogranulomatous pneumonia in foals and immunocompromised people. Currently, there is no experimental model of R. equi pneumonia other than intra-bronchial experimental infection of foals with R. equi, which is labor-intensive and costly. This study's objective was to develop a guinea pig (GP) model of R. equi pneumonia that would facilitate development of novel approaches for controlling and preventing this disease. Guinea pigs were infected with either 10¹, 10², 10³, or 10⁴ colony forming units (CFUs) of a virulent strain of R. equi using a Madison aerosol chamber, or 10⁶ or 10⁷ CFUs of this strain intratracheally. Animals were monitored daily for clinical signs of pneumonia, and were euthanized and necropsied on days 1, 3, 7, or 35 post-infection (PI). Lung homogenates were plated onto selective agar to determine bacterial load. No clinical signs of disease were observed regardless of the inoculum dose or infection method. No bacteria were recovered from GPs euthanized at 35 days PI. Histology and immunostaining of T-cells, B-cells, and macrophages in lungs showed that inflammatory responses in infected GPs were similarly unremarkable irrespective of dose or route of infection. Guinea pigs appear to be resistant to pulmonary infection with virulent R. equi even at doses that reliably produce clinical pneumonia in foals.

In contrast to other species, α-Galactosylceramide (α-GalCer) is not an immunostimulatory NKT cell agonist in horses

α-GalCer is a potent immunomodulatory molecule that is presented to NKT cells via the CD1 antigen-presenting system. We hypothesized that when used as an adjuvant α-GalCer would induce protective immune responses against Rhodococcus equi, an important pathogen of young horses. Here we demonstrate that the equine CD1d molecule shares most features found in CD1d from other species and has a suitable lipid-binding groove for presenting glycolipids to NKT cells. However, equine CTL stimulated with α-GalCer failed to kill cells infected with R. equi, and α-GalCer did not increase killing by CTL co-stimulated with R. equi antigen. Likewise, α-GalCer did not induce the lymphoproliferation of equine PBMC or increase the proliferation of R. equi-stimulated cells. Intradermal injection of α-GalCer in horses did not increase the recruitment of lymphocytes or cytokine production. Furthermore, α-GalCer-loaded CD1d tetramers, which have been shown to be broadly cross-reactive, did not bind equine lymphocytes. Altogether, our results demonstrate that in contrast to previously described species, horses are unable to respond to α-GalCer. This raises questions about the capabilities and function of NKT cells and other lipid-specific T lymphocytes in horses.

Analyses of lipid rafts, Toll-like receptors 2 and 4, and cytokines in foals vaccinated with Virulence Associated Protein A/CpG oligonucleotide vaccine against Rhodococcus equi

Rhodococcus equi establishes long-term pulmonary infection, survives in phagolysosomes of alveolar macrophages and causes pneumonia in foals. The failure of the foal to clear R. equi bacteria is believed to be due to its inability to produce IFN-γ and defects in Toll-like receptor(TLR) signaling. Lipid rafts sequester immune receptors such as TLRs and facilitate efficient cell signaling and therefore, a deficiency in accumulation of receptors in lipid rafts may result in failure to activate. We tested whether a Virulence Associated Protein A (VapA)/CpG vaccine against R. equi would impact the production of IL-10, IFN-γ and TNF-α in lung tissue and fluid samples, alter expression of TLR2 and TLR4 and alter their association with the lipid rafts in broncho-alveolar lavage (BAL) cells. Eight foals, 1–6 days of age, were vaccinated against R. equi followed by a booster at day 14 and challenged with R. equi (5 x 10(6) CFU/ml;10 ml) on day 28. This group was termed "vaccinated pre-challenge" before the infection and "vaccinated post-challenge" after the infection. A second group of foals (n = 7) was not vaccinated but challenged with R. equi on day 28 of the study. This group was termed "non-vaccinated pre-challenge" and after infection with R. equi was named "non-vaccinated post-challenged. We report adaptation of previous protocols to isolate plasma membrane fractions from BAL cells and identification of lipid raft fractions based on the presence of flotillin-1 and GM-1 and absence of transferrin receptor. TLR2 and TLR4 were restricted to plasma membrane fractions 7–9 of alveolar cells collected from vaccinated foals before and after the challenge. Western blots showed that vaccinated post-challenge foals had higher expression of TLR2 in their lung tissues compared to non-vaccinated pre-challenge foals. TNF- concentration was higher in BAL fluid collected from the vaccinated compared to the non-vaccinated foals on day 28. Lung tissue extracts collected on day 49 from the non-vaccinated R. equi challenged foals showed higher expression of IL-10 compared to the vaccinated-challenged foals. However, there were no differences among the groups with respect to the concentration of IFN-γ in BAL fluid or lung tissue extracts. Taken together, we modified previous protocols to isolate plasma membrane fractions from BAL cells of foals and report that the vaccination with a VapA/CPG vaccine increases association of TLR2 and TLR4 with lipid raft fractions and alters expression of TNF-α and IL-10. The data point to a subtle effect of vaccination on the association of TLR2 and TLR4 with lipid rafts in BAL cells.

