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genus is commonly found amongst soil bacteria. Species that are members of this genus are gram positive,
obligate aerobes. They are chemoorganotrophs and have an oxidative metabolism
(Holt et al., 1994). Arthrobacter
species have been particularly useful in the bioremediation of groundwater
contaminated with pesticides and herbicides; this is made possible because of
their adaptable genomes, which can handle stressful conditions and environments.
(Mongodin et al., 2006).
Motility was originally discovered to be associated with the
distinct morphogenic cycles of three Arthrobacter
species: Arthrobacter atrocyaneus, Arthrobacter citreus, and
Arthrobacter simplex. These members of the genus displayed motility beginning at
the time of induction to the rod-shaped morphology, yet were later non-motile
when in the coccoid morphology (Stanlake and Clark, 1976).
Although not normally considered to be a threat to our health,
there have been cases where Arthrobacter species
were identified as the causative agent of disease. In particular, Arthrobacter woluwensis was determined
to be the cause of infective endocarditis in a 39-year-old patient who had used intravenous drugs for many years. Outside of this case, Arthrobacter species have only been
reported five times as the cause of human disease (Bernasconi et al., 2004). Arthrobacter species have also been
isolated in the past from patients with compromised immune systems, but
pathogenicity was low (Conn and Dimmick, 1947). Another species, Arthrobacter cumminsii, has actually
become the most commonly encountered strain, but is difficult to identify and
therefore may be underdiagnosed (Funke et al., 1998).
Characteristics of Arthrobacter
species were being described by H.J. Conn as early as 1928, but it was not
until 1957 that the Arthrobacter
genus was included as part of the Corynebacteriaceae family (Breed et al.,
1957). The major distinguishing feature for Arthrobacter
species was the way the species could alternate between rods in young
cultures and coccoids in older cultures. This reliance on the morphological
characteristics inevitably led to confusion over what actually constituted a
member of the genus and led to many misclassifications of species. Over time,
more characteristics were outlined as being necessary for inclusion as a member
of the Arthrobacter genus, and two groups,
which all species must be classified under, were developed. These groups are
the A. globiformis/A. citreus group and the A. nicotianae group, and they
differ in three important areas including their peptidoglycan construction,
teichoic acid content, and lipid composition (Jones and Keddie, 2007).
Based off the results of 16s rRNA studies, Arthrobacter species are related to the
following coryneform genera: Aureobacterium,
Cellulomonas, Curtobacterium and
Microbacterium. Arthrobacter species
are also related, albeit less closely, to Brevibacterium
(Stackebrandt et al., 1980; Stackebrandt and Woese, 1981). They all appear on
the actinomycete branch of a phylogenetic tree for gram-positive eubacteria
with high GC content. Also, these Arthrobacter species cannot be separated
phylogenetically from members of the Micrococcus
genus due to similarities. However, both Arthrobacter and Micrococcus
are considered their own taxa (Jones and Keddie, 2007).
were first found in soils in the 19th century and have since
established themselves as one of the dominant bacterial genera found there. The
Arthrobacter genus has demonstrated
that it is resourceful metabolically because of its ability to grow on many
different types of substrates. This genus is not only resourceful
metabolically, but also resilient to undesirable environmental conditions. Arthrobacter have been found to survive
in places such as the deep subsurface, arctic ice, chemically contaminated
sites, and radioactive environments; they can handle starvation, radiation,
exposure to oxygen radicals and even toxic chemicals. This incredible ability
to survive has been attributed to Arthrobacter’s
unique morphological cycle, which allows the species to remain in a stable,
coccoid configuration at stressful times (Mongodin et al., 2006).
Tests have been performed to determine what Arthrobacter species can utilize as
their carbon sources for growth. Of 130 Arthrobacter
isolates tested, seventy-seven percent were able to survive and grow off of at
least two aromatic substrates (Stevenson, 1967). Arthrobacter with simple nutritional needs can make use of the most
aromatic hydrocarbons and these hydrocarbons can even pass as the lone source
of carbon for these species. It is also important to know that Arthrobacter species have an optimal
growth temperature of between 20 and 30 C. A neutral to slightly alkaline pH is
also preferred for growth (Holt et al., 1994).
genomes are available via the National Center for Biotechnology Information (NCBI).
* Genome sequencing in progress.
FB24 was isolated from soil that had been polluted with chromate, lead and
hydrocarbons. This particular strain was interesting to scientists conducting
experiments because it had an extremely high tolerance for the chromium. In
fact, it was found to be resistant to other heavy metals and may be radiation
resistant as well. Further study of this species may help reveal, from a
molecular standpoint, why Arthrobacter
can survive such taxing conditions (Nakatsu et al., 2005; Joynt et al., 2006).
A very important feature of the Arthrobacter genus is their morphology. Specifically, Arthrobacter are observed to be rod-like
during the exponential growth phase and coccoid during the stationary phase.
This change from rod-like to coccoid during the Arthrobacter growth cycle is influenced by a key nutrient called
biotin. The age of a particular culture media is also important; rod-like
configurations are associated with fresh, “young” media, whereas the coccoid
configuration dominates in older media (Mullakhanbhai and Bhat, 1967). When
being observed, Arthrobacter species
range from yellow to white in their coloration and measure 2 mm in diameter, on
average. They are gram-positive and nonsporulating as well.
are obligate aerobes with respiratory metabolisms; there are no species in this
genus that have fermentative metabolisms (Holt et al., 1994). Nicotine, nucleic
acids, herbicides and pesticides can all be used as substrates for the
oxidative metabolism of different Arthrobacter
species. Two species have been found to supplement their normal metabolisms
with an anaerobic metabolism. This is necessary for Arthrobacter globiformis and Arthrobacter
nicotianae, two species that encounter environments that are sometimes
limited in oxygen. Also, Arthrobacter
can transform hexavalent chromium, which is toxic, into trivalent chromium,
which is its less toxic form (Megharaj et al., 2003).
Because of their metabolic diversity, Arthrobacter
species have been taken advantage of for their ability to biodegrade
various types of pollutants in our environment. Species of the genus, like Arthrobacter
sp. FB24, are quite resistant to certain heavy metals that are toxic, which
is useful. Arthrobacter aurescens strain TC1 can metabolize more
than 23 kids of s-triazine compounds, an important fact to consider since these
compounds are found in pesticides, resins, dyes, and explosives (Mongodin et
al., 2006). Arthrobacter crystallopoietes can reduce hexavalent chromium
levels in soil, which may mean that there are potential future applications for
it in bioremediation as well (Camargo et al., 2003).
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