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Infection and Immunity, May 2001, p. 3423-3426, Vol. 69, No. 5
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.5.3423-3426.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Key Role of Teichoic Acid Net Charge in
Staphylococcus aureus Colonization of Artificial
Surfaces
Matthias
Gross,
Sarah E.
Cramton,
Friedrich
Götz, and
Andreas
Peschel*
Microbial Genetics, University of
Tübingen, D-72076 Tübingen, Germany
Received 31 October 2000/Returned for modification 8 January
2001/Accepted 12 February 2001
 |
ABSTRACT |
Staphylococcus aureus is responsible for a large
percentage of infections associated with implanted biomedical devices.
The molecular basis of primary adhesion to artificial surfaces is not
yet understood. Here, we demonstrate that teichoic acids, highly
charged cell wall polymers, play a key role in the first step of
biofilm formation. An S. aureus mutant bearing a
stronger negative surface charge due to the lack of
D-alanine esters in its teichoic acids can no longer
colonize polystyrene or glass. The mutation abrogates primary adhesion
to plastic while production of the glucosamine-based polymer involved
in later steps of biofilm formation is not affected. Our data suggest
that repulsive electrostatic forces can lead to reduced staphylococcal
biofilm formation, which could have considerable impact on the design
of novel implanted materials.
 |
TEXT |
Staphylococcus aureus is
one of the most frequently isolated bacterial pathogens, causing severe
morbidity and often fatal infections. Like coagulase-negative
Staphylococcus epidermidis, S. aureus has the
capacity to adhere to catheters and other indwelling devices and form a
biofilm, which is then difficult to combat with host defenses or
antibiotics (for recent reviews, see references 7,
24, 26, and 28). The alarming
rise in nosocomial staphylococcal bacteremia can be largely attributed
to the increasing use of intravascular catheters (14).
Biofilm formation is thought to be a two-step process that requires the
primary adhesion of bacteria to a substrate surface followed by the
formation of multiple cell layers (28). The
glucosamine-based polysaccharide intercellular adhesin (PIA), or
poly-N-succinylglucosamine (PNSG), is responsible for
cell-cell adhesion; mutant S. epidermidis or S. aureus strains that no longer produce PIA (PNSG) are unable
to form a biofilm, while the initial adhesion to plastic surfaces is
unaffected (4, 10, 29). PIA (PNSG) is a linear
-1,6-linked glucosaminoglycan, a high percentage of which is
N-acetylated and/or succinylated (20, 21). The
icaADBC operon is responsible for its biosynthesis (6).
The mechanisms responsible for initial adhesion to a plastic surface,
however, are not yet well understood. They are influenced by the
physicochemical properties of both the plastic material and the
bacterial cell surface. In S. epidermidis, the cell wall lytic enzyme AtlE, which affects the hydrophobicity of the cell surface, has been implicated in the initial adhesion to plastic and
glass (8, 9). The role of teichoic acids, highly charged cell wall polymers, in biofilm formation, however, has remained elusive. S. aureus teichoic acids are composed of
alternating phosphate and ribitol (wall teichoic acids) or glycerol
(lipoteichoic acids) groups, which are replaced with
D-alanine and N-acetylglucosamine (5). We have recently described an S. aureus
mutant lacking D-alanine in the teichoic acids
due to a disruption in the dltABCD operon, which is
responsible for D-alanine incorporation. The lack
of D-alanine esters caused a stronger negative
net charge on the bacterial cell surface that affected the resistance
to cationic antimicrobial peptides such as defensins from human
phagocytes (22), the susceptibility to vancomycin, and the
activity of autolysins (23).
The D-alanine substituents of S. aureus
teichoic acids are necessary for biofilm formation.
The
biofilm-forming capacities of the S. aureus Sa113
dltA mutant (ATCC 35556 dltA::spc) and its wild-type parental
strain (ATCC 35556) were determined as described previously
(4). Briefly, tryptic soy broth (Life Technologies,
Karlsruhe, Germany) supplemented with 0.25% glucose was inoculated
with 1/200 volume of overnight cultures, which had been adjusted to the
same A578 of 0.1. Samples (200 µl)
were added to the wells of 96-well polystyrene (Greiner Labortechnik,
Frickenhausen, Germany) or glass (Dynatec, Denkendorf, Germany)
microtiter plates. After cultivation for 24 h at 37°C, biofilm
formation was detected by (i) staining bacteria that remained attached
to the surface of flat-bottomed polystyrene or glass microtiter plates
with 0.1% safranin after the plates were gently washed twice with
phosphate-buffered saline and (ii) observing confluent growth in
U-bottomed polystyrene microtiter plates. Wild-type and mutant cells
showed the same capacity to bind safranin (data not shown). These
experiments as well as the studies described below were carried out at
least three times and yielded reproducible results.