Immunogenicity of an electron beam inactivated Rhodococcus equi vaccine in neonatal foals

Rhodococcus equi is an important pathogen of foals that causes severe pneumonia. To date, there is no licensed vaccine effective against R. equi pneumonia of foals. The objectives of our study were to develop an electron beam (eBeam) inactivated vaccine against R. equi and evaluate its immunogenicity. A dose of eBeam irradiation that inactivated replication of R. equi while maintaining outer cell wall integrity was identified. Enteral administration of eBeam inactivated R. equi increased interferon-γ production by peripheral blood mononuclear cells in response to stimulation with virulent R. equi and generated naso-pharyngeal R. equi-specific IgA in newborn foals. Our results indicate that eBeam irradiated R. equi administered enterally produce cell-mediated and upper respiratory mucosal immune responses, in the face of passively transferred maternal antibodies, similar to those produced in response to enteral administration of live organisms (a strategy which previously has been documented to protect foals against intrabronchial infection with virulent R. equi). No evidence of adverse effects was noted among vaccinated foals.

The effectiveness of anti-R. equi hyperimmune plasma against R. equi challenge in thoroughbred Arabian foals of mares vaccinated with R. equi vaccine

This study aimed to determine the effectiveness of a pregnant mare immunization of a Rhodococcus equi (R. equi) vaccine candidate containing a water-based nanoparticle mineral oil adjuvanted (Montanide IMS 3012) inactive bacterin and virulence-associated protein A (VapA), as well as the administration of anti-R. equi hyperimmune (HI) plasma against R. equi challenge in the mares' foals. The efficacy of passive immunizations (colostral passive immunity by mare vaccination and artificial passive immunity by HI plasma administration) was evaluated based on clinical signs, complete blood count, blood gas analysis, serological response (ELISA), interleukin-4 (IL-4) and interferon gamma (IFN-γ), total cell count of the bronchoalveolar lavage fluids (BALF) samples, reisolation rate of R. equi from BALF samples (CFU/mL), lung samples (CFU/gr), and lesion scores of the organs and tissue according to pathological findings after necropsy in the foals. The vaccination of pregnant mares and HI plasma administration in the foals reduced the severity of R. equi pneumonia and lesion scores of the organs and tissue by 3.54-fold compared to the control foals. This study thus indicates that immunization of pregnant mares with R. equi vaccine candidate and administration of HI plasma in mares' foals effectively protect foals against R. equi challenge.

Mouse lung infection model to assess Rhodococcus equi virulence and vaccine protection

