1. IDENTIFYING
CHARACTERISTICS
The above picture shows Open lung biopsy showing coalescent microabscesses
with numerous histiocytes containing Rhodococcus equi organisms. PAS stain.
Original magnification x 250. Figure provided by Dr. Margie Scott, Vanderbilt
University Medical Center.
2. TAXONOMIC DESCRIPTION
Growth cycle ranges from cocci or short rods to more complex growth phases that form filaments with short projections, elementary branching or, in some species, extremely branched hyphae. They exhibit an unusual stock of novel enzymatic capabilities for the transformation and degradation of diverse classes of substrates, and some transformations result in useful commercial processes. They are able to metabolize various gaseous and liquid hydrocarbons such as acetylene, propane, and a variety of alkanes (C2-C8, C13-C14). Several species are capable of producing surface-active glycolipids by alkane-grown rhodococci. Some species produce microbial flocculants consisting of proteins and carbohydrates, which flocculate particles such as kaolin or bacteria such as Escherichia coli. The rhodococci exhibit a broad substrate diversity for the degradation of phenols, aromatic acids, halogenated phenols and alkanes, substituted benzenes, anilines, and quinolines.
Some strains are pathogenic for animals, including human beings.
R. equi has been recognized as an important veterinary pathogen causing
broncho pneumonia, enteritis, lymphadenitis, abortion, and other diseases
in animals. Many infections are being caused by organisms previously considered
nonpathogenic to humans and animals.
The above picture shows a macrophage engulfing the yeast Candida albicans.
Rhodococci are widely distributed in nature and have been frequently
isolated from soil, fresh water, herbivore dung, and from the gut of some
insects. Classical selective isolation methods have depended on the ability
of rhodococci to use hydrocarbons as sole sources of carbon for energy
and growth. Rhodococci grow well on most standard laboratory media including
Bennett's, modified Sauton's supplemented with thiamine, and glucose-yeast
extract agars. Analysis of fatty-acid and menaquinone composition can lead
to the identification of rhodococci. More detailed studies of mycolic acid
structure and phospholipid composition enable distinctions to be made at
the species level.
Because the rhodococci are widely distributed in soil and sludge,
they may play an important role in the biotransformation of chlorinated
phenols in the environment. Commercial applications include the production
of acrylic acid and acrylimide in Japan and the use of specific rhodococci
for steroid modifications. They will probably attract increased commercial
interest in the areas of bioremediation and chemical production in future
years, based on their ability to catalyze the oxidation and metabolism
of diverse and unusual substrates.
Balows, A. ed., et al. 1992. The Prokaryotes: A handbook on the biology
of bacteria. New York
Finnerty, W.R. 1992. The Biology and Genetics of the Genus Rhodococcus. Annual Review of Microbiology. Vol. 46. pp. 193-218.
Holt, J.G., Krieg, N.R., Sneath, P.H.A., Staley, J.T., and Williams, S.T. 1994. Bergey's Manual of Determinative Bacteriology. Ninth edition. Williams and Wilkins. Baltimore, Maryland.
Lederberg, J. ed. 1992. Encyclopedia of Microbiology. Academic Press. San Diego.
Rhodococcus equi: This site contains information on the emergence of this pathogen.
UWM-Dept.of Biol. Sciences: Link to sites on Rhodococcus and other microorganisms
.
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