Streptomyces scabies

Streptomyces scabies or Streptomyces scabiei is a streptomycete bacterium species found in soils around the world. Unlike most of the 500 or so Streptomyces species it is a plant pathogen causing corky lesions to form on tuber and root crops as well as decreasing the growth of seedlings. Along with other closely related species it causes the potato disease common scab, which is an economically important disease in many potato growing areas. It was first described in 1892, being classified as a fungus, before being renamed in 1914 and again in 1948. Several other species of Streptomyces cause similar diseases to S. scabies but other, more closely related species, do not. The genome of S. scabies has been sequenced and is the largest Streptomyces genome known so far. The genome contains a pathogenicity island containing the genes required for S. scabies to infect plants, and which can be transferred between different species. S. scabies can produce several related toxins which are the most responsible for its pathogenicity, but several other systems have also been identified which contribute. It can infect young seedlings of all plants, as well as mature root and tuber crops, but is most often associated with causing common scab of potato. The first known reference to common scab of potatoes, dates back to 1825, but it was not initially thought to have a biological cause. Isolates of an organism that causes common scab of potato were first isolated by Roland Thaxter in Connecticut in 1890 and in 1892 he described the primary strain as Oospora scabies. The original culture was not maintained. In 1914 H. T. Gussow renamed the species Actinomyces scabies, noting that Oospora was an incorrect classification since the disease was not caused by a fungus. The genus Streptomyces was first proposed by Waksman and Henrici in 1943, meaning 'pliant or bent fungus'. Most species of Streptomyces are saprotrophic feeding off dead matter with relatively few being causing disease. In 1948 Waksman and Henrici used the name Streptomyces scabies to describe the species and this name was revived in 1989 by Lambert and Loria, who bought together 12 different strains that formed one homogeneous group. In 1997 the name was changed to Streptomyces scabiei following a grammatical convention as set out in Rule 12c of the International Code of Nomenclature of Bacteria. In 2007 Lambert and Loria recommended that the original name of Streptomyces scabies be kept due to its long-established use and it continues to be used today. In 1979 Elesawy and Szabo proposed it be assigned to the Diastatochromogenes cluster along with S. neyagawaensis, S. bottropensis, S. diastatochromogenes, S. eurythermus and S. griseosporeus, which was later confirmed by other authors based on morphological and genetic analyses. At least four other species of Streptomyces also cause diseases on potato tubers. The most widespread species other than S. scabies are S. turgidiscabies and S. acidiscabies, which can be distinguished based on their morphology, the way they utilise food sources and their 16S RNA sequences. Unlike S. scabies, S. acidiscabies is predominantly seed-borne rather than soil-borne and be suppressed using insecticides and nematicides, suggesting that microfauna play a role in its transmission. In 2003 three other species of Streptomyces that cause common scab symptoms were isolated in Korea and named S. luridiscabiei, S. puniciscabiei and S. niveiscabiei. They differ from S. scabies by having spores that are of different colours. S. ipomoea causes a similar disease on sweet potato tubers. There are also other species of Streptomyces found in scab lesions on potato tubers that do not cause disease. 16 distinct strains have been isolated from tubers and based on a genetic analysis of them, they are most similar to S. griseoruber, S. violaceusniger, S. albidoflavus and S. atroolivaceus. Streptomyces scabies is a streptomycete bacteria which means it forms a mycelium made of hyphae, a growth form more usually associated with fungi. The hyphae of Streptomyces, are much smaller than those of fungi (0.5–2.0 μm) and form a heavily branched mycelium. They are Gram-positive and have a high proportion of the DNA bases guanine and cytosine (71%) in their genome. The genome of strain 87.22 has been sequenced and it is 10.1 Mbp, encoding 9,107 provisional genes. All Streptomyces genomes sequenced so far are relatively large for bacteria, but the genome of S. scabies is the largest. When cultured on agar the hyphae develop aerial fragments which bear chains of spores, giving the culture a fuzzy appearance. The chains of spores have the appearance of corkscrews and are grey in colour. These chains allow it to be differentiated from other species that are virulent on potatoes. Each chain contains 20 or more spores that are 0.5 by 0.9–1.0 μm, smooth and grey. Bacteria are often distinguished by their ability to grow on media containing different substances, which they either feed on or that inhibit their growth. Defining characteristics of strains of S. scabies are that they grow on the sugar raffinose, are unable to degrade xanthine and when grown on media containing the amino acid tyrosine, they produce the pigment melanin, the same chemical that gives humans their skin colour. This trait is often associated with their ability to cause disease, but it is not always present and is considered a secondary trait. They are killed by 10 IU of the antibiotic penicillin G per ml, 25 μg of oleandomycin per ml, 20 μg of streptomycin per ml, 10 μg of thallous acetate per ml, 0.5 μg of crystal violet per ml, and 1,000 μg of phenol per ml. The lowest pH at which they will grow varies slightly between strains but is between 4 and 5.5. When it infects crops it causes corky lesions to form in the tuber or tap root. The lesions are typically brown, with a diameter of several millimetres but the size and colour can vary depending on environmental conditions. The disease does not affect the yield nor make the tubers inedible, but decreases the quality of the crop, which lessens their value or even makes them unmarketable.