The pathogenic actinomycete Rhodococcus equi causes severe purulent lung infections in foals and immunocompromised people. Although relatively unsusceptible to R. equi, mice are widely used for in vivo studies with this pathogen. The most commonly employed mouse model is based on systemic (intravenous) infection and determination of R. equi burdens in spleen and liver. Here, we investigated the murine lung for experimental infection studies with R. equi. Using a 10(7)CFU intranasal challenge in BALB/c mice, virulent R. equi consistently survived in quantifiable numbers up to 10 days in the lungs whereas virulence-deficient R. equi bacteria were rapidly cleared. An internally controlled virulence assay was developed in which the test R. equi strains are co-inoculated and monitored in the same mouse. Isogenic R. equi bacteria lacking either the plasmid vapA gene or the entire virulence plasmid were compared using this competitive assay. Both strains showed no significant differences in in vivo fitness in the lung, indicating that the single loss of the virulence factor VapA was sufficient to account for the full attenuation seen in the absence of the virulence plasmid. To test the adequacy of the lung infection model for monitoring R. equi vaccine efficacy, BALB/c mice were immunized with live R. equi and challenged intranasally. Vaccination conferred protection against acute pulmonary challenge with virulent R. equi. Our data indicate that the murine lung infection model provides a useful tool for both R. equi virulence and vaccine studies.

Rhodococcus equi: the many facets of a pathogenic actinomycete

Rhodococcus equi is a soil-dwelling pathogenic actinomycete that causes pulmonary and extrapulmonary pyogranulomatous infections in a variety of animal species and people. Young foals are particularly susceptible and develop a life-threatening pneumonic disease that is endemic at many horse-breeding farms worldwide. R. equi is a facultative intracellular parasite of macrophages that replicates within a modified phagocytic vacuole. Its pathogenicity depends on a virulence plasmid that promotes intracellular survival by preventing phagosome-lysosome fusion. Species-specific tropism of R. equi for horses, pigs and cattle appears to be determined by host-adapted virulence plasmid types. Molecular epidemiological studies of these plasmids suggest that human R. equi infection is zoonotic. Analysis of the recently determined R. equi genome sequence has identified additional virulence determinants on the bacterial chromosome. This review summarizes our current understanding of the clinical aspects, biology, pathogenesis and immunity of this fascinating microbe with plasmid-governed infectivity.

Development of a live, attenuated, potential vaccine strain of R. equi expressing vapA and the virR operon, and virulence assessment in the mouse

Pneumonia caused by Rhodococcus equi remains a significant problem in foals. The objective of this study was to develop a safe and efficacious attenuated strain of R. equi for eventual use in oral immunization of foals. The approach involved expression of vapA in a live, virulence plasmid-negative, strain of R. equi (strain 103-). PCR-amplified fragments of the vapA gene, with and without the upstream genes virR, orf5, vapH, orf7 and orf8 (orf4-8), were cloned into a shuttle vector pNBV1. These plasmids, named pAW48A and pAWVapA respectively, were electroporated into strain 103-. The presence of the recombinant vectors in the attenuated strain (103-) and the integrity of the inserted genes were confirmed, and both constructs expressed VapA. The virulence of the two strains was compared to that of wild type R. equi 103+ and negative controls by their intravenous inoculation into mice, followed by examination of liver clearance 4 days later. Mice inoculated with R. equi 103-, 103-/pAWVapA and 103-/pNBV1 completely cleared infection, whereas strain 103-/pAW48A persisted in 47% of mice.

Mucosal co-immunization of mice with recombinant lactococci secreting VapA antigen and leptin elicits a protective immune response against Rhodococcus equi infection

Rhodococcus equi causes severe pneumonia in foals and has recently gained attention as a significant opportunistic pathogen in immunocompromised humans. However, no effective vaccine to prevent rhodococcosis is currently available. In this study, we have engineered the food-grade bacterium Lactococcus lactis to secrete the virulence-associated protein A from R. equi (LL-VapA). The immunogenic potential of LL-VapA strain was then evaluated after either intragastric or intranasal immunization in mice either alone or in combination with LL-Lep, a recombinant strain of L. lactis secreting biologically active leptin, a pleiotropic hormone with significant immunomodulatory properties. Intragastric administration of LL-VapA led to the highest VapA-specific mucosal response whereas intranasal administration led to the highest systemic immune responses. Cytokines released from in vitro-stimulated spleen cells show both a strong IFN-γ response and an increase of IL-4 level in all immunized groups, except for the group intranasally co-administered with both LL-VapA and LL-Lep. Strikingly, a significant reduction in R. equi viable counts in liver and spleen was observed four days after intravenous challenge with a virulent strain of R. equi in all immunized groups except for the group vaccinated by intragastric route with LL-VapA. Altogether, our results demonstrate that LL-VapA can evoke a T(H)1-based protective immune response in intranasally immunized mice. This response is enhanced when co-administered with LL-Lep strain, whereas only co-administration of LL-VapA and LL-Lep can induce a protective immune response in intragastric vaccinated mice, associated with a T(H)1/T(H)2 cytokine response.

