study of these microorganisms. Although, most newly discovered Shewanella
strains were initially classified as S. putrefaciens, DNA:DNA hybridization and 16S
rRNA sequences resulted in the identification of more than 60 species within this
genus [71].
It was only in 1988, with the discovery of members of the Shewanella genus
with the capacity to perform EET, that these microorganisms started to find a
prominent position within the scientific community [26]. These findings strongly
suggested that this genus could play important roles in the biogeochemical cycles of
the elements and in biotechnological applications, such as in BES [23, 32].
Members of the genus Shewanella are facultative anaerobic Gram negative
c-Proteobacteria. They generally possess a single polar flagellum and a rod shape
with 2–3 lm in length and 0.5–0.6 lm in diameter [72]. The vast majority of the
Shewanella isolates were obtained from marine environments, where they are
sometimes found as fish pathogens with important impact in the aquaculture
industry [73, 74]. Shewanella are also found in other habitats such as the freshwater
S. oneidensis MR-1 [72].
Numerous Shewanella species are capable of growing at low temperatures
(<5 °C) even though their optimal growth temperature is above 16 °C [75]. By
contrast, those species found to be opportunistic human pathogens such as S. algae
can grow at the relatively high temperature of 42 °C [76]. Another major aspect of
the versatility of Shewanella is their ability to utilize a broad variety of organic and
inorganic compounds as a final electron acceptor [77]. This allows them to thrive in
a wide range of aquatic habitats, both marine and freshwater, and play a significant
role in several biogeochemical redox cycles, including those of iron and manganese
[26]. Since many of these organic and inorganic compounds are toxic or highly
insoluble, they do not enter the bacteria and are extracellularly reduced by terminal
reductases localized on the surface of the cell [31].
In 2001, the genome of S. oneidensis MR-1 was sequenced, primarily due to its
position as a model organism for dissimilatory metal reduction and its potential role
in several biotechnological applications [78]. Since then, approximately 40 other
Shewanella genomes have been sequenced [79]. Analysis of the S. oneidensis
MR-1 genome revealed that the chromosome encodes for 41 putative c-type
cytochromes (9 in the cytoplasmic membrane, 27 in the periplasm, and 5 in the
outer membrane) [80, 81]. The capability to transfer electrons to a vast range of
electron acceptors and perform EET is linked to this large number of c-type
cytochromes, which spans from the cytoplasmic membrane to the outer membrane
[82, 83]. Using a variety of genetic (e.g., knock-out studies) and biochemical
techniques (e.g., protein characterization), some of the components involved have
been identified and characterized in detail (Fig. 3). The so called “minimal setup” of
redox proteins which are assigned to the EET process will be discussed below.
222
B. M. Fonseca et al.
strains were initially classified as S. putrefaciens, DNA:DNA hybridization and 16S
rRNA sequences resulted in the identification of more than 60 species within this
genus [71].
It was only in 1988, with the discovery of members of the Shewanella genus
with the capacity to perform EET, that these microorganisms started to find a
prominent position within the scientific community [26]. These findings strongly
suggested that this genus could play important roles in the biogeochemical cycles of
the elements and in biotechnological applications, such as in BES [23, 32].
Members of the genus Shewanella are facultative anaerobic Gram negative
c-Proteobacteria. They generally possess a single polar flagellum and a rod shape
with 2–3 lm in length and 0.5–0.6 lm in diameter [72]. The vast majority of the
Shewanella isolates were obtained from marine environments, where they are
sometimes found as fish pathogens with important impact in the aquaculture
industry [73, 74]. Shewanella are also found in other habitats such as the freshwater
S. oneidensis MR-1 [72].
Numerous Shewanella species are capable of growing at low temperatures
(<5 °C) even though their optimal growth temperature is above 16 °C [75]. By
contrast, those species found to be opportunistic human pathogens such as S. algae
can grow at the relatively high temperature of 42 °C [76]. Another major aspect of
the versatility of Shewanella is their ability to utilize a broad variety of organic and
inorganic compounds as a final electron acceptor [77]. This allows them to thrive in
a wide range of aquatic habitats, both marine and freshwater, and play a significant
role in several biogeochemical redox cycles, including those of iron and manganese
[26]. Since many of these organic and inorganic compounds are toxic or highly
insoluble, they do not enter the bacteria and are extracellularly reduced by terminal
reductases localized on the surface of the cell [31].
In 2001, the genome of S. oneidensis MR-1 was sequenced, primarily due to its
position as a model organism for dissimilatory metal reduction and its potential role
in several biotechnological applications [78]. Since then, approximately 40 other
Shewanella genomes have been sequenced [79]. Analysis of the S. oneidensis
MR-1 genome revealed that the chromosome encodes for 41 putative c-type
cytochromes (9 in the cytoplasmic membrane, 27 in the periplasm, and 5 in the
outer membrane) [80, 81]. The capability to transfer electrons to a vast range of
electron acceptors and perform EET is linked to this large number of c-type
cytochromes, which spans from the cytoplasmic membrane to the outer membrane
[82, 83]. Using a variety of genetic (e.g., knock-out studies) and biochemical
techniques (e.g., protein characterization), some of the components involved have
been identified and characterized in detail (Fig. 3). The so called “minimal setup” of
redox proteins which are assigned to the EET process will be discussed below.
222
B. M. Fonseca et al.