Rhodococcus bacteremia in cancer patients is mostly catheter related and associated with biofilm formation

Complementary and Alternative Therapies for Psoriasis

Despite recent advancements in psoriasis treatment, challenges in management persist. Recently, there has been a rising interest amongst patients in complementary and alternative medicines (CAM), driven by the desire for more natural, holistic approaches and dissatisfaction with conventional treatments. Up to 41% of patients with psoriasis reported using alternative therapies and 39.5% use complementary therapies (Murphy EC, Nussbaum D, Prussick R, Friedman AJ (2019) Use of complementary and alternative medicine by patients with psoriasis. J Am Acad Dermatol 81:280-283). Despite their rapidly growing prevalence, literature on CAM therapies for psoriasis is lacking, making their recommendation difficult. Since the last systematic review on this topic published in 2018, evidence for new alternative therapies has emerged, promoting a further investigation of their efficacy (Gamret AC, Price A, Fertig RM, Lev-Tov H, Nichols AJ (2018) Complementary and Alternative Medicine Therapies for Psoriasis: A Systematic Review. JAMA Dermatol 154:1330-1337). This systematic review aims to compile recent literature on the most studied alternative therapies for psoriasis and further discuss their effectiveness in order to counsel clinicians in guiding patients on the use of these non-standard approaches. A literature search was conducted in the PubMed, EMBASE, Cochrane Central Register of Controlled Trials, and Clinicaltrials.gov databases for randomized controlled trials (RCT) on complementary and alternative therapies in psoriasis from March 2018 through April 2024, resulting in 12 studies being included in this review. The preliminary results for many treatments such as curcumin, dietary modification and additions, indigo naturalis, meditation, acupuncture, and balneotherapy showed positive clinical effects. However, additional well-designed randomized trials are needed to confirm the potential beneficial effects and to establish safety of use.

Peritoneal Dialysis-Related Recurrent Rhodococcus corynebacterioides Peritonitis: A Case Report and Review of Literature

Rhodococcus corynebacterioides is a Gram-positive bacterium known to cause bacteremia and oligoarthritis. There have been only a few case reports in the literature that describe its association with peritoneal dialysis (PD)-related peritonitis. We report a case of recurrent peritonitis caused by R. corynebacterioides. The patient presented with abdominal pain, and PD fluid analysis was positive for infection, with cultures growing R. corynebacterioides. The patient was treated with multiple courses of intraperitoneal antibiotics due to recurrent episodes of PD-associated peritonitis from this bacterium, ultimately necessitating the removal of the PD catheter and the transition to hemodialysis.

Bloodstream infection by Rhodococcus corynebacterioides in a pediatric patient diagnosed with high-risk retinoblastoma

Rhodococcus is a pathogen that is known to cause infections in animals and humans, mainly in cases of immunocompromised patients. A case of a pediatric cancer patient suffering from a bloodstream infection caused by Rhodococcus corynebacterioides was described in this work. Gram positive rods were isolated from blood cultures. The target bacterium was identified using a combination of biochemical tests, the MALDI-TOF mass spectrometry technique, and the analysis of the 16S rRNA sequence. Moreover, an antimicrobial susceptibility test was performed using the E-test. The isolated bacterium was identified as R. corynebacterioides. The 3-year-old patient was successfully treated with vancomycin and meropenem. This is the first published report of R. corynebacterioides in a pediatric patient diagnosed with retinoblastoma that developed a bloodstream infection. R. corynebacterioides should be considered among the opportunistic infectious agents affecting pediatric cancer patients.

Laboratory Plasticware Induces Expression of a Bacterial Virulence Factor

Pollution with microplastic has become a prime environmental concern. The various ways in which human-made polymers and microorganisms interact are little understood, and this is particularly true for microplastic and pathogenic microorganisms. Previous reports demonstrated that expression of central virulence-associated protein A (VapA) of the pathogenic bacterium Rhodococcus equi is shut off at 30°C, whereas it is strongly expressed at 37°C, a temperature which may serve as an intrahost cue. Here, we show that cultivation at 30°C in disposable plastic tubes increases mRNA levels of vapA 70-fold compared to growth in conventional glass tubes. Strong expression of vapA in plastic tubes does not seem to be caused by a compound leaching from plastic but rather by tube surface properties. Expression stimulation during growth in plastic is regulated by the R. equi transcription regulators VirR and VirS, indicating that plastic-induced vapA expression is (co)regulated through the canonical vapA expression pathway. Our observations have important implications for the future analysis and assessment of environmental microplastic contaminations in that they show that, in principle, contact of pathogens with environmental plastic can increase their virulence.