The steroid catabolic pathway of the intracellular pathogen Rhodococcus equi is important for pathogenesis and a target for vaccine development

Rhodococcus equi causes fatal pyogranulomatous pneumonia in foals and immunocompromised animals and humans. Despite its importance, there is currently no effective vaccine against the disease. The actinobacteria R. equi and the human pathogen Mycobacterium tuberculosis are related, and both cause pulmonary diseases. Recently, we have shown that essential steps in the cholesterol catabolic pathway are involved in the pathogenicity of M. tuberculosis. Bioinformatic analysis revealed the presence of a similar cholesterol catabolic gene cluster in R. equi. Orthologs of predicted M. tuberculosis virulence genes located within this cluster, i.e. ipdA (rv3551), ipdB (rv3552), fadA6 and fadE30, were identified in R. equi RE1 and inactivated. The ipdA and ipdB genes of R. equi RE1 appear to constitute the α-subunit and β-subunit, respectively, of a heterodimeric coenzyme A transferase. Mutant strains RE1ΔipdAB and RE1ΔfadE30, but not RE1ΔfadA6, were impaired in growth on the steroid catabolic pathway intermediates 4-androstene-3,17-dione (AD) and 3aα-H-4α(3′-propionic acid)-5α-hydroxy-7aβ-methylhexahydro-1-indanone (5α-hydroxy-methylhexahydro-1-indanone propionate; 5OH-HIP). Interestingly, RE1ΔipdAB and RE1ΔfadE30, but not RE1ΔfadA6, also displayed an attenuated phenotype in a macrophage infection assay. Gene products important for growth on 5OH-HIP, as part of the steroid catabolic pathway, thus appear to act as factors involved in the pathogenicity of R. equi. Challenge experiments showed that RE1ΔipdAB could be safely administered intratracheally to 2 to 5 week-old foals and oral immunization of foals even elicited a substantial protective immunity against a virulent R. equi strain. Our data show that genes involved in steroid catabolism are promising targets for the development of a live-attenuated vaccine against R. equi infections.

Rhodococcus equi causes fatal pyogranulomatous bronchopneumonia in young foals and is an emerging opportunistic pathogen of immunocompromised humans. Despite its importance, there is currently no safe and effective vaccine against R. equi infections. Like Mycobacterium tuberculosis, the causative agent of human tuberculosis, R. equi is able to infect, survive and multiply inside alveolar macrophages. Recently we have shown that essential steps in the cholesterol catabolic pathway (encoded by the rv3551, rv3552, fadE30 genes) are involved in the pathogenicity of M. tuberculosis. We hypothesized that the orthologous genes in the cholesterol catabolic gene cluster of R. equi also are essential for its virulence mechanism. Analysis of the respective R. equi strain RE1 mutants revealed that they were impaired in growth on intermediates of the steroid catabolic pathway and had attenuated phenotypes in a macrophage infection assay. Mutant RE1ΔipdAB, carrying a deletion of the orthologs of rv3551 and rv3552, could be safely administered to 2–5 week-old foals intratracheally and oral immunization provided a substantial protection against infection by a virulent R. equi strain. Our data show that genes important for methylhexahydroindanone propionate degradation, part of the steroid catabolic pathway, are promising targets for the development of a live-attenuated vaccine against R. equi infections.

