A Microbial Biorealm page on the genus Spiroplasma kunkelii
Higher order taxa
Bacteria; Firmicutes; Mollicutes; Entomoplasmatales; Spiroplasmataceae
Description and significance
Spiroplasmas are prokaryotic gram positive bacteria within the class of Mollicutes with a defining feature of being cell wall-less3. It was the first mollicute to be obtained in culture after being isolated from a corn plant infected with corn stunt disease in Costa Rica. Strain CR2-3x was cloned three times and was grown within a serum containing medium LD8A3 at 30°C. The concentration of the environment for growth was approximately 5 x 108 cells per mL. It was also found to be the first mollicute to illustrate a helical shape2. All spiroplasmas since have been studied from being isolated from plants, ticks, and insects. Spiroplasmas are characterized by a helical morphology and movement and are bound by a membrane. Cells of Spiroplasma kunkelii are just 0.15-0.2 µm in diameter while being 2.0-15 µm in length, making it simple to cross a membrane filter up to 220 nm in size9.
S. kunkelii is the causative agent of corn stunt disease. It can be transmitted by insects from corn husk to corn husk through the vector of the insect1. It can be found within the vector of the corn leafhopper Dalbulus maidis in irrigated low elevation areas within dry seasons and un-irrigated high elevation areas after the dry seasons6. Spiroplasmas reside in the gut of the insect vector after crossing the insect-gut barrier. From there, the microbe travels into the hemolymph where it multiplies and becomes large in numbers. When the vector comes in contact with a plant, the spiroplasmas then attach on to the leaves and begin infecting8.
It is an important organism for study due to its ability to multiply fast and reach high abundances as well as because of the effect it has on plants pertaining to corn stunt disease2. As of April 28, 2013, estimates of growth rate have not been found along with estimates of generation time. Because the disease is spread by insect vectors, it is hard to manage in tropical parts of South America. Corn stunt disease is also a large threat to corn primarily in parts of Central and South America, but it is working its way into the United States2. S. kunkelii can be isolated in the lab in a medium containing gamma globulin-free horse serum in order to investigate the growth patterns of different helical structures. In a serum free medium, large diffuse colonies could be produced by strains that had normal helicity. In any culture that contained one helical strain or partially helical strains, the colonies produced were either small or illustrated little motility2.
Since the full genome of S. kunkelii has not been fully sequenced yet, not a lot of information is known about its genome. Due to reductive or degenerative evolution, the genome size of mollicutes in general is smaller (0.14 Mbp) when compared to most prokaryotes. Gene loss in spiroplasmas in particular isn’t as large as found for other mollicutes. The gene content is so miniscule that the gene set approaches the minimal set necessary for growth and pathogenesis. The key division gene for S. kunkelii has been found to be FtsZ9sk. CR2-3X is the one major chromosome that is under investigation. The size of this chromosome has been estimated to be about 1550 kb. The shape of this particular chromosome has been noted to be circular, which is homologous to the chromosome shape of Mycoplasma pneumonia. Plasmids are currently found to be absent from the chromosome.There are 27 chromosomal genes located on the CR2-3X chromosome. Of these 27 genes, 23 of them are involved in chromosome partitioning, DNA replication, protein translation, and RNA transcription, nucleotide, carbohydrate, and co-enzyme metabolism, protein secretion and membrane structure. Membrane structure involves the spiralin protein. There are a total of 66 restriction sites also detected on chromosome CR2-3X2.
There was only one RNA operon found in the genome of S. kunkelii2.
Cell and colony structure
S. kunkelii has the ability to have spherical and helical morphology. As mentioned above, cells of S. kunkelii are just 0.15-0.2 µm in diameter while being 2.0-15 µm in length, making it simple to cross a membrane filter up to 220 nm in size9. They have a distinct feature of being a small cell wall-less bacteria with motility being in a corkscrew like motion1. Colonies of the microbe are produced in the insect vector. Colony size is based on the helical structure of S. kunkelii, but absolute values have not yet been found9. Depending on the species of mollicute, the cells of S. kunkelii can move anywhere from 0.1 to 71 m/s1. In the tip structure, where cells are flask shaped and are used for attachment, proteins similar to microtubules function to move the membrane proteins while attached to adherin-like proteins1.
