Rathayibacter toxicus

Rathayibacter toxicus is a phytopathogenic bacterium known for causing annual ryegrass toxicity (ARGT) commonly found in South and Western Australia. 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. The species name, toxicus, stems from the Latin word meaning 'poison', due to Rathayibacter toxicus's ability to produce corynetoxins. Rathayibacter toxicus has been previously classified as “Corynebacterium sp.”, “Corynebacterium rathayi”, “Clavibacter sp.”, “Clavibacter rathayi”, and “Clavibacter toxicus.” The organism is a member of the Microbacteriaceae family. Microbacteriaceae contains twenty-eight other genera, though a distinct clade is formed between genus Rathayibacter and genus Clavibacter. Genera that are closer related to Rathayibacter are Frigoribacterium, Curtobacterium, and Clavibacter; while genus Leifsonia is more distantly related to Rathayibacter. 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. In 1956, the first reported livestock deaths due to annual ryegrass (Lolium rigidum) toxicity (ARGT), later discovered to be the work of the ARGT bacterium now known to be Rathayibacter toxicus, were found in the “wheat-sheep belt” in Black Springs, South Australia. 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. It was not until 1968 that the bacterium responsible for ARGT was isolated and identified as Corynebacterium sp. by A. Kerr, and mistakenly identified as Corynebacterium rathayi later in 1977. The principal investigator and discovery date of the organism are known, but the original isolation method is obscured; however, the isolation technique utilized to perform a morphological assessment of a different strain of the same organism was undergone by Bird and Stynes. The researchers identified the organism of interest by the characteristic yellow slime and it was removed from a nematode gall, placed into distilled water, and plated on a unique media (10 grams of sucrose, 8 grams of caseine hydrolyzate, 4 grams of yeast extract, 2 grams of KH2PO4, 0.3 grams of MgSO4 7H2O, 15 grams of agar, and distilled water was added until reaching 1L). Pure yellow colonies formed within 24 hours. The identification of the bacterium as Corynebacterium rathayi was insufficiently supported, and the transfer of “Corynebacterium rathayi” into the genus Clavibacter was urged by Davis et al.. in 1984 after the cell wall peptidoglycan layer was found to have 2,4-diaminobutyric acid (DAB). In 1987, Riley found that the bacteria associated with ARGT were distinguishable from not only Corynebacterium rathayi but other phytopathogenic coryneforms through immunological assays. Riley, in support of Davis’ findings, also identified DAB in the ARGT bacterium’s peptidoglycan layer through amino acid analysis, further supporting the reclassification into Clavibacter as Clavibacter sp. Due to differences in serology, allozyme analysis, bacteriophage susceptibility, vector adhesion, and biochemical properties that distinguished the new Clavibacter sp. associated with ARGT from other members of the genus, Riley and Ophel (1992) proposed Clavibacter toxicus as a new species. In 1993, Zgurskaya et al. proposed a new genus, “Rathayibacter,” and desired to reclassify the “Clavibacter sp.” associated with ARGT into this genus based on differences in menaquinone composition, morphological and physiological characteristics, DNA-DNA relatedness, chemotaxonomy, serology, allozyme / protein patterns, and 16S rRNA gene sequences. In 1998, Clavibacter toxicus was reclassified as Rathayibacter toxicus by Sasaki and colleagues. 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. 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. It does not produce spores or display any mobility. The cell wall of R. toxicus is characterized by the presence of the L-isomer of DAB. 4 strains of Rathayibacter toxicus (WAC3373, 70137, DSM 7488, FH142) have had their genomes completely sequenced, assembled, annotated, and published. 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%. 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.