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Applied and Environmental Microbiology, September 2002, p. 4653-4657, Vol. 68, No. 9
0099-2240/02/$04.00+0 DOI: 10.1128/AEM.68.9.4653-4657.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Characterization of the Lipopolysaccharides and Capsules of Shewanella spp.
Anton A. Korenevsky,1 Evgeny Vinogradov,2 Yuri Gorby,3 and Terry J. Beveridge1*
Department of Microbiology, College of Biological Science, University of Guelph, Guelph, Ontario, Canada, N1G 2W1,1
Institute for Biological Science, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6,2
Pacific Northwest National Laboratory, Richland, Washington 993523
Received 22 February 2002/
Accepted 4 June 2002

ABSTRACT
Electron microscopy, sodium dodecyl sulfate-polyacrylamide gel
electrophoresis with silver staining and
1H,
13C, and
31P-nuclear
magnetic resonance (NMR) were used to detect and characterize
the lipopolysaccharides (LPSs) of several
Shewanella species.
Many expressed only rough LPS; however, approximately one-half
produced smooth LPS (and/or capsular polysaccharides). Some
LPSs were affected by growth temperature with increased chain
length observed below 25°C. Maximum LPS heterogeneity was
found at 15 to 20°C. Thin sections of freeze-substituted
cells revealed that
Shewanella oneidensis,
S. algae,
S. frigidimarina,
and
Shewanella sp. strain MR-4 possessed either O-side chains
or capsular fringes ranging from 20 to 130 nm in thickness depending
on the species. NMR detected unusual sugars in
S. putrefaciens CN32 and
S. algae BrY
DL. It is possible that the ability of
Shewanella to adhere to solid mineral phases (such as iron oxides)
could be affected by the composition and length of surface polysaccharide
polymers. These same polymers in
S. algae may also contribute
to this opportunistic pathogen's ability to promote infection.

INTRODUCTION
Shewanella organisms are generally associated with aquatic habitats
and play important roles in the cycling of particulate iron
and organic matter, but they can also be opportunistic pathogens
(
33,
45). Because of their environmental significance, they
are presently under vigorous investigation and many new species
were recently included in the genus (
45). Most
Shewanella organisms
are capable of dissimilatory reduction of a wide range of electron
acceptors, including metal oxides [e.g., those of Fe(III) and
Mn(IV)]. Several reductive mechanisms are possible. Electron
flux from the organism to the solid oxide may occur via (i)
direct contact of bacterial outer membrane and oxide surface
(
2,
9,
21,
31), (ii) organic shuttles (such as humic acids or
quinones to mediate electron flow (
22,
32), or (iii) a combination
of both processes. For the direct contact model, metal reductases
(such as
c-type cytochromes) seem to be embedded in the outer
membrane of dissimilatory metal-reducing bacteria to facilitate
electron flow (
13,
29,
30). Lipopolysaccharide (LPS) and outer
membrane proteins could play principal roles in establishing
and maintaining contact with oxide minerals so that electron
transport to the terminal acceptor occurs. The junction between
cell and mineral must be tight so as to ensure that the reductase
functions effectively (
23). However, the cell surface structure
and LPS of
Shewanella, which could affect the cell-mineral connection,
are poorly understood (
28,
39,
40,
48). In this present article
we characterize those structural elements that can extend beyond
the outer face of the outer membrane, i.e., the LPS O-side chains
and capsular polymers.

Bacterial strains and growth conditions.
The strains used in this study are shown in Table
1. Most of
these strains were kindly provided by Doug Lies (Jet Propulsion
Laboratory, Pasadena, Calif.).
S. algae BrY was supplied by
both D. Lies and F. Caccavo, Jr. (while he was at the Department
of Microbiology, University of New Hampshire, Durham); the former
is designated
S. algae BrY
DL and the latter BrY
FC. All were
cultured on either tryptic soy broth or tryptic soy broth supplemented
with 2% (wt/vol) NaCl (i.e., for
S. pealeana and
S. woodyi).
Cultures were grown aerobically on a rotary shaker (150 rpm)
at temperatures from 5 to 37°C and were harvested at a mid-exponential
growth phase (optical density at 470 nm [OD
470] =

0.8).

