Recent isolations of Rhodococcus equi from cavitatory pulmonary disease in patients with AIDS have aroused interest among medical microbiologists in this unusual organism. Earlier isolations from humans had also been in immunosuppressed patients following hemolymphatic tumors or renal transplantation. This organism has been recognized for many years as a cause of a serious pyogranulomatous pneumonia of young foals and is occasionally isolated from granulomatous lesions in several other species, in some cases following immunosuppression. The last decade has seen many advances in understanding of the epidemiology, pathogenesis, diagnosis, treatment, and immunity to infection in foals. The particular susceptibility of the foal is not understood but can be explained in part by a combination of heavy challenge through the respiratory route coinciding with declining maternally derived antibody in the absence of fully competent foal cellular immune mechanisms. R. equi is largely a soil organism but is widespread in the feces of herbivores. Its growth in soil is considerably improved by simple nutrients it obtains from herbivore manure. About one-third of human patients who have developed R. equi infections had contact in some way with herbivores or their manure. Others may have acquired infection from contact with soil or wild bird manure. R. equi is an intracellular parasite, which explains the typical pyogranulomatous nature of R. equi infections, the predisposition to infection in human patients with defective cell-mediated immune mechanisms, and the efficacy of antimicrobial drugs that penetrate phagocytic cells.

Necrotic enteritis (NE) is one of the most important enteric diseases in poultry and is a high cost to the industry worldwide. It is caused by avian-specific, Necrotic Enteritis Beta toxin (NetB)-producing, strains of Clostridium perfringens that also possess in common other virulence-associated genes. In Europe the disease incidence has increased since the ban on in-feed “growth promoting” antibiotics. Because of this, many recent studies of NE have focused on finding different ways to control the disease, and on understanding its pathogenesis. Frustratingly, reproduction of the disease has proven impossible for some researchers. This review describes and discusses factors known to be important in reproducing the disease experimentally, as well as other considerations in reproducing the disease. The critical bacterial factor is the use of virulent, netB-positive, strains; virulence can be enhanced by using tpeL- positive strains and by the use of young rather than old broth cultures to increase toxin expression. Intestinal damaging factors, notably the use of concurrent or preceding coccidial infection, or administration of coccidial vaccines, combined with netB-positive C. perfringens administration, can also be used to induce NE. Nutritional factors, particularly feeding high percentage of cereals containing non-starch polysaccharides (NSP) (wheat, rye, and barley) enhance disease by increasing digesta viscosity, mucus production and bacterial growth. Animal proteins, especially fish meal, enhance C. perfringens proliferation and toxin production. Other factors are discussed that may affect outcome but for which evidence of their importance is lacking. The review compares the different challenge approaches; depending on the aim of particular studies, the different critical factors can be adjusted to affect the severity of the lesions induced. A standardized scoring system is proposed for international adoption based on gross rather than histopathological lesions; if universally adopted this will allow better comparison between studies done by different researchers. Also a scoring system is provided to assist decisions on humane euthanasia of sick birds.

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The virulence plasmids of the equine virulent strains Rhodococcus equi ATCC 33701 and 103 were sequenced, and their genetic structure was analyzed. p33701 was 80,610 bp in length, and p103 was 1 bp shorter; their sequences were virtually identical. The plasmids contained 64 open reading frames (ORFs), 22 of which were homologous with genes of known function and 3 of which were homologous with putative genes of unknown function in other species. Putative functions were assigned to five ORFs based on protein family characteristics. The most striking feature of the virulence plasmids was the presence of a 27,536-bp pathogenicity island containing seven virulence-associated protein (vap) genes, including vapA. These vap genes have extensive homology to vapA, which encodes a thermoregulated and surface-expressed protein. The pathogenicity island contained a LysR family transcriptional regulator and a two-component response regulator upstream of six of the vap genes. The vap genes were present as a cluster of three (vapA, vapC, and vapD), as a pair (vapE and vapF), or individually (vapG; vapH). A region of extensive direct repeats of unknown function, possibly associated with thermoregulation, was present immediately upstream of the clustered and the paired genes but not the individual vap genes. There was extensive homology among the C-terminal halves of all vap genes but not generally among the N-terminal halves. The remainder of the plasmid consisted of a large region which appears to be associated with conjugation functions and a large region which appears to be associated with replication and partitioning functions.Rhodococcus equi is an important pulmonary pathogen of foals and is increasingly isolated from pneumonic infections and other infections in human immunodeficiency virus (HIV)-infected patients (19,33). Isolates from foals possess a large virulence plasmid, varying in size from 80 to 90 kb (45,47,49). Isolates lacking the plasmid are avirulent to foals (16,51). Little is known about the function of the plasmid apart from its encoding a virulence-associated surface protein (VapA) (45,49), the presence of a family of four vap genes (5), and the origin of replication (53). Infection with R. equi bacteria carrying the virulence plasmid may lead to immunomodulation in foals by causing failure to mount an effective Th1-based cellular immune response, but the basis of this effect is undefined (17). The expression of VapA is thermoregulated (Ն34°C) and pH regulated (41, 42), so that in this respect the plasmid has similarities to the virulence plasmids of pathogenic Yersinia species, such as Yersinia pestis, and of Shigella species (11,22,30). The plasmid is of significant interest, since it is associated with survival of the bacterium inside macrophages (16,21,33). Understanding its structure and function may therefore yield insights not only into the basis of virulence of this organism but also into the mechanisms of macrophage survival of other facultative intracellular pathogens, including Mycobacterium tub...

We report the genome of the facultative intracellular parasite Rhodococcus equi, the only animal pathogen within the biotechnologically important actinobacterial genus Rhodococcus. The 5.0-Mb R. equi 103S genome is significantly smaller than those of environmental rhodococci. This is due to genome expansion in nonpathogenic species, via a linear gain of paralogous genes and an accelerated genetic flux, rather than reductive evolution in R. equi. The 103S genome lacks the extensive catabolic and secondary metabolic complement of environmental rhodococci, and it displays unique adaptations for host colonization and competition in the short-chain fatty acid–rich intestine and manure of herbivores—two main R. equi reservoirs. Except for a few horizontally acquired (HGT) pathogenicity loci, including a cytoadhesive pilus determinant (rpl) and the virulence plasmid vap pathogenicity island (PAI) required for intramacrophage survival, most of the potential virulence-associated genes identified in R. equi are conserved in environmental rhodococci or have homologs in nonpathogenic Actinobacteria. This suggests a mechanism of virulence evolution based on the cooption of existing core actinobacterial traits, triggered by key host niche–adaptive HGT events. We tested this hypothesis by investigating R. equi virulence plasmid-chromosome crosstalk, by global transcription profiling and expression network analysis. Two chromosomal genes conserved in environmental rhodococci, encoding putative chorismate mutase and anthranilate synthase enzymes involved in aromatic amino acid biosynthesis, were strongly coregulated with vap PAI virulence genes and required for optimal proliferation in macrophages. The regulatory integration of chromosomal metabolic genes under the control of the HGT–acquired plasmid PAI is thus an important element in the cooptive virulence of R. equi.