IMPORTANCE Millions of tons small plastic pieces (microplastic) find their way into the environment every year. They pose digestive and toxicity problems to various life forms in soil, freshwater, and seawater. Additionally, microplastic offers an opportunity for microorganisms to attach and to become an important part of a “plastisphere community.” The significance of our study lies in the documentation of a sharp increase in production of a central virulence factor by a bacterial pathogen when the bacterium is in touch with certain makes of plastic. Although this feature may not reflect an increased health risk in case of this particular soilborne pathogen, our data disclose a new facet of how microplastics can endanger life.

Clinicopathological and radiographic features in 40 cats diagnosed with pulmonary and cutaneous Rhodococcus equi infection (2012-2018)

OBJECTIVES

This retrospective study aimed to describe clinical manifestations, diagnostic options, radiological features, therapeutic plans and outcomes for cats infected with Rhodococcus equi.

METHODS

Forty cats aged between 2 months and 11 years old (median 6 months) that were definitively diagnosed with rhodococcosis between 2012 and 2018 were recruited in this study. Medical records were reviewed for information on signalment, history, clinical presentation, diagnostic testing, treatment plans and clinical outcomes.

RESULTS

Of the 40 cats, 36 showed the pulmonary form of the disease, with 35 (87.5%) presenting with dyspnoea, while four cats presented with only cutaneous lesions. Mean body temperature was 38.7 ± 0.2°C. Dyspnoea was noted in 87.5% of the cats. Leukocytosis (58.3%) with band neutrophilia (83.3%), monocytosis (58.3%) and thrombocytopenia (55.5%) were prominent findings in the haematology reports. Hyperproteinaemia (61.1%) with hypoalbuminaemia (22.2%) and hyperglobulinaemia (63.8%) with a low albumin:globulin ratio (38.9%) were prominent features of blood biochemistry reports. An alveolar-interstitial pattern was noted in 75% of pre-thoracocentesis radiographs. Pleural effusion, hepatomegaly, thoracic lymphadenopathy and atelectasis of any lung lobe were seen in 88.9%, 75%, 41.7% and 36.1% of cats, respectively. Overall, the mortality rate was 67.5% in both forms.

CONCLUSIONS AND RELEVANCE

Clinicians should be aware that feline rhodococcosis manifests as a pulmonary disease at a much higher rate than previously reported. Further studies are required to address the epidemiology, pathophysiology, disease management and prognosis of feline rhodococcosis. The role of immunosuppression as a predisposing factor in feline rhodococcosis requires further investigation.

Functional Genomics Insights Into the Pathogenicity, Habitat Fitness, and Mechanisms Modifying Plant Development of Rhodococcus sp. PBTS1 and PBTS2

Pistachio Bushy Top Syndrome (PBTS) is a recently emerged disease that has strongly impacted the pistachio industry in California, Arizona, and New Mexico. The disease is caused by two bacteria, designated PBTS1 that is related to Rhodococcus corynebacterioides and PBTS2 that belongs to the species R. fascians. Here, we assessed the pathogenic character of the causative agents and examined their chromosomal sequences to predict the presence of particular functions that might contribute to the observed co-occurrence and their effect on plant hosts. In diverse assays, we confirmed the pathogenicity of the strains on "UCB-1" pistachio rootstock and showed that they can also impact the development of tobacco species, but concurrently inconsistencies in the ability to induce symptoms were revealed. We additionally evidence that fas genes are present only in a subpopulation of pure PBTS1 and PBTS2 cultures after growth on synthetic media, that these genes are easily lost upon cultivation in rich media, and that they are enriched for in an in planta environment. Analysis of the chromosomal sequences indicated that PBTS1 and PBTS2 might have complementary activities that would support niche partitioning. Growth experiments showed that the nutrient utilization pattern of both PBTS bacteria was not identical, thus avoiding co-inhabitant competition. PBTS2 appeared to have the potential to positively affect the habitat fitness of PBTS1 by improving its resistance against increased concentrations of copper and penicillins. Finally, mining the chromosomes of PBTS1 and PBTS2 suggested that the bacteria could produce cytokinins, auxins, and plant growth-stimulating volatiles and that PBTS2 might interfere with ethylene levels, in support of their impact on plant development. Subsequent experimentation supported these in silico predictions. Altogether, our data provide an explanation for the observed pathogenic behavior and unveil part of the strategies used by PBTS1 and PBTS2 to interact with plants.

