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:
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
Streptomyces is the largest genus of