Streptomyces spp: Culture; Molecular diagnosis (PCR).
Streptomyces is the broadest genus within the Actinomycetales order. It includes more than 500 species of bacteria that are characterized by being gram-positive, aerobic and having a filamentous appearance, which form well-developed vegetative hyphae with ramifications. Its genome has a high content of guanine and cytosine, and they have a very complex secondary metabolism. They are found mainly in the soil and among the decaying plant matter, and have a characteristic "wet earth" smell that is due to the production of a volatile metabolite, geosmine.
The hyphae of these bacteria form a mycelium complex, similar to that of fungi, which helps in the search for organic compounds from their substrates. Mycelia and hyphae are nonmobile, but they produce spores that contribute to their dispersion. The spores are formed by the fragmentation of the hyphae and are born in straight, wavy or helical chains. Streptomyces colonies are slow growing, and produce a wide variety of pigments, responsible for the color of both the vegetative part and the mycelium.
Streptomyces species are catalase positive, reduce nitrates to nitrites and degrade adenine, esculin, casein, gelatin, hypoxanthine, starch and L-tyrosine. The peptidoglycan of its cell wall contains large amounts of L-diaminopimelic acid (L-DAP), and does not contain mycolic acids.
The most important characteristic of Streptomyces species is their ability to produce secondary metabolites with antibacterial properties (chloramphenicol, daptomycin, fosfomycin, streptomycin, tetracycline, etc.), antifungals (nystatin, amphotericin B, natamycin), antiparasitic (ivermectin), antitumor agents (bleomycin) and immunosuppressants (tacrolimus, sirolimus). Genomic analysis has shown that any strain has the potential to produce dozens of such secondary metabolites, and metagenomic analysis has revealed a large number of sets of relevant biosynthetic genes. Therefore, these organisms are being studied more and more with the expectation that they will contribute significantly to the development of new therapeutic agents to combat the global emergence of antibiotic resistance in pathogenic bacteria, as well as provide other bioactive substances with diverse applications, both in medicine as in agriculture and other fields.
However, this is only one aspect of the interest and importance of these bacteria. Ecologically, streptomycetes have key roles in the natural recycling of fungal and plant cell walls. Some of them have developed intimate associations with insects or plants, and a few have acquired pathogenic characteristics.
Most of the Streptomyces are efficient colonizers of the rhizosphere, although they can also be endophytes, colonizing the internal tissues of the host plants. These attributes may be due to characteristics such as gene expression controlled by "quorum sensing", the multiplication rate, the production of antibiotics, siderophores, cellulases, phytohormones, amino acid synthesis, chitinase, lipase and β-1,3-glucanase production. Thus, microorganisms of great importance are considered to favor the growth of vegetables, as well as to protect plants from the action of different pathogens.
Of particular relevance is the ability of some Streptomyces species to establish parasitic interactions with plants, which leads to the development of economically important crop diseases. This is a rare attribute among Streptomyces, since only a few species of the hundreds described are known to function as plant pathogens.
Most of the Streptomyces phytopathogenic species cause a disease called "common scab", which is especially important at an economic level because it affects the potato, although it can also affect other crops such as beets, radishes or carrots. The main symptom of this disease is the appearance of dark scab lesions on the surface of the tuber, which may be superficial, elevated or deep. It is believed that the type of injury produced depends on factors such as environmental conditions, the pathogen load and its aggressiveness, and the susceptibility of the crop. The disease affects only the underground parts of susceptible hosts, and is restricted to those rapidly growing plant tissues, such as developing tubers.
The first species that was described as causing "common scab" was Streptomyces scabiei, but there are other species that have been related to this disease, such as S. turgidiscabies, S. steliscabiei, S. niveiscabiei, S. luridiscabiei and S. puniciscabiei. A similar disease called "acid scab" is caused by S. acidiscabies and is identical to the "common scab" in its symptoms, but occurs in low pH soils (pH ≤ 5.2) where the growth of S. scabiei and other species is inhibited
S. scabiei also causes a peanut disease called "peanut wart." A closely related species, S. europaeiscabiei, which has been found in Europe, North America and Korea, causes "common scabies", but some strains can also cause a "netted scab", which is characterized by superficial lesions with a net appearance Network This disease can also be caused by S. reticuliscabiei and S. aureofaciens, and is distinguished from "common scab" because it also produces severe necrosis of the fibrous roots and leads to significant yield losses.
Another disease, associated with S. ipomoeae, is the so-called “sweet potato soil rot”. In this case, the bacterium infects both the fibrous roots and the fleshy storage roots of the sweet potato. Infected fibrous roots become necrotic and die, resulting in a reduction of plant support and growth, as well as a decrease in planting yield and marketability.
Cucurbitaceae root tumor was first discovered in melons in 1982 on the island of Kyushu, Japan. This disease is characterized by large galls in the fibrous roots, lower plant growth and wilting, which indicates a reduced root function. Subsequently, root tumor was found in cucumbers in the same region of Japan. The symptoms of this disease are different from those previously attributed to other Streptomyces species, and the morphological similarity of galls with those caused by Agrobacterium tumefaciens and Pseudomonas savastanoi suggests the participation of plant growth regulators. The bacterium that causes the melon root tumor is present only in the outer cells of the gills. The cells inside the tumor are not infected. It was suggested that the increase in cell division or enlargement is due to a physiologically active substance produced by the pathogen.
Early research on the mechanisms of pathogenicity of Streptomyces in plants revealed the role of specialized phytotoxic metabolites (phytotoxins) as key pathogenicity factors in these organisms. The first phytotoxins associated with the pathogenicity of these bacteria were thaxtomine A and B, which were isolated from tuber tissue infected by S. scabiei. Although thaxtomins are now considered the main determinant of pathogenicity associated with diseases such as "common scab" and "acid scab", there is now evidence that other phytotoxins can play a key role in the development or severity of diseases of the plants caused by Streptomyces spp.
Tests performed in IVAMI:
- Isolation in culture and presumptive identification by microscopic examination.
- Isolation in culture and identification by molecular methods (PCR and sequencing).
- Tubers with suspected involvement by Streptomyces spp.
- Land or crop water where crop disruption occurs.
- Cultures of bacteria isolated from plants, land, water, ..., compatible with Streptomyces spp. for identification
Storage and shipment of the sample:
- Refrigerated (preferred) for less than 2 days.
- Frozen: more than 48 hours.
Delivery of results:
- Cultivation of Streptomyces spp. from samples (land, plants, water, ...): 7 days.
- Molecular detection (PCR) of some Streptomyces species: 48 to 72 hours.
- Molecular identification of bacteria supposedly corresponding to Streptomyces isolated in culture: 48 to 72 hours.
Cost of the test:
- Cultivation: Consult email@example.com.
- Molecular diagnosis (PCR): Consult firstname.lastname@example.org.