SDS-PAGE and nuclear magnetic resonance (NMR) LPS analyses.
Proteinase K/sodium dodecyl sulfate-polyacrylamide gel electrophoresis
(SDS-PAGE) was used to analyze LPS (
16,
24), and the gels were
silver stained (
41). Gels of
S. putrefaciens,
S. oneidensis,
S. baltica, and
Shewanella sp. strains MR-4 and CL 256/73 displayed
only low-
Mr bands when grown at 26°C. Gels did not display
high-
Mr bands or "laddering" (indicative of smooth LPS [S-LPS]),
suggesting that these contained rough LPS (R-LPS) with core
oligosaccharide and no O-side chains (Fig.
1, lane 1, is representative).
S. amazonensis,
S. frigidimarina,
S. pealeana,
S. woodyi, and
most
S. algae organisms produced S-LPS (or possibly capsular
polysaccharides) as evidenced by bands located in the upper
regions of the gel (Fig.
1, lanes 2 to 6, is representative).
These bands sometimes were smeared (Fig.
1, lane 3) suggesting
that each band possessed similar but not identical polymer lengths.
At this point in our study, it was impossible to distinguish
these S-LPSs from capsular material. By definition capsular
macromolecules contain long-branched or unbranched homo- or
heteropolysaccharides linked to a lipid substituent, which anchors
them to the outer membrane. Capsular polymers can have either
lipid A or entirely different lipid anchors, such as diacylglycerolphosphate,
but the major differentiating property is polymer length (
46).
Capsular polysaccharides are longer than those of LPS and, as
such, should not enter the gels. Strangely,
S. algae BrY
FC lacked
bands in the top region of the gel (Fig.
1, lane 1), being different
from all other tested strains of
S. algae, including BrY
DL.
1H-,
13C-, and
31P-NMR was performed on selected strains and
spectra analyzed according to the method described by Vinogradov
et al. using a Varian Inova spectrometer (
44). Most cores that
have been studied possess high concentrations of carboxyl and
phosphoryl sites, making them highly polar so as to interact
with metal ions (
10,
20) and inanimate surfaces (
26).
S. putrefaciens CN32 (and related strains [
28,
47]) and
S. oneidensis MR-1 have
R-LPS (Table
2), which may aid their adhesion and close fit
to iron oxide minerals.
1H-,
13C-, and
31P-NMR analyses on CN32
indicated that its oligosaccharide backbone consisted of ß-Galf-(1-3)-ß-Gal-(1-4)-ß-Glc-

-DDHep2PEtN-(1-5)-

-Kdo4P-(1-6)-ß-GlcN4P-(1-6)-

-GlcN1P,
which possesses phosphate and carboxylate groups (i.e., 3-deoxy-
D-manno-2-octulosonic
acid [Kdo]) suggesting that this region is polar and can be
ionized. Phosphorylated Kdo is rare among extensively studied
bacteria, such as enterobacteria, and difficult to detect by
conventional colorimetric means (
17). At this point, since so
few
Shewanella LPSs have been studied in detail (
39,
40), it
is impossible to say if phosphorylated Kdo could be a common
trait of this genus or species.
From a dissimilatory medal-reducing bacterium point of view,
it may be advantageous for a bacterium to have its length of
LPS constrained so that the microbe can fit closely to the oxide,
thereby more efficiently using the mineral as a terminal electron
acceptor. Indeed, for a close fit to any inert surface, as long
as the hydophobicity and hydrophilicity properties are appropriate,
short LPSs should be more beneficial. Dissimilatory reduction
of metal oxides requires anaerobiosis, and our
Shewanella was
grown aerobically, but even under these conditions, close oxide-bacterium
union is seen (
14). Almost one-half of our strains possessed
R-LPS (Table
2), suggesting that core oligosaccharide was the
LPS terminus. Because of the high potential charge and relatively
short terminus (core oligosaccharide) on CN32 LPS, it is attractive
to suggest that these two characteristics allow a tight union
between bacterium and iron oxide, allowing good contact of the
putative outer membrane iron reductase so that electron flow
is ensured (
23,
29).
Many enterobacteria and pseudomonads are noted for the strong antigenicity that is expressed by their LPSs. Although endotoxicity is more a function of lipid A, O-side chains differentiate these bacteria into specific immunodominant groupings. For example, Pseudomonas aeruginosa PAO1 belongs to serogroup 05 and expresses two separate LPSs under normal growth conditions, A-band LPS ("common antigen") and B-band LPS ("serotype-specific antigen") (37). Here there are not only distinct differences in chemistry of the O-polysaccharides but also in their polymer lengths. LPSs frequently form distinctive ladderlike patterns on silver-stained gels. Many of our strains showed an S-LPS phenotype, often without a ladderlike appearance, suggesting that the O-polymers are of a relatively constant length. It was interesting that most S. algae strains contained S-LPS, since this species, unlike others, has been implicated in certain infections (33, 45). Here, the O-side chain could be a contributing virulence factor. NMR analysis of the polysaccharide chain of BrYDL LPS revealed -3)-
-D-BacNAc4Nbu-(1-3)-
-L-Rha-(1-2)-
-L-Rha-(1,2)-L-malyl-(4-2)-
-L-FucN-(1- to be the basic backbone structure with a malic acid linkage unit (44), which is distinctive for a gram-negative pathogen and different from S. algae strain 48055 (40). Interestingly, the distinctive malic acid linkage unit should provide increased flexibility of the O-polysaccharide chain, thereby allowing easier bending and compression than side chains of enteric bacteria, a feature that could also aid tighter bonding to mineral surfaces under environmental settings.