While the S. aureus wild-type strain was positive in both
assays, the dltA mutant lacking
D-alanine esters in its teichoic acids formed
cell pellets in U-bottomed plates instead of confluent cell layers
(data not shown) and staining with safranin demonstrated that the
bacteria did not adhere to the plastic surface (Fig. 1A). Biofilm formation by the
dltA mutant was even weaker than that of the intercellular
adhesion mutant (ATCC 35556 icaADBC::tet) (4) (Fig.
1A, wells 2 and 3, respectively); adhesion of the dltA
mutant was comparable to the phenotype of the nonadhering species
Staphylococcus carnosus (Fig. 1A, well 5). Upon
complementation of the dltA mutant with plasmid pRBdlt1
bearing a wild-type copy of the dltABCD operon, the capacity
to form a biofilm was restored (Fig. 1A, well 4). Very similar results
were obtained with glass wells, indicating that
D-alanine-modified teichoic acids play a role in
adherence to polar as well as apolar surfaces (Fig. 1B). Interestingly,
dltA mutant cells bearing pRBdlt1 formed a stronger biofilm
on plastic and glass surfaces than did the wild-type strain. This
increase corresponds to the higher number of
D-alanine esters in the teichoic acids of the
complemented mutant (22), which is probably due to the
relatively high copy number of plasmid pRBdlt1.

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FIG. 1.
Biofilm formation by S. aureus strains in
polystyrene (A) and glass wells (B). Bacterial biofilms formed during
overnight cultivation in microtiter plates were stained with safranin.
Wells: 1, S. aureus ATCC 35556 wild type; 2, icaADBC mutant; 3, dltA mutant; 4, dltA mutant complemented with plasmid pRBdlt1; 5, S. carnosus TM300.
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|
Production of PIA/PNSG is not affected in the dltA
mutant.
In order to elucidate which phase of biofilm formation is
abrogated in the dltA mutant, we first analyzed production
of the polysaccharide responsible for cell-cell adhesion, essentially as previously described (4). Briefly, polysaccharide was
released from equal numbers of bacteria grown as described above by
being boiled in 0.5 M EDTA (pH 8.0) for 5 min. Traces of protein A were degraded by incubation with 4 mg of proteinase K (Roche Biochemicals, Mannheim, Germany)/ml for 30 min at 37°C. The proteinase was
subsequently removed by extraction with a
phenol-chloroform-isoamylalcohol solution (25:24:1
[vol/vol/vol]) buffered with 20 mM Tris-HCl (pH 8.0). Five
microliters from each sample was then spotted onto a nitrocellulose
membrane and subjected to antibody detection analysis using a rabbit
antiserum raised against S. epidermidis PIA (a gift from
Dietrich Mack, University Hospital Eppendorf, Hamburg, Germany)
(6). Both the wild-type and dltA mutant strains produced a product detectable with PIA-specific antiserum (Fig. 2, spots 1 and 3), while the
icaADBC mutant remained negative (Fig. 2, spot 2),
demonstrating that abrogation of plastic colonization in the
dltA mutant is not due to the absence of intercellular adhesin.

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FIG. 2.
Detection of PIA (PNSG). Cell surface extracts from
overnight cultures of S. aureus ATCC 35556 were spotted
onto a nitrocellulose filter and detected using an antiserum raised
against S. epidermidis PIA. Spots: 1, wild type; 2, icaADBC mutant; 3, dltA mutant.
Rows: results of two experiments with same samples.
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|
The initial binding to plastic surfaces is impaired in the
dltA mutant.
To analyze the initial binding of the
bacteria to polystyrene, equal amounts of wild-type and mutant cells
were prepared as described above for biofilm formation assays but were
incubated for only 30 min in flat-bottomed polystyrene microtiter
plates. After the washing step, the wells were air-dried and stained
with safranin, and A490 was determined
in a microtiter plate reader (SpectraMax 340; Molecular Devices,
Sunnyvale, Calif.). The absorbance of wells incubated with 200 µl of
sterile medium was subtracted. While similar amounts of wild-type and
icaADBC mutant bacteria adhered to the wells, the
dltA mutant exhibited more than 50% reduced level of
initial binding, indicating that the initial step of biofilm formation
was affected by the loss of the D-alanine esters
(Fig. 3).

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FIG. 3.
Initial adhesion to a polystyrene surface. Equal numbers
of bacteria were incubated in polystyrene microtiter plates for 30 min.
Adhering bacteria were visualized with safranin and quantified using a
microtiter plate reader. The mean values and standard deviations of at
least six wells from a representative experiment are shown. The mean
value obtained for the wild-type strain was defined as 100%. Bars: 1, S. aureus ATCC 35556 wild type; 2, icaADBC mutant; 3, dltA mutant; 4, dltA mutant complemented with plasmid pRBdlt1; 5, S. carnosus TM300.
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The lack of D-alanine esters affects the pattern of
surface-bound proteins.