Phenotypic Characterization of Rhodococcus equi Biofilm Grown In Vitro and Inhibiting and Dissolving Activity of Azithromycin/Rifampicin Treatment

Microbial biofilm has been implicated in a wide range of chronic infections. In spite of the fact that Rhodococcus equi is a recognized cause of chronic disease in animals and humans, few studies have focused on the sessile phenotype of R.equi. The aim of this research was to phenotypically characterize the biofilm development of R. equi and its answerability for hypo-responsiveness to macrolides and rifampicin. Biofilm formation is initiated by bacterial adhesion to the surface. In this work, the ability of R. equi to adhere to the surface of human lung epithelial cells was detected by a fluorometric adhesion test performed on 40 clinical isolates. Subsequently, the capability of R. equi to produce biofilm was investigated by colorimetric, fluorescence and scanning electron microscopy analysis, revealing a general slow growth of rhodococcal biofilm and different sessile phenotypes among field isolates, some also including filamented bacteria. Azithromycin treatment produced a higher long-term inhibition and dissolution of R. equi biofilms than rifampicin, while the two antibiotics combined boosted the anti-biofilm effect in a statistically significant manner, although this was not equally effective for all R. equi isolates. Increasing the MIC concentrations of drugs tenfold alone and in combination did not completely eradicate pre-formed R. equi biofilms, while a rifampicin-resistant isolate produced an exceptionally abundant extracellular matrix. These results have strengthened the hypothesis that biofilm production may occur as an antibiotic tolerance system in R. equi, potentially determining persistence and, eventually, chronic infection.

Strain-to-strain variation of Rhodococcus equi growth and biofilm formation in vitro

OBJECTIVE

Rhodococcus equi is an opportunistic pathogen that causes disease worldwide in young foals and immunocompromised humans. The interactions of R. equi with the host immune system have been described; however, most studies have been conducted using a few well-characterized strains. Because biological differences between R. equi strains are not well characterized, it is unknown if experimental results will replicate when different strains are used. Therefore, our objective was to compare the growth and biofilm formation of low-passage-rate clinical isolates of R. equi to higher-passage-rate, commonly studied isolates to determine whether strain-to-strain variation exists.

RESULTS

Twelve strains were used: 103+, ATCC 33701, UKVDL206 103S harboring a GFP-expressing plasmid, a plasmid-cured 33701 (higher-passage-rate) and seven low-passage clinical isolates. Generation time in liquid revealed fast, moderate-fast, moderate-slow, and slow-growing isolates. The higher-passage-rate isolates were among the moderate-slow growing strains. A strain's rate of growth did not correspond to its ability to form biofilm nor to its colony size on solid media. Based on our results, care should be taken not to extrapolate in vitro work that may be conducted using different R. equi strains. Further work is needed to evaluate the effect that the observed differences may have on experimental results.

Rhodococcus Infection in Solid Organ and Hematopoietic Stem Cell Transplant Recipients1

We conducted a case–control study of 18 US transplant recipients with Rhodococcus infection and 36 matched controls. The predominant types of infection were pneumonia and bacteremia. Diabetes mellitus and recent opportunistic infection were independently associated with disease. Outcomes were generally favorable except for 1 relapse and 1 death.