Rhodococcus equi venous catheter infection: a case report and review of the literature

Introduction

Rhodococcus equi is an animal pathogen that was initially isolated from horses and is being increasingly reported as a cause of infection in humans with impaired cellular immunity. However, this pathogen is underestimated as a challenging antagonist and is frequently considered to be a mere contaminant despite the potential for life-threatening infections. Most case reports have occurred in immunocompromised patients who have received organ transplants (for example kidney, heart, bone marrow) or those with human immunodeficiency virus infection. Infections often manifest as pulmonary involvement or soft tissue abscesses. Bacteremia related to R. equi infections of tunneled central venous catheters has rarely been described.

Case presentation

We report the case of a 63-year-old non-transplant recipient, non-HIV infected Caucasian woman with endometrial carcinoma who developed recurrent bloodstream infections and septic shock due to R. equi and ultimately required the removal of her port catheter, a subcutaneous implantable central venous catheter. We also review the medical literature related to human infections with R. equi.

Conclusion

R. equi should be considered a serious pathogen, not a contaminant, particularly in an immunocompromised patient who presents with a central venous catheter-related bloodstream infection. Counseling patients with central venous catheters who participate in activities involving exposure to domesticated animals is recommended.

An atypical riboflavin pathway is essential for Brucella abortus virulence

Brucellosis is a worldwide zoonosis that affects livestock and humans and is caused by closely related Brucella spp., which are adapted to intracellular life within cells of a large variety of mammals. Brucella can be considered a furtive pathogen that infects professional and non-professional phagocytes. In these cells Brucella survives in a replicative niche, which is characterized for having a very low oxygen tension and being deprived from nutrients such as amino acids and vitamins. Among these vitamins, we have focused on riboflavin (vitamin B2). Flavin metabolism has been barely implicated in bacterial virulence. We have recently described that Brucella and other Rhizobiales bear an atypical riboflavin metabolic pathway. In the present work we analyze the role of the flavin metabolism on Brucella virulence. Mutants on the two lumazine synthases (LS) isoenzymes RibH1 and RibH2 and a double RibH mutant were generated. These mutants and different complemented strains were tested for viability and virulence in cells and in mice. In this fashion we have established that at least one LS must be present for B. abortus survival and that RibH2 and not RibH1 is essential for intracellular survival due to its LS activity in vivo. In summary, we show that riboflavin biosynthesis is essential for Brucella survival inside cells or in mice. These results highlight the potential use of flavin biosynthetic pathway enzymes as targets for the chemotherapy of brucellosis.

Current understanding of the equine immune response to Rhodococcus equi. An immunological review of R. equi pneumonia

Rhodococcus equi is recognised to cause chronic purulent bronchopneumonia in foals of less than 6 months of age. Virulent strains of the bacteria possess a large 80-90 kb plasmid encoding several virulence-associated proteins, including virulence-associated protein A (VapA), which is associated with disease. R. equi pneumonia can represent significant costs and wastage to the equine breeding industry, especially on stud farms where the disease is endemic. This article reviews knowledge of the equine immune response, both in the immune adult and susceptible neonate, with respect to this pathogen. Humoral immune responses are addressed, with a discussion on the use of hyperimmune and normal adult equine plasma as prophylactic tools. The role that innate immune mechanisms play in the susceptibility of some foals to R. equi infection is also highlighted. Likewise, cell-mediated immune components are reviewed, with particular attention directed towards research undertaken to develop an effective vaccine for foals. It is possible that the implementation of a single immunoprophylaxis strategy to prevent R. equi infection on farms will yield disappointing results. Combined prophylactic protocols that address husbandry practices, environmental and aerosol contamination levels, enhancement of innate immunity, good quality hyperimmune plasma for the neonate, and vaccinal efficacy in the developing foal may be required.

Molecular and infection biology of the horse pathogen Rhodococcus equi

The soil actinomycete Rhodococcus equi is a pulmonary pathogen of young horses and AIDS patients. As a facultative intracellular bacterium, R. equi survives and multiplies in macrophages and establishes its specific niche inside the host cell. Recent research into chromosomal virulence factors and into the role of virulence plasmids in infection and host tropism has presented novel aspects of R. equi infection biology and pathogenicity. This review will focus on new findings in R. equi biology, the trafficking of R. equi-containing vacuoles inside host cells, factors involved in virulence and host resistance and on host-pathogen interaction on organismal and cellular levels.