Sulfur cycle in microorganisms WORLD OF MICROBIOLOGY AND IMMUNOLOGY536•
SSulfur cycle in microorganismsULFUR CYCLE IN MICROORGANISMS
Sulfur is a key constituent of certain amino acids, proteins, and
other biochemicals of both eukaryotesand prokaryotes. For
example, sulfur is a component of an enzyme called coenzyme
A, which is vital for respirationof plant and animal cells.
Plants are not able to directly use elemental sulfur.
Instead, they rely on the ability of certain types of bacteriato
convert elemental sulfur to another form. Bacteria that are
known as chemoautotrophic bacteriacan combine sulfur with
water and oxygen to produce hydrogen sulfate. Plants are able
to incorporate the sulfate compound into proteins.
Bacteria can participate in the reduction of sulfur, in
which the sulfur compounds act as an electron receptor, or in
the oxidation of sulfur, in which an electron is removed from
the sulfur compound.
Hydrogen sulfide, a gas that has the characteristic smell
of rotten eggs, is toxic to air-requiring plant and animal tissue.
However, the gas can be utilized by oxygen-requiring bacteria
such as Thiothrixand Beggiatoa, and by the anaerobic purple
sulfur bacteria. These bacteria utilize the hydrogen sulfide and
carbon dioxide to produce elemental sulfur.
Sulfur can occur in many chemically reduced mineral
forms, or sulfides, in association with many metals. The most
common metal sulfides in the environment are iron sulfides
(called pyrites when they occur as cubic crystals), but all
heavy metals can occur in this mineral form. Whenever metal
sulfides are exposed to an oxygen-rich environment, certain
bacteria begin to oxidize the sulfide, generating sulfate as a
product, and tapping energy from the process that is used to
sustain their own growth and reproduction. This autotrophic
process is called chemosynthesis, and the bacteria involved
are named Thiobacillus thiooxidans.When a large quantity of
sulfide is oxidized in this way, an enormous amount of acidity
is associated with the sulfate product. Indeed, Thiobacillus
prosperushas an optimum pHof between pH=1 and pH=4,
and Thiobacillus ferroxidanshas an optimum pH range of
between pH=2 and pH=4.
Some species of the genus Thiobacillus, including
Thiobacillus thiooxidans and Thiobacillus ferroxidansare
able to process elemental sulfur and iron sulfate, respectively.
Within the past several decades, the existence of bacte-
ria that utilize sulfur at hydrothermal ventsdeep within the
ocean has been chronicled. These bacteria form the basis of
the entire complex ecosystem that springs up, in the total
absence of light, around the sulfur-rich emission form the
vents. Some of the bacteria live in symbiosis with the so-
called tubeworms that thrive in this ecosystem. The worms
provide protection and an incoming supply of nutrients to the
bacteria. In turn, the bacteria metabolize the sulfur to forms
usable to the worms. The discovery of the bacterial basis of
this undersea ecosystem greatly increased human awareness
of the microbial diversity on Earth.See alsoBiogeochemical cycles; Economic uses and benefits
of microorganismsSUPERFUND• seeBIOREMEDIATION
SSwine feverWINE FEVER
Swine fever is a viral disease that afflicts swine. The disease
is also known as hog cholera. A related form of the disease is
called African swine fever.
The virus that causes swine fever is a member of the
family Flaviviridae and the genus Pestivirus. A virus causes
African swine fever from the family Iridovirisae. The virus
itself is designated Asfarviridae, a name derived from
“African Swine Fever and Related Viruses.” The virus is so far
the sole member of the newly created genus Asfivirus.
The two virusesare quite different from one another in
structure and behavior. Yet, the diseases they cause are very
similar with respect to their transmission and the symptoms of
infection. Both viruses can be easily passed from an infected
pig to a healthy pig. Contact can be direct or via body secre-
tions or feces. The resulting infection can be mild or more
severe. Also a long lasting form of infection can result. The
more severe form of the infection results in a very high fever
that can lead to convulsions. Often the skin appears discolored
and pigs will huddle together. Death usually results a week or
two weeks after the appearance of symptoms. The chronic
form of the infection displays similar but less severe symp-
toms. The symptoms can persist for months before the swine
succumbs. Finally, an infection can display few if any symp-
toms. However, this mild bout of the disease can caused
reduce number of live births.
Swine that survive the infections can be life-long carri-
ers of the viruses.
Distinction of swine fever from African swine fever is
only possible by the direct examination of the viruses. The
examinations typically involve the isolation of the virus in an
appropriate cell cultureand the use of fluorescent-labeled anti-
bodies and the enzyme-linked immunosorbant assay(ELISA).
Both viruses are easily spread from hog to hog. Pigs and
related animals such as wild boar are the only natural reser-
voirs of Pestivirus. Asfiviruscan reside in species such as
ticks. The viruses can also be accidentally carried from an
infected swine to a susceptible swine via humans, animals,
and birds. This is in part due to the environmental persistence
of the viruses. For example, Pestivirusis able to survive cold
conditions, and so can survive in a refrigerated carcass during
transport. As well, the virus is able to survive some forms of
meat processing (e.g., curing and smoking). However,
Pestivirusis susceptible to various disinfectants (e.g., sodium
hydroxide, Formalin, various detergents).
Pestivirusinfects the blood and virtually all body fluids
of an infected animal. Furthermore, the animal can excrete the
virus for months.
The adverse effects to the health of the swine, and to
their economic value, has made the eradication of swine fevers
a priority in many countries around the world. In the United
States, for example, a concerted effort by State and Federal
governments and industry over almost two decades has virtu-womi_S 5/7/03 8:20 AM Page 536