Rhodococcus

Rhodococcus is a genus of aerobic, nonsporulating, nonmotile Gram-positive bacteria closely related to Mycobacterium and Corynebacterium.[1][2] While a few species are pathogenic, most are benign, and have been found to thrive in a broad range of environments, including soil, water, and eukaryotic cells. Some species have large genomes, including the 9.7 megabasepair genome (67% G/C) of Rhodococcus sp. RHA1.[3]

Strains of Rhodococcus are important owing to their ability to catabolize a wide range of compounds and produce bioactive steroids, acrylamide, and acrylic acid, and their involvement in fossil fuel biodesulfurization.[3] This genetic and catabolic diversity is not only due to the large bacterial chromosome, but also to the presence of three large linear plasmids.[1] Rhodococcus is also an experimentally advantageous system owing to a relatively fast growth rate and simple developmental cycle, but is not well characterized.[3]

Another important application of Rhodococcus comes from bioconversion, using biological systems to convert cheap starting material into more valuable compounds, such as its ability to metabolize harmful environmental pollutants, including toluene, naphthalene, herbicides, and PCBs. Rhodococcus species typically metabolize aromatic substrates by first oxygenating the aromatic ring to form a diol (two alcohol groups). Then, the ring is cleaved with intra/extradiol mechanisms, opening the ring and exposing the substrate to further metabolism. Since the chemistry is very stereospecific, the diols are created with predictable chirality. While controlling the chirality of chemical reaction presents a significant challenge for synthetic chemists, biological processes can be used instead to faithfully produce chiral molecules in cases where direct chemical synthesis is not feasible or efficient. An example of this is the use of Rhodococcus to produce indene, a precursor to the AIDS drug indinavir, a protease inhibitor, and containing two of the five chiral centers needed in the complex.[4]

Rhodococcus has been greatly researched as a potential agent for the bioremediation of pollutants as it is commonly found in the natural environment, and they possess certain characteristics that allow them to thrive under a variety of conditions, and they have the capability to metabolize many hydrocarbons.[5]

Rhodococci possess many properties that makes them suitable for bioremediation under a range of environments. Their ability to undergo microaerophilic respiration allows them to survive in environments containing low oxygen concentrations, and their ability to undergo aerobic respiration also allows them to survive in oxygenated environments.[6] They also undergo nitrogen fixation, which allows them to generate their own nutrients in environments with low nutrients.[7]

Rhodococci also contain characteristics that enhances their ability to degrade organic pollutants. Their hydrophobic surface allows for adhesion to hydrocarbons, which enhances its ability to degrade these pollutants.[8] They have a wide variety of catabolic pathways and many unique enzyme functions.[9] This gives them the ability to degrade many recalcitrant, toxic hydrocarbons. For example, Rhodococci expresses dioxygenases, which can be used to degrade benzotrifluoride, a recalcitrant pollutant.[10] Rhodococcus sp. strain Q1, a strain naturally found in soil and paper mill sludge, contains the ability to degrade quinoline, various pyridine derivatives, catechol, benzoate, and protocatechuic acid.[11] Rhodococci are also capable of accumulating heavy metal ions, such as radioactive caesium, allowing for easier removal from the environment.[12] Other pollutants, such as azo dyes,[13] pesticides[14] and polychlorinated biphenyls[15] can also be degraded by Rhodococci.

Scanning electron micrograph of Rhodococcus sp. strain Q1 grown on quinoline - the organism can use quinoline as a sole source of carbon, nitrogen, and energy, tolerating concentrations up to 3.88 millimoles per liter.

The genus Rhodococcus has two pathogenic species: R. fascians and R. equi. The former, a plant pathogen, causes leafy gall disease in both angiosperm and gymnosperm plants.[16] R. equi is the causative agent of foal pneumonia (rattles) and mainly infects foals up to three months in age. However, it has a wide host range, sporadically infecting pigs, cattle, and immunocompromised humans, in particular AIDS patients and those undergoing immunosuppressive therapy.[17] Both pathogens rely on a conjugative virulence plasmid to cause disease. In case of R. fascians, this is a linear plasmid, whereas R. equi harbors a circular plasmid. Both pathogens are economically significant. R. fascians is a major pathogen of tobacco plants. R. equi, one of the most important foal pathogens, is endemic on many stud farms around the world.

Rhodococcus has also been identified as a contaminant of DNA extraction kit reagents and ultrapure water systems, which may lead to its erroneous appearance in microbiota or metagenomic datasets.[18]