Streptomyces

Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content. Found predominantly in soil and decaying vegetation, most streptomycetes produce spores, and are noted for their distinct "earthy" odor that results from production of a volatile metabolite, geosmin.

Streptomycetes are characterised by a complex secondary metabolism. They produce over two-thirds of the clinically useful antibiotics of natural origin (e.g., neomycin, chloramphenicol). The now uncommonly-used streptomycin takes its name directly from Streptomyces. Streptomycetes are infrequent pathogens, though infections in human such as mycetoma can be caused by S. somaliensis and S. sudanensis, and in plants can be caused by S. caviscabies and S. scabies.

Selected species
Roughly 550 species of Streptomyces are recognized at present, but new ones are still being discovered (e.g., S. aomiensis and S. hyderabadensis, both described in 2010). Many species are named after their colorful hyphae and/or spores.

Significant Streptomyces species include:


 * Streptomyces achromogenes
 * Streptomyces alboniger
 * Streptomyces albus
 * Streptomyces ambofaciens
 * Streptomyces aomiensis
 * Streptomyces aureomonopodiales
 * Streptomyces aureofaciens
 * Streptomyces avermitilis
 * Streptomyces avidinii
 * Streptomyces bikiniensis
 * Streptomyces caespitosus
 * Streptomyces cattleya
 * Streptomyces caviscabies
 * Streptomyces chartreusis
 * Streptomyces chusanensis
 * Streptomyces clavuligerus
 * Streptomyces coelicolor
 * Streptomyces diastaticus
 * Streptomyces exfoliatus
 * Streptomyces faecalis
 * Streptomyces faecium
 * Streptomyces felleus
 * Streptomyces ferralitis


 * Streptomyces fimbriatus
 * Streptomyces filementosus
 * Streptomyces fradiae
 * Streptomyces fulvissimus
 * Streptomyces globisporus
 * Streptomyces griseoruber
 * Streptomyces griseoviridis
 * Streptomyces griseus
 * Streptomyces hyderabadensis
 * Streptomyces hygroscopicus
 * Streptomyces iysosuperficus
 * Streptomyces lactamdurans
 * Streptomyces lavendulae
 * Streptomyces lincolnensis
 * Streptomyces lividans
 * Streptomyces lusitanus
 * Streptomyces mediterranei
 * Streptomyces niveus
 * Streptomyces nodosus
 * Streptomyces noursei
 * Streptomyces novocastria
 * Streptomyces olivochromogenes
 * Streptomyces orientalis'


 * ''Streptomyces peucetius
 * ''Streptomyces phaeochromogenes'
 * Streptomyces platensis
 * Streptomyces pulveraceus
 * Streptomyces rimosus
 * Streptomyces roseosporus
 * Streptomyces sannurensis
 * Streptomyces scabies
 * Streptomyces somaliensis
 * Streptomyces stanford
 * Streptomyces stramineus
 * Streptomyces sudanensis
 * Streptomyces tendae
 * Streptomyces thermodiastaticus
 * Streptomyces thermoviolaceus
 * Streptomyces toxytricini
 * Streptomyces tsukubaensis
 * Streptomyces tubercidicus
 * Streptomyces venezuelae
 * Streptomyces verticillus
 * Streptomyces violaceoruber
 * Streptomyces violaceusniger
 * Streptomyces violochromogenes
 * Streptomyces viridochromeogenes

Saccharopolyspora erythraea was formerly placed in the present genus too (as Streptomyces erythraeus).

Genomics
The complete genome of "S. coelicolor" strain A3(2) was published in 2002. At the time, the S. coelicolor genome was thought to contain the largest number of genes of any bacterium. The chromosome is 8,667,507 bp long with a GC-content of 72.1% and is predicted to contain 7,825 protein encoding genes. In terms of taxonomy, "S. coelicolor" A3(2) belongs to the species of S. violaceoruber and not a validly described separate species; "S. coelicolor" A3(2) is not to be mistaken for the actual S. coelicolor (Müller).

The first complete genome sequence of S. avermitilis was completed in 2003. Each of these genomes forms a chromosome with a linear structure, unlike most bacterial genomes, which exist in the form of circular chromosomes. The genome sequence of S. scabies, a member of the genus with the ability to cause potato scab disease, has been determined at the Wellcome Trust Sanger Institute and is currently in annotation, with publication scheduled for 2009.

Biotechnology
In recent years, biotechnology researchers have begun using Streptomyces species for heterologous expression of proteins. Traditionally, Escherichia coli was the species of choice to express eukaryotic genes since it was well understood and easy to work with. Expression of eukaryotic proteins in E. coli may be problematic however, due to the inability of bacteria to glycosylate proteins. And there may also be issues with incorrectly folded proteins, which may lead to insolubility, deposition in inclusion bodies, and loss of bioactivity of the product. Though E. coli have secretion mechanisms, these are of low efficiency and result in secretion into the periplasmic space, whereas secretion by a Gram-positive bacterium such as a Streptomyces spp. results in secretion directly into the extracellular medium. In addition, Streptomyces spp. have more efficient secretion mechanisms than E.coli. The properties of the secretion system is an advantage for industrial production of heterologously expressed protein because it simplifies subsequent purification steps and may increase yield. These properties among others make Streptomyces spp. an attractive alternative to other bacteria such as E. coli and Bacillus subtilis. As is true for all bacteria, Streptomyces spp. does not have the ability to glycosylate proteins, which may necessitate using an eukaryotic host, if this is required for bioactivity of the product.

Medicine
Streptomyces is the largest antibiotic-producing genus, producing antibacterial, antifungal, and anti-parasitic drugs, and also a wide range of other bioactive compounds such as immunosuppressants.

Antifungals
Streptomycetes produce numerous antifungal compounds of medicinal importance, including nystatin (from S. noursei), amphotericin B (from S. nodosus), and natamycin (from S. natalensis).

Antibacterials
Members of the Streptomyces genus are the source for numerous antibacterial pharmaceutical agents; among the most important of these are:


 * Cefoxitin (from S. lactamdurans)
 * Chloramphenicol (from S. venezuelae)
 * Daptomycin (from S. roseosporus)
 * Fosfomycin (from S. fradiae)
 * Lincomycin (from S. lincolnensis)
 * Neomycin (from S. fradiae)
 * Puromycin (from S. alboniger)
 * Rifamycin (from S. mediterranei)
 * Streptomycin (from S. griseus)
 * Tetracycline (from S. rimosus)
 * Vancomycin (from S. orientalis)

Clavulanic acid (from S. clavuligerus) is a drug used in combination with some antibiotics (like amoxicillin) in order to block and/or weaken some bacterial resistance-mechanisms by irreversible beta-lacatamase inhibition.

Antiparasitic drugs
S. avermitilis is responsible for the production of one of the most widely employed drugs against nematode and arthropod infestations, ivermectin.

Other
Less commonly, streptomycetes produce compounds used in other medical treatments: migrastatin (from S. platensis) and bleomycin (from S. verticillus) are antineoplastic (anti-cancer) drugs.

S. hygroscopicus and S. viridochromeogenes produce the natural herbicide bialaphos.