Differences in Rhodococcus equi Infections Based on Immune Status and Antibiotic Susceptibility of Clinical Isolates in a Case Series of 12 Patients and Cases in the Literature

Rhodococcus equi is an unusual zoonotic pathogen that can cause life-threatening diseases in susceptible hosts. Twelve patients with R. equi infection in Kentucky were compared to 137 cases reported in the literature. Although lungs were the primary sites of infection in immunocompromised patients, extrapulmonary involvement only was more common in immunocompetent patients (P < 0.0001). Mortality in R. equi-infected HIV patients was lower in the HAART era (8%) than in pre-HAART era (56%) (P < 0.0001), suggesting that HAART improves prognosis in these patients. Most (85–100%) of clinical isolates were susceptible to vancomycin, clarithromycin, rifampin, aminoglycosides, ciprofloxacin, and imipenem. Interestingly, there was a marked difference in susceptibility of the isolates to cotrimoxazole between Europe (35/76) and the US (15/15) (P < 0.0001). Empiric treatment of R. equi infection should include a combination of two antibiotics, preferably selected from vancomycin, imipenem, clarithromycin/azithromycin, ciprofloxacin, rifampin, or cotrimoxazole. Local antibiograms should be checked prior to using cotrimoxazole due to developing resistance.

Rhodococcus erythropolis cells adapt their fatty acid composition during biofilm formation on metallic and non-metallic surfaces

Several parameters are involved in bacterial adhesion and biofilm formation including surface type, medium composition and cellular surface hydrophobicty. When the cells are placed inside tubes, parameters such as oxygen availability should also influence cell adhesion. To understand which cellular lipids are involved in the molecular events of biofilm formation in Rhodococcus erythropolis, cell adhesion was promoted on different metallic and non-metallic surfaces immersed in culture media. These cells were able to modulate the fatty acid composition of the cell membrane in response to both the surface to which they adhered and the growth medium used. To assess the response of the cells to both surfaces and operational conditions, biofilms were also promoted inside a reactor built with five different types of tubes and with medium recirculation. The biofilm biomass could be directly related not to the hydrophobicity of the tubes used but to the oxygen permeability of the tubes. Besides this, cell age influenced the adhesion of the R. erythropolis cells to the tubes. Principal component analysis showed that the lipid composition of the cells could separate cells attached to metallic from those on non-metallic surfaces in the plane formed by PC1 and PC2, and influence biofilm biomass.

Biofilm formation by Rhodococcus equi and putative association with macrolide resistance

Abstract: Rhodococcus equi is a facultative intracellular pathogen, which cause severe pyogranulomatous pneumonia in foals and tuberculosis-like lesions in humans. Its ability to form biofilm was described in strains isolated from chronic diseases associated to treatment failures in humans. This study aimed to verify the biofilm formation by 113 R. equi isolated from equine samples (clinical and fecal) using two different methods (biofilm-culturing with and without additional glucose and epifluorescence microscopy). We also aimed to determine the efficacy of azithromycin, clarithromycin and erythromycin on R. equi in established biofilm. We found 80.5% (26/41) and 63% (58/72) biofilm-positive isolates, in fecal and clinical samples, respectively. The additional glucose increased the biofilm formation by R. equi fecal samples, but not by clinical samples. The antimicrobials tested herein were not able to eradicate R. equi in biofilm even at higher concentrations. This is the first study showing the biofilm formation by R. equi isolated from equine samples. Our findings indicate that R. equi biofilm-producers may be more resistant to the antimicrobials evaluated. Further studies are warranted to test this hypothesis.

Biofilm production by multiresistant Corynebacterium striatum associated with nosocomial outbreak

Corynebacterium striatum is a potentially pathogenic microorganism that causes nosocomial outbreaks. However, little is known about its virulence factors that may contribute to healthcare-associated infections (HAIs). We investigated the biofilm production on abiotic surfaces of multidrug-resistant (MDR) and multidrug-susceptible (MDS) strains of C. striatum of pulsed-field gel electrophoresis types I-MDR, II-MDR, III-MDS and IV-MDS isolated during a nosocomial outbreak in Rio de Janeiro, Brazil. The results showed that C. striatum was able to adhere to hydrophilic and hydrophobic abiotic surfaces. The C. striatum 1987/I-MDR strain, predominantly isolated from patients undergoing endotracheal intubation procedures, showed the greatest ability to adhere to all surfaces. C. striatum bound fibrinogen to its surface, which contributed to biofilm formation. Scanning electron microscopy showed the production of mature biofilms on polyurethane catheters by all pulsotypes. In conclusion, biofilm production may contribute to the establishment of HAIs caused by C. striatum.