S. kunkelii tend to grow well at around 37°C, the normal temperature of the human body. Located within the class of Mollicutes means they share some common features. Some of these features include a simple metabolism and parasitic characteristics. Since this microbe is similar to a parasite, it uses the host insect vector for multiplication and uses the plant for survival. The environment of the insect vector allows for optimal replication1.
The habitat of S. kunkelii is located within the insect vector. The microbe resides within the intestinal area and the mid gut epithelium of the insect and enters the hemolymph where it will divide and grow into abundant numbers. Corn stunt disease limits the production of corn and is more prevalent in South America and tropical regions of the world. There is no “cure” for corn stunt disease and no treatments that will get rid of the spiroplasma once it has infected the host plant or insect vector1.
All hosts of S. kunkelii are within plants. Zea mays remains the primary host while Euchleana Mexicana (Zea Mexicana) and E. perennis (Z. perennis) are also known hosts. To determine if a plant is infected with the microbacterium, ELISA and DFM are two methods commonly used. A common symptom of a plant infected with corn stunt disease resides in the color of the leaves. If a plant is infected, purplish and reddish leaves will result. This disease is more common at lower elevations when compared to the disease at higher elevations9.
Pathogenically, spiroplasmas are found within the microvilli and vesicles of the endothelium in the gut region of the intestines within D. maidis. Spiroplasmas with extensions similar to tubes were contained within cytoplasmic vesicles and then accumulated in between the densa and laminae rara layers of the basal lamina. The spiroplasmas are also found in the hemolymph and then crossed the malpighian tubule epithelium of the basal lamina to accumulate in muscle cells that experienced cytopathogenic changes. Once S. kunkelii has reached the hemolymph, the spiroplasmas multiply and eventually kill the insect host8.
Bai, X., Hogenhout, S. A genome sequence survey of the mollicute corn stunt spiroplasma Spiroplasma kunkelii. FEMS Microbiology. 2002. 210: 7-17.
Barros, Thereza., Dally, E., Davis, R., Lin, S., Roe, B., Zhao, Y. Physical and genetic map of the Spiroplasma kunkelii CR2-3x chromosome. MicroBiology. 2006. 52: 857-865. Accessed 24 April 2013.
Bove, J.M. Spiroplasmas: infections agents of plants, arthropods, and vertebrates. Pubmed. August 1997. 109: 14-15, 604-612.
Danet, J., Duret, S., et al. Gene Disruption through Homologous Recombination in Spiroplasma citri: an scm1-Disrupted Motility Mutant is Pathogenic. Journal of Bacteriology. December 1999. 181:24, 7449-7456.
Davis, E., Hammond, R.W., et al. Predicted ATP-binding cassette systems in the phytopathogenic mollicute Spiroplasma kunkelii. Molecular Genetics and Genomics. April 2004. 271:3, 325-338.
Davis, R. Corn Stunt. UC IPM Online. 2012. Accessed 24 April 2013.
Davis, R.E., Hammond, R.W., et al. Cell division gene cluster in Spiroplasma kunkelii: functional characterization of ftsZ and the first report of ftsA in mollicutes. PubMed. February 2004. 23:2, 127-134.
Hogenhout, S.A., Meulia, T., et al. Infection and replication sites of Spiroplasma kunkelii (class: mollicutes) in midgut and Malpighian tubules of the leafhopper Dalbulus maidis. PubMed. March 2003. 82:3, 167-175.
 Invasive Species Compendium. Spiroplasma kunkelii (corn stunt spiroplasma). Cabi. April 2011. Accessed March 2013.
Edited by Gina Johnson of Dr. Lisa R. Moore, University of Southern Maine, Department of Biological Sciences, http://www.usm.maine.edu/bio