Aerobic Actinomycetes of Clinical Significance

The group of Gram-positive bacillary organisms broadly known as "aerobic actinomycetes" consists of heterogeneous and taxonomically divergent genera. They are found in a wide variety of natural and man-made environments but are rarely considered a part of the normal human flora, with infections normally originating from exogenous sources. An extensive number of genera have been described, but only a minority of these has been associated with human or veterinary health. The association with human disease is usually of an opportunistic nature, either through accidental means of inoculation or through involvement with immunocompromising conditions in the host. They cause a wide spectrum of diseases in humans, which may differ greatly between the genera and even between species, but which also may have a great amount of overlap. The occurrence of such infections is probably greater than appreciated, since many may go unrecognized. Etiologic prevalence of specific genera and species varies geographically within the United States and worldwide. Traditional phenotypic identification methods for separation of the many genera and species of aerobic actinomycetes have found great difficulties. Recent use of chemotaxonomic analyses and emerging technologies such as molecular analysis of nucleic acids, and more recently proteomics for identification to the genus/species level, has provided a far more robust technique to understand the organisms' relatedness, distribution, epidemiology, and pathogenicity in humans.

Rhodococcus equi granulomatous mastitis in an immunocompetent patient

A 37-year-old immunocompetent woman was evaluated for progressive swelling of her left breast. Magnetic resonance imaging (MRI) showed multiple hypo-intense solid lesions and could not exclude breast cancer. Tissue biopsy was suggestive of granulomatous mastitis without any evidence of malignancy. Culture of the specimen in brain heart infusion broth grew Rhodococcus equi. The patient responded well to combination therapy with ciprofloxacin and azithromycin, and the lesions regressed in follow-up MRI. To the best of our knowledge, this is the first report of R. equi granulomatous mastitis. Accurate identification of this rare pathogen is necessary to provide appropriate treatment in granulomatous mastitis.

Brief Tale of a Bacteraemia by Rhodococcus equi, With Concomitant Lung Mass: What Came First, the Chicken or The Egg?

Rhodococcus equi is an uncommon Gram positive, variably acid-fast pathogen, that appears as hard to treat mostly owing to the establishment of intracellular niches. Lack of interpretive criteria for susceptibility testing may lead to under-reporting or overestimation of resistances, whereas knowledge about this pathogen’s clinical impact may be affected by erroneous phenotype-based characterization at a genus and species level.

We present the case of a bacteraemia with a concomitant lung mass in a lymphoma patient, that further highlights the emergence of rhodococcal diseases as a matter for concern in the fields of infectious diseases and haematology.

Characterization of the role of the pathogenicity island and vapG in the virulence of the intracellular actinomycete pathogen Rhodococcus equi

Rhodococcus equi, a facultative intracellular pathogen of macrophages, causes severe, life-threatening pneumonia in young foals and in people with underlying immune deficiencies. R. equi virulence is dependent on the presence of a large virulence plasmid that houses a pathogenicity island (PAI) encoding a novel family of surface-localized and secreted proteins of largely unknown function termed the virulence-associated proteins (VapACDEFGHI). To date, vapA and its positive regulators virR and orf8 are the only experimentally established virulence genes residing on the virulence plasmid. In this study, a PAI deletion mutant was constructed and, as anticipated, was attenuated for growth both in macrophages and in mice due to the absence of vapA expression. Expression of vapA in the PAI mutant from a constitutive promoter, thereby eliminating the requirement for the PAI-encoded vapA regulators, resulted in delayed bacterial clearance in vivo, yet full virulence was not restored, indicating that additional virulence genes are indeed located within the deleted pathogenicity island region. Based on previous reports demonstrating that the PAI-carried gene vapG is highly upregulated in macrophages and in the lungs of R. equi-infected foals, we hypothesized that vapG could be an important virulence factor. However, analysis of a marked vapG deletion mutant determined the gene to be dispensable for growth in macrophages and in vivo in mice.