The composition and amount of cell
wall-associated proteins can considerably influence the hydrophobicity
and net charge of the bacterial cell envelope and the interaction with
substrate surfaces (17). In order to compare the patterns
of surface proteins, wild-type and dltA mutant bacteria were
grown to stationary phase in tryptic soy broth and equal numbers of
cells were boiled in 1% sodium dodecyl sulfate (SDS) for 5 min as
described previously (23). Intact cells were removed by
centrifugation, and surface proteins were separated on
Tricine-SDS-polyacrylamide gels according to standard methods
(1).
The patterns of surface-associated proteins in the two strains revealed
only slight differences. Three bands at about 19,
24, and 26 kDa were
more pronounced in the mutant while one band
at about 33 kDa was
stronger in the wild type (Fig.
4). These
deviations may be the result of different capacities to retain
the
proteins in the cell wall or differences in proteolytic activities
in
the two strains. We have recently demonstrated that smaller
amounts of
autolytic enzymes, which bind to teichoic acids (
3)
and
which may affect the primary adhesion to plastic or glass
(
9), are released by SDS treatment from the
dltA mutant than
from wild-type bacteria (
23).
The four prominent protein bands
indicated in Fig.
4, however,
exhibited no autolytic activity
in zymographic analyses (data not
shown), which were performed
with SDS-polyacrylamide gels containing
Micrococcus luteus or
S. carnosus cells as
described previously (
23).

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FIG. 4.
Surface-associated proteins. Surface proteins from
S. aureus ATCC 35556 strains were released with 1% SDS,
separated on an SDS-15% polyacrylamide gel, and stained with
Coomassie blue. Lanes: 1 and 2, extracts from the wild type; 3 and 4, extracts from dltA mutant. Protein extracts (50 or 25 µl) were applied to lanes 1 and 3 or 2 and 4, respectively. The
masses of protein markers (M) are indicated on the left. Protein bands
with different intensities in the two strains are indicated by closed
(more pronounced in the wild type) and open (more pronounced in the
dltA mutant) arrows.
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|
Conclusions.
We show here for the first time that the charge
of teichoic acids plays a pivotal role in the initial step of biofilm
formation. The cell surface of S. aureus, as in most
bacteria, has a moderately negative net charge at neutral pH
(27), which is probably due to the fact that the teichoic
acids contain fewer positively charged D-alanine
residues than negatively charged phosphate groups (22). Nevertheless, S. aureus can adhere to hydrophobic or
slightly negatively charged surfaces such as polystyrene or glass,
respectively. The direct interaction of bacteria and surfaces is
dependent on van der Waals forces, which are generally attractive, and
interionic forces, which can be either attractive or repulsive
(25). Even if bacteria and surfaces are charged alike, van
der Waals forces can overcome repulsion and lead to adhesion (18,
19). The much stronger net negative charge of the
dltA mutant, however, probably leads to a pronounced
increase in the repulsive forces, thereby disabling any adherence of
the bacteria to polystyrene or glass. On the other hand, altered
teichoic acid net charge may affect the adhesive properties of
bacterial cells in an indirect way. For instance, the absence of
D-alanine esters in teichoic acids has been shown
to alter the folding of exoproteins in Bacillus subtilis
(13). Although the pattern of cell wall-associated proteins was only slightly different in the S. aureus
mutant, altered protein conformations might lead to altered
physicochemical properties of the cell surface and thereby compromise
the interaction with artificial surfaces.
Our data suggest that increasing the repulsive forces between the
plastic surface and the bacteria by modifying the properties
of
implanted materials may lead to reduced capacity to form a
biofilm.
Accordingly, several studies have demonstrated that the
use of
negatively charged materials, such as ionized plastics,
Teflon, or
heparinized surfaces, is of particular benefit in reducing
colonization
(
2,
11,
12,
15). Moreover, since the teichoic
acid content
and the degree of
D-alanylation vary among
S. aureus strains (
16), increased amounts of
D-alanine esters may contribute
to the capacity
of staphylococci to colonize indwelling
devices.
 |
ACKNOWLEDGMENTS |
We thank Matthias Herrmann and Michael Otto for helpful
discussions, Dietrich Mack for providing the PIA-specific antiserum, and Ulrike Pfitzner for photography.
This work was supported by the Interdisciplinary Clinical Research
Center Tübingen and by grants from the Deutsche
Forschungsgemeinschaft (GO 371/3-1) to A.P. and F.G.
 |
FOOTNOTES |
*
Mailing address: Microbial Genetics, University of
Tübingen, Auf der Morgenstelle 28, D-72076 Tübingen,
Germany. Phone: 49-7071-297-2611. Fax: 49-7071-29-5065. E-mail:
andreas.peschel{at}uni-tuebingen.de.
Editor:
E. I. Tuomanen
 |
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Infection and Immunity, May 2001, p. 3423-3426, Vol. 69, No. 5
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.5.3423-3426.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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