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 GC 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. The now uncommonly used
streptomycin takes its name directly from Streptomyces. Streptomycetes are
infrequent pathogens, though infections in humans, such as mycetoma, can be
caused by S. somaliensis and S. sudanensis, and in plants can be caused
by S. caviscabies, S. acidiscabies, S. turgidiscabies and S. scabies.
Taxonomy Streptomyces is the type genus of the
family Streptomycetaceae and currently covers close to 576 species with the
number increasing every year. Acidophilic and acid-tolerant strains
that were initially classified under this genus have later been moved to
Kitasatospora and Streptacidiphilus. Species nomenclature are usually based
on their color of hyphae and spores. Saccharopolyspora erythraea was formerly
placed in the present genus, too. Morphology
The genus Streptomyces includes aerobic, Gram-positive, filamentous bacteria that
produce well-developed vegetative hyphae with branches. They form a complex
substrate mycelium that aids in scavenging organic compounds from their
substrates. Although the mycelia and the aerial hyphae that arise from them are
amotile, mobility is achieved by dispersion of spores. Spore surfaces may
be hairy, rugose, smooth, spiny or warty. In some species, aerial hyphae
consist of long, straight filaments, which bear 50 or more spores at more or
less regular intervals, arranged in whorls. Each branch of a verticil
produces, at its apex, an umbel, which carries from two to several chains of
spherical to ellipsoidal, smooth or rugose spores. Some strains form short
chains of spores on substrate hyphae. Sclerotia-, pycnidia-, sporangia-, and
synnemata-like structures are produced by some strains.
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 S. violaceoruber, and is not a validly described separate
species; "S. coelicolor A3(2)" is not to be mistaken for the actual S.
coelicolor, although it is often referred to as S. coelicolor for
convenience. 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. At 10.1 Mbp long and encoding 9,107 provisional
genes, it is the largest known Streptomyces genome sequenced, probably
due to the large pathogenicity island. 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. Sometimes, proteins do not fold
properly, which may lead to insolubility, deposition in inclusion
bodies, and loss of bioactivity of the product. Though E. coli strains 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 species results in secretion directly into the
extracellular medium. In addition, Streptomyces species 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.
Plant pathogenic bacteria So far, ten species belonging to this
genus have been found to be pathogenic to plants:
S. scabiei S. acidiscabies
S. europaeiscabiei S. luridiscabiei
S. niveiscabiei S. puniciscabiei
S. reticuliscabiei S. stelliscabiei
S. turgidiscabies S. ipomoeae
Medicine Streptomyces is the largest
antibiotic-producing genus, producing antibacterial, antifungal, and
antiparasitic drugs, and also a wide range of other bioactive compounds, such
as immunosuppressants. Almost all of the bioactive compounds produced by
Streptomyces are initiated during the time coinciding with the aerial hyphal
formation from the substrate mycelium. = Antifungals=
Streptomycetes produce numerous antifungal compounds of medicinal
importance, including nystatin, amphotericin B, and natamycin.
= Antibacterials= Members of the Streptomyces genus are
the source for numerous antibacterial pharmaceutical agents; among the most
important of these are: Chloramphenicol
Daptomycin Fosfomycin
Lincomycin Neomycin
Nourseothricin Puromycin
Streptomycin Tetracycline
Oleandomycin Tunicamycin
Mycangimycin Boromycin
Clavulanic acid is a drug used in combination with some antibiotics to
block and/or weaken some bacterial-resistance mechanisms by
irreversible beta-lactamase inhibition. Novel antiinfectives currently being
developed include Guadinomine, a compound that blocks the Type III
secretion system of Gram-negative bacteria.
= 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 and bleomycin are antineoplastic drugs; boromycin
exhibits antiviral activity against the HIV-1 strain of HIV, as well as
antibacterial activity. S. hygroscopicus and S.
viridochromogenes produce the natural herbicide bialaphos.
See also Antimycin A - compound produced by this
bacterium used in piscicides Streptomyces isolates
References Further reading
Baumberg S. Genetics and Product Formation in Streptomyces. Kluwer
Academic. ISBN 978-0-306-43885-1. Gunsalus IC. Bacteria:
Antibiotic-producing Streptomyces. Academic Press. ISBN 978-0-12-307209-2.
Hopwood DA. Streptomyces in Nature and Medicine: The Antibiotic Makers. Oxford
University Press. ISBN 978-0-19-515066-7.
Dyson P, ed.. Streptomyces: Molecular Biology and Biotechnology. Caister
Academic Press. ISBN 978-1-904455-77-6. External links
Current research on Streptomyces coelicolor at the Norwich Research Park
Some current Streptomyces Research & Methods / Protocols / Resources
S. avermitilis genome homepage S. coelicolor A3(2) genome homepage
Streptomyces.org.uk homepage StrepDB - the Streptomyces genomes
annotation browser Streptomyces Genome Projects from
Genomes OnLine Database