Rathayibacter toxicus

The genus Rathayibacter is an homage to E. Rathay, the plant pathologist who first isolated strains of the genus combined with the suffix -bacter meaning "rod" in Latin.[5] The species name, toxicus, stems from the Latin word meaning "poison", due to R. toxicus's ability to produce corynetoxins.[1]

Rathayibacter toxicus has been previously classified as "Corynebacterium rathayi", "Clavibacter rathayi", and "Clavibacter toxicus".[5][1][6] The organism is a member of the family Microbacteriaceae.[5][1][6][7] Microbacteriaceae contains twenty-eight other genera, though a distinct clade is formed between genus Rathayibacter and genus Clavibacter.[5] Genera that are closer related to Rathayibacter are Frigoribacterium, Curtobacterium, and Clavibacter; while genus Leifsonia is more distantly related to Rathayibacter.[5] In genus Rathayibacter there are six species that cluster together within Microbacteriaceae and Rathayibacter toxicus has the deepest branching as it is least related to the other species.[5][6][8]

In 1956, the first reported livestock deaths due to annual ryegrass toxicity (ARGT) were found in the "wheat-sheep belt" in Black Springs, South Australia.[1] ARGT is caused by grazing on Lolium rigidum infected with a particular bacterium, now known to be R. toxicus.[1][9] In the late 1950s, J. M. Fisher identified a gall-forming nematode (Anguina sp.) and a yellow-slime bacterium, both pathogens of the seed-heads of annual ryegrass.[10] It was not until 1968 that the bacterium responsible for ARGT was isolated, and later in 1977 mistakenly identified as a Corynebacterium sp. (Corynebacterium rathayi) by A. Kerr.[4][1][9][10]

Rathayibacter toxicus is a Gram-positive, obligate aerobe with irregular rod morphology, usually 0.5 to 0.7 µm in diameter by ~1.1 to 2.0 µm, and ends that are blunt and rounded.[5] It possesses a capsule around the cell that is 0.08-0.2 µm thick, allowing the microorganism to survive hot and arid conditions during the summer or in the absence of a host plant.[5][1] It does not produce spores or display any mobility.[5] The cell wall of R. toxicus is characterized by the presence of the L-isomer of DAB.[5][6]

4 strains of Rathayibacter toxicus (WAC3373, 70137, DSM 7488, FH142) have had their genomes completely sequenced, assembled, annotated, and published.[7] R. toxicus has been found to have a single circular chromosome with an average genome size of 2.325 Megabases and an average GC content of 61.5%.[7][14] Strain WAC3373 serves as the reference organism with a genome size of 2.35 Mb, GC content of 61.5%, 2165 total genes, 2069 protein coding genes, 54 total RNA genes (45 tRNA, 6 rRNA, 3 other RNA), and 42 pseudogenes.[7]

Rathayibacter toxicus is a chemoorganotroph that utilizes oxygen as its terminal electron acceptor.[5] Using tubes of Medium C containing a variety of carbon sources, each 0.5% weight per volume concentration, noting growth and acid production for 4 weeks, it was determined that R. toxicus utilizes galactose, mannose, and xylose as carbon sources forming acidic byproducts.[5][1] The production of acids from carbohydrates occurs oxidatively and weakly.[5]

Rathayibacter toxicus is mesophilic with optimum growth at 26 °C (79 °F) and no growth at 37 °C (99 °F).[5][1] This was determined by examination of bacterial growth on streak-plated 523M agar, incubated at 26 °C (79 °F) and 37 ± 0.5 °C (32.9 °F) after 3, 7, and 14 days.[5][1] The organism has responded well to 523M agar, CB agar, R agar, and other basic media containing yeast extract, peptone, and glucose when grown at pH 7.[5] R. toxicus requires 0.1% yeast extract for growth.[5][1] Cultures grown in YSB medium, ranging from 0 to 10% weight per volume concentrations of NaCl, observed after 3, 7, and 14 days revealed that R. toxicus is only able to withstand a maximum of 1% NaCl concentration.[5][1] The generation time of R. toxicus is approximately 18 hours in 523M broth at 25 °C (77 °F) based on optical density measurements via a spectrophotometer.[1] Colony morphology on 523M agar is convex, smooth, mucoid with yellow, rose-orange, or pink pigmentation.[5][1]

Anguina sp. (seed gall nematodes) are natural vectors for transmission of the pathogen.[3] R. toxicus is obligately conveyed by nematode.[17] The organism is known to only infect the floral parts of Poaceae species, a ubiquitous family of grasses, in Australia and parts of South Africa.[3] Lolium rigidum (annual ryegrass) has been found to be commonly infected with R. toxicus from November to March.[13] Other grass species such as Agrostis avenacea (annual blown grass), Ehrharta longiflora (annual veldtgrass), and Polypogon monspeliensis (annual beard grass) were also susceptible to infection by nematode galls carrying R. toxicus.[13]

Rathayibacter toxicus, transported by the parasitic nematode Anguina funesta, is infectious to annual ryegrass and is the principal cause of annual ryegrass toxicity (ARGT).[1] ARGT is a neurological disorder caused by R. toxicus’ secretion of a lethal glycolipid toxin (structurally similar to tunicamycin) in infected livestock.[3][9] The toxin induces convulsions and/or development of unusual gait which typically ends in death of cattle and sheep grazing on infected plants.[18][19] Many other organisms have shown vulnerability including horses, pigs, and “other laboratory animals” with sheep having a 90% mortality rate and death occurring within 24 hours of poisoning.[9] ARGT has been a major concern in Western and South Australia for the past 50 years, but symptoms have been identified in regions as far off as South Africa where it was linked to deaths of grazing thoroughbred horses.[3][18][20] Although the pathogen requires transmission via the mechanical vector (Anguina funesta), R. toxicus has shown the ability to attach to other Anguina species and infect a variety of plants (i.e. annual beard grass, bent grass, wild oats (Avena spp.), and winged canary grass), as aforementioned.[3][20] Introduction of R. toxicus to other regions is a current concern due to the economic costs of livestock loss, pasture treatment, and livestock inspections and maintenance.[9][19]