Stability and compatibility of antimicrobial lock solutions

PURPOSE

Published stability and compatibility data on a growing array of solutions used for antimicrobial lock therapy (ALT) are reviewed.

SUMMARY

ALT involves the instillation of a highly concentrated antimicrobial, often in combination with an anticoagulant, into a central venous catheter (CVC) lumen; this technique is often used for prophylaxis after CVC insertion or as an adjunctive treatment in cases of central line-associated bloodstream infection (CLABSI) if catheter removal is not feasible. Optimal selection of stable and compatible antimicrobials and additives can maximize catheter dwell times, streamline pharmacy compounding practices, and help ensure patient safety. Of 98 articles on ALT solutions identified in a literature search, 17 met the prespecified criteria for the use of validated stability and compatibility methodology. Antimicrobials active against common CLABSI pathogens that may be appropriate for ALT include cefazolin, cefotaxime, ceftazidime, ciprofloxacin, daptomycin, gentamicin, linezolid, telavancin, ticarcillin-clavulanic acid, and vancomycin; validated data demonstrate the stability of these agents in solution with heparin or nonheparin anticoagulants over 72-96 hours or longer. Other antifungal agents and antiinfectives (e.g., ethyl alcohol) have been used in specific patients and ALT situations. The prolonged stability of several antimicrobial-additive combinations may allow for extended dwell times and less frequent lock solution exchanges.

CONCLUSION

Pharmacists' knowledge of diverse combinations of antimicrobial agents and additives in lock solutions, including several shown to be stable and compatible for extended periods, can help expand and optimize the use of ALT in both treatment and prophylactic modalities.

Detection and quantification of bacterial biofilms combining high-frequency acoustic microscopy and targeted lipid microparticles

BACKGROUND

Immuno-compromised patients such as those undergoing cancer chemotherapy are susceptible to bacterial infections leading to biofilm matrix formation. This surrounding biofilm matrix acts as a diffusion barrier that binds up antibiotics and antibodies, promoting resistance to treatment. Developing non-invasive imaging methods that detect biofilm matrix in the clinic are needed. The use of ultrasound in conjunction with targeted ultrasound contrast agents (UCAs) may provide detection of early stage biofilm matrix formation and facilitate optimal treatment.

RESULTS

Ligand-targeted UCAs were investigated as a novel method for pre-clinical non-invasive molecular imaging of early and late stage biofilms. These agents were used to target, image and detect Staphylococcus aureus biofilm matrix in vitro. Binding efficacy was assessed on biofilm matrices with respect to their increasing biomass ranging from 3.126 × 103 ± 427 UCAs per mm(2) of biofilm surface area within 12 h to 21.985 × 103 ± 855 per mm(2) of biofilm matrix surface area at 96 h. High-frequency acoustic microscopy was used to ultrasonically detect targeted UCAs bound to a biofilm matrix and to assess biofilm matrix mechanoelastic physical properties. Acoustic impedance data demonstrated that biofilm matrices exhibit impedance values (1.9 MRayl) close to human tissue (1.35 - 1.85 MRayl for soft tissues). Moreover, the acoustic signature of mature biofilm matrices were evaluated in terms of integrated backscatter (0.0278 - 0.0848 mm(-1) × sr(-1)) and acoustic attenuation (3.9 Np/mm for bound UCAs; 6.58 Np/mm for biofilm alone).

CONCLUSIONS

Early diagnosis of biofilm matrix formation is a challenge in treating cancer patients with infection-associated biofilms. We report for the first time a combined optical and acoustic evaluation of infectious biofilm matrices. We demonstrate that acoustic impedance of biofilms is similar to the impedance of human tissues, making in vivo imaging and detection of biofilm matrices difficult. The combination of ultrasound and targeted UCAs can be used to enhance biofilm imaging and early detection. Our findings suggest that the combination of targeted UCAs and ultrasound is a novel molecular imaging technique for the detection of biofilms. We show that high-frequency acoustic microscopy provides sufficient spatial resolution for quantification of biofilm mechanoelastic properties.

