Rhodococcus equi is mainly a cause for disease in animals. However, this bacterium can be fatal in immunocompromised individuals with infections from Rhodococcus equi. These individuals may have numerous problems ranging from caviatary pulmonary disease to Necrotizing lobar pneumonia. Rhodococcus equi is in the Corynebacterium genus. An infection caused by this group of bacteria, also known as the "coryneform" use rod-shaped gram-positive pleomorphic microbes that closely resemble Corynebacterium diphteriae and changes them taxonomically. It has recently been reclassified due to more findings and put into the suprageneric taxon nocardioform actinomycetes group.
Rhodococcus equi is an aerobic bacterium. This microbe is gram positive and contains mycolic acids in its cell wall, it shares many traits with Mycobacterium tuberculosis. Rhodococcus equi is a facultative intracellular bacterium with rod to coccus pleomorphism. Sources and sites of Rhodococcus equi are animals with the bacteria, horses, soil, LRT, lung, and immunosuppressed humans. Some diagnostic factors that help distinguish this bacterium from others are catalase positive, mucoid and coalescing colonies, obligate aerobe, teardrop colonies, and weakly acid fast. Rhodococcus equi is nonfastidious and will grow colonies with a salmon pink pigmentation on blood agar after 2 to 3 days in incubation.
Bronchial tissue Gram stain showing intrahistiocytic coccobacillary forms of Rhodococcus equi. Original magnification, x 1,000. Open lung biopsy showing coalescent microabscesses with numerous histiocytes containing Rhodococcus equi organisms. PAS stain. Original magnification x 250. Figure and description provided by Dr. Margie Scott, Vanderbilt University Medical Center. Another helpful identification factor of Rhodococcus equi is the fact that it undergoes synergistic hemolysis. This can be demonstrated through the cross-streaking of Rhodococcus equi on sheep blood agar with another bacterium (i.e. A. haemolyticum, Staphylococcus aureus, and Corynebacterium pseudoteberculosis). The picture below shows some mechanisms of synergistic hemolysis. A hemolytic system is one that is genetically identical on the surface of other microbes. Strains of Rhodocococcus equi can also be identified when imipenem and b-lactam antibiotics are used against them.
Cooperative (and antagonistic) hemolytic reactions on sheep blood agar, demonstrating cooperative hemolysis between Rhodococcus equi, Arcanobacterium haemolyticum, and Staphylococcus aureus. Partial hemolysis by S. aureus (cross-hatched on diagram) is inhibited in the proximity of A. haemolyticum. (Figure and description from http://www.cdc.gov/ncidod/eid/vol3no2/linder.htm)
Magnusson isolated Rhodococcus equi in 1923 from the granulomatous lung infections of foals. This bacterium is still a big pathogen found in foals. Therefore, many veterinary studies are related to this common animal disease. Rhodococcus equi can be found in the soil in areas where livestock graze. This is how infection is spread from animal to animal. Once a young foal or animal breathes these bacteria into their lungs, the bacteria multiply in macrophages leading to pneumonia. When a human gets this bacterium, it is usually from environmental exposure. Only 25% of the cases reported have been from exposure to horses. We know it must be able to adapt to harsh changing environments. It is hard to follow the pathogenesis of this disease; one factor that is certain is its transmission route includes the respiratory system.
An effective treatment for Rhodococcus equi is a glycopeptide antibiotic (i.e. vancomycin, teicoplanin and rifampin) or a macrolide antibiotic (i.e. erythromycin and clarithromycin. Often a combination of these two drugs is used to ensure that the bacteria are destroyed. The treatments do not work very well yet because the disease is so new to humans. Therefore, patients diagnosed with this are carefully monitored. Mortality rates in immunosuppressed patients are high for this bacterium. More studies are being conducted to help those humans and animals that contract Rhodococcus equi.
Sources used:
http://hermes.ucd.ie/~molmicro/, Molecular Microbiology, University College Dublin http://www.cdc.gov/ncidod/eid/vol3no2/linder.htm#ref23, Rhodococcus equi and Arcanobacterium haemolyticum: Two "Coryneform" Bacteria Increasingly Recognized as Agents of Human Infection, Regina Linder, Hunter College, New York, New York, USA
http://www.medinfo.ufl.edu/year2/mmid/bms5300/bugs/rhoequi.html, Rhodococcus equi
http://www3.interscience.wiley.com/cgi-bin/abstract/76507356/START, Article from Diagnostic Cytopathology, 18 January 2001, Volume 24, Issue 2, 2001, Rhodococcus equi and cytomegalovirus pneumonia in a renal transplant patient: Diagnosis by fine-needle aspiration biopsy, Copyright © 2001 Wiley-Liss, Inc., p. 129-31
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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