Influence of growth temperature.
Growth temperature can markedly influence the polymeric organization
of LPS (
1,
18,
25,
27,
35,
42), and this was true with some
Shewanella strains. Temperatures below 25°C resulted in
S- to semirough LPS in
S. oneidensis MR-1, which corresponded
to the appearance of one to three bands above the putative core
(Fig.
2A). Maximum size heterogeneity of LPS was observed at
15 to 20°C. Most apparent was
S. frigidimarina, where a
characteristic ladderlike banding pattern appeared at 15°C
(Fig.
2B). A further decrease of growth temperature caused a
decrease in this LPS size heterogeneity; however, the LPS profiles
of most strains were not affected by temperature. The low temperature
effect on some strains was not surprising, since many
Shewanella organisms are either psychrotolerant or psychrophilic (
7,
43).

Ultrastructural analyses.
For conventional embeddings for thin sections the glutaraldehyde-osmium
tetroxide protocol of Beveridge et al. (
5) was followed and
cells were embedded in LR White. Ruthenium red was also used
as suggested by Beveridge et al. (
5). Freeze substitution was
according to the method described by Graham et al. (
15). Sections
were imaged with a Philips EM300 under standard operating conditions.
All conventionally processed strains possessed envelope profiles
typical of gram-negative cells having an outer membrane, periplasmic
space, peptidoglycan layer, and plasma membrane (Fig.
3A is
representative). No specific surface structures, such as capsules,
S-layers, exopolymeric substances, sheaths, spinae, pili (fimbriae),
or flagella (see references
4 and
6), could be discerned.
Since freeze substitution has been shown to preserve finely
detailed surface structures such as capsules and LPS O-side
chains (
3,
15,
19), it was also used to examine the surfaces
of these
Shewanella organisms. All strains contained a periplasmic
gel (
3,
6).
S. oneidensis strains revealed a fibrous fringe
extending about 20 to 30 nm and 60 to 80 nm from the cell surfaces
of MR-1 and DLM 7, respectively (Fig.
3B is representative).
A more extensive fringe was seen on
S. algae BrY
DL and BrY
FC reaching up to 60 to 90 nm. However, cell populations in the
samples were very heterogeneous, since in each population, some
cells possessed different fringe heights, some possessed only
patches of fringe, and some had no fringe at all (Fig.
3C is
representative), suggesting that there was unequal expression
of fringe within a single culture.
Shewanella sp. strain MR-4
possessed the most extensive fringe (70 to 130 nm) of all (Fig.
3D). Taken together, the variance of fringe thickness and patchiness
and the heterogeneity of fringe expression corroborated the
(sometimes) smeared appearance of bands within SDS-PAGE gels,
since a variation of polymer lengths would be expected.
Some variations in fiber arrangement of the fringes were also observed. The fibers of MR-4 extended directly away from the cell envelope in quite an organized manner (Fig. 3D), while those of S. oneidensis and S. algae (Fig. 3B and C) were more randomly arranged, forming a netlike mesh; these could be a softer, less ordered matrix than that of MR-4. Taking into account the structure and substantial thickness of these three fibrous layers, it would be reasonable to designate them as comprising capsules based solely on polymer length. In contrast, the cell surfaces of S. putrefaciens (CN32 and NCTC 10695), S. baltica, and S. amazonensis were devoid of any fibrous material and were not capsulated (Fig. 3E).
Ruthenium red is a stain that is frequently used to contrast acidic surface polymeric substances, such as capsules (4, 5). Strangely, when this stain was used with all our strains, no fringes were seen, even on those strains shown to have capsules by freeze substitution (data not shown). It is possible that the capsules of Shewanella are too delicate to withstand conventional fixation-ruthenium red processing or that the capsules' overall electronegative charge density is too low to bind the stain.
Because the electron microscopy and LPS analyses have produced many data points in our study, we have tabulated results in Table 2. Here it is revealed that
50% of the strains have R-LPS and that about one-half of these possess a fringe (freeze substitution) that is presumably a capsule. The other strains possess S-LPS or low-Mr polysaccharides (since they enter the SDS-PAGE gel) that can only infrequently be seen by freeze substitution.