Epidemiology of infections in cancer patients

Although major advances in the care of cancer patients over the past several decades have resulted in improved survival, infectious complications remain a significant cause of morbidity and mortality. To successfully identify, treat, and prevent infections, a comprehensive understanding of risk factors that predispose to infection and of commonly encountered pathogens is necessary. In addition, clinicians must keep abreast of the changing epidemiology of infections in this population. As therapeutic modalities continue to evolve, as established pathogens become increasingly drug resistant, and as new pathogens are discovered, successful management of infections will continue to present challenges in the years to come.

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.

Complete genome sequence analysis of Nocardia brasiliensis HUJEG-1 reveals a saprobic lifestyle and the genes needed for human pathogenesis

Nocardia brasiliensis is an important etiologic agent of mycetoma. These bacteria live as a saprobe in soil or organic material and enter the tissue via minor trauma. Mycetoma is characterized by tumefaction and the production of fistula and abscesses, with no spontaneous cure. By using mass sequencing, we determined the complete genomic nucleotide sequence of the bacteria. According to our data, the genome is a circular chromosome 9,436,348-bp long with 68% G+C content that encodes 8,414 proteins. We observed orthologs for virulence factors, a higher number of genes involved in lipid biosynthesis and catabolism, and gene clusters for the synthesis of bioactive compounds, such as antibiotics, terpenes, and polyketides. An in silico analysis of the sequence supports the conclusion that the bacteria acquired diverse genes by horizontal transfer from other soil bacteria, even from eukaryotic organisms. The genome composition reflects the evolution of bacteria via the acquisition of a large amount of DNA, which allows it to survive in new ecological niches, including humans.

Induction of proinflammatory cytokines in human lung epithelial cells during Rhodococcus equi infection

Rhodococcus equi is an opportunistic human pathogen associated with immunosuppressed people. While the interaction of R. equi with macrophages has been comprehensively studied, little is known about its interactions with non-phagocytic cells. Here, we characterized the entry process of this bacterium into human lung epithelial cells. The invasion is inhibited by nocodazole and wortmannin, suggesting that the phosphatidylinositol 3-kinase pathway and microtubule cytoskeleton are important for invasion. Pre-incubation of R. equi with a rabbit anti-R. equi polyclonal antiserum resulted in a dramatic reduction in invasion. Also, the invasion process as studied by immunofluorescence and scanning electron microscopy indicates that R. equi make initial contact with the microvilli of the A549 cells, and at the structural level, the entry process was observed to occur via a zipper-like mechanism. Infected lung epithelial cells upregulate the expression of cytokines IL-8 and IL-6 upon infection. The production of these pro-inflammatory cytokines was significantly enhanced in culture supernatants from cells infected with non-mucoid plasmid-less strains when compared with cells infected with mucoid strains. These results demonstrate that human airway epithelial cells produce pro-inflammatory mediators against R. equi isolates.

Conjugal transfer of a virulence plasmid in the opportunistic intracellular actinomycete Rhodococcus equi

Rhodococcus equi is a facultative intracellular, Gram-positive, soilborne actinomycete which can cause severe pyogranulomatous pneumonia with abscessation in young horses (foals) and in immunocompromised people, such as persons with AIDS. All strains of R. equi isolated from foals and approximately a third isolated from humans contain a large, ~81-kb plasmid which is essential for the intramacrophage growth of the organism and for virulence in foals and murine in vivo model systems. We found that the entire virulence plasmid could be transferred from plasmid-containing strains of R. equi (donor) to plasmid-free R. equi strains (recipient) at a high frequency and that plasmid transmission reestablished the capacity for intracellular growth in macrophages. Plasmid transfer required living cells and cell-to-cell contact and was unaffected by the presence of DNase, factors pointing to conjugation as the major means of genetic transfer. Deletion of a putative relaxase-encoding gene, traA, located in the proposed conjugative region of the plasmid, abolished plasmid transfer. Reversion of the traA mutation restored plasmid transmissibility. Finally, plasmid transmission to other Rhodococcus species and some additional related organisms was demonstrated. This is the first study showing a virulence plasmid transfer in R. equi, and it establishes a mechanism by which the virulence plasmid can move among bacteria in the soil.