Possible implications.
Surface polysaccharides, whether they are capsular or LPS, strongly
affect the physicochemistry and adhesion qualities of gram-negative
bacteria (
8,
10,
11,
12,
34,
38,
48). Often it has been assumed
that strains possessing R-LPS are more hydrophobic than their
S-LPS counterparts, but this is not necessarily true (
19,
25).
Rough strains often have more exposed ionizable groups, and
O-side chains can be so rapidly in motion that long-lived interactions
between ions or surfaces can be rare (
3,
49). Capsules, which
are typically considered to aid adhesion, can be either adhesive
(via charge-charge interaction) or nonadhesive (
34,
46). Chemistry
and length of both capsular and O-polysaccharides and core oligosaccharides
all come to bear as they help to produce the net physicochemistry
of a bacterial surface. In some instances, strong polarity would
be required for adhesion, whereas in others, strong hydrophobicity
would be required. Phenotypic plasticity is important (
36),
and the most successful bacteria are those that can modulate
their surface properties according to the environment and the
chosen attachment surface. For this reason, we propose that
certain
Shewanella organisms, such as CN32, have developed short
R-LPS of high charge character to aid close adherence to inanimate
surfaces, such as iron oxides. Under anaerobic conditions this
would allow the Fe(III)-Fe(II) couple to aid electron capture
during respiration. MR-1, on the other hand, retains the R-LPS
phenotype but also requires the aid of a capsule of low charge
density to aid its adhesion. BrY
DL has adapted itself to live
under natural environmental conditions but can also be an opportunistic
pathogen. Possibly, the O-side chains aid in pathogenicity,
but its flexible malic acid hinge-like linkage could also allow
close contact to hard materials. There is no doubt that the
surfaces of the other
Shewanella organisms in our study will
utilize their structural combinations in similar self-serving
ways.

ACKNOWLEDGMENTS
We acknowledge the excellent technical assistance of Bob Harris,
Dianne Moyles, and Sean Langley of our laboratory, and we thank
Chris Whitfield of our Department for his helpful comments on
capsular material.
This work was funded by NABIR-DOE grants to T.J.B. and Y.G. The electron microscopy was done in the NSERC Guelph Regional STEM Facility (GRSF), which is partially funded by an NSERC-Major Facilities Access grant to T.J.B.

FOOTNOTES
* Corresponding author. Mailing address: Dept. of Microbiology, CBS, University of Guelph, Guelph, Ontario, Canada, N1G 2W1. Phone: (519) 824-4120. Fax: (519) 837-1802. E-mail:
tjb{at}uoguelph.ca.


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Applied and Environmental Microbiology, September 2002, p. 4653-4657, Vol. 68, No. 9
0099-2240/02/$04.00+0 DOI: 10.1128/AEM.68.9.4653-4657.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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