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Journal of Bacteriology, November 2006, p. 7785-7795, Vol. 188, No. 22
0021-9193/06/$08.00+0     doi:10.1128/JB.00673-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Biofilm Formation by Streptococcus pneumoniae: Role of Choline, Extracellular DNA, and Capsular Polysaccharide in Microbial Accretion

Miriam Moscoso, Ernesto García,^* and Rubens López

Departamento de Microbiología Molecular, Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, 28040 Madrid, Spain

Received 11 May 2006/ Accepted 13 August 2006

Top Abstract Introduction Materials and Methods Results Discussion References

 
Streptococcus pneumoniae colonizes the human upper respiratory tract, and this asymptomatic colonization is known to precede pneumococcal disease. In this report, chemically defined and semisynthetic media were used to identify the initial steps of biofilm formation by pneumococcus during growth on abiotic surfaces such as polystyrene or glass. Unencapsulated pneumococci adhered to abiotic surfaces and formed a three-dimensional structure about 25 µm deep, as observed by confocal laser scanning microscopy and low-temperature scanning electron microscopy. Choline residues of cell wall teichoic acids were found to play a fundamental role in pneumococcal biofilm development. The role in biofilm formation of choline-binding proteins, which anchor to the teichoic acids of the cell envelope, was determined using unambiguously characterized mutants. The results showed that LytA amidase, LytC lysozyme, LytB glucosaminidase, CbpA adhesin, PcpA putative adhesin, and PspA (pneumococcal surface protein A) mutants had a decreased capacity to form biofilms, whereas no such reduction was observed in Pce phosphocholinesterase or CbpD putative amidase mutants. Moreover, encapsulated, clinical pneumococcal isolates were impaired in their capacity to form biofilms. In addition, a role for extracellular DNA and proteins in the establishment of S. pneumoniae biofilms was demonstrated. Taken together, these observations provide information on conditions that favor the sessile mode of growth by S. pneumoniae. The experimental approach described here should facilitate the study of bacterial genes that are required for biofilm formation. Those results, in turn, may provide insight into strategies to prevent pneumococcal colonization of its human host.

Top Abstract Introduction Materials and Methods Results Discussion References

 
It is now widely recognized that biofilm formation is the natural form of growth for most microbial species in their natural habitats (27). A biofilm is a highly structured, sessile microbial community characterized by bacterial cells attached to a surface or interface and embedded in a matrix of extracellular polymeric substances (10, 17). Biofilm development is a multistep process, and the study of the genetic basis of colonization of abiotic surfaces has become an important aspect in the search for genes involved in biofilm formation and maintenance in gram-positive and gram-negative bacteria (6, 40, 55). The importance of biofilms is best illustrated by the finding that more than 60% of bacterial infections are considered to involve biofilms, and these communities exhibit an inherent tolerance to antibiotic therapies and host immune attack (11, 56). The dynamics of biofilm formation constitute a simple strategy of microbial survival that facilitates the transmission of pathogens by providing a stable protective environment and by acting as a reservoir for the dissemination of a great number of microorganisms to new surfaces (28).

Streptococcus pneumoniae (pneumococcus) is a major gram-positive human pathogen and currently the leading cause of community-acquired pneumonia, meningitis, and bloodstream infections in the elderly, the young, and patients with immunosuppressive illness and chronic diseases; it is also the main causative agent of middle-ear infections in children. More than 500,000 cases of pneumococcal pneumonia occur each year in the United States, causing almost 50,000 deaths in adults and children. Moreover, since the introduction of Haemophilus influenzae type Ib conjugate vaccine, S. pneumoniae has become the principal cause of bacterial meningitis (5). The global importance of S. pneumoniae as the cause of illness, sequelae, and death, together with the emergence of strains resistant to ß-lactam antibiotics and quinolones, has conferred upon this microorganism the status of "superbug" (30).

Members of the genus Streptococcus are important human pathogens, and their ability to reach a high density may be due to the formation of biofilm-like populations in confined areas of the body, such as the heart valves, tonsillar crypts, and implanted medical devices. Streptococcus mutans, the major etiological agent of dental caries, provides a notable example of the formation of complex biofilms by the oral microbiota. However, it has been suggested that in contrast to the very dense dental plaques formed by S. mutans and other oral streptococci, pathogenic streptococci that infect mucosal tissues colonize environments where bacterial microcolonies are usually less dense due to the bathing effects of secretions and epithelial desquamation (12).

Until now, few studies have documented the potential ability of pneumococci to produce biofilms in vitro (8, 18, 75). However, it was shown that pneumococcal growth on Sorbarod filters in a continuous culture-like system mimics nasopharyngeal carriage. In that model, type 3 pneumococci generated tandem duplications within the cap3A gene of the capsule locus, leading to the formation of unencapsulated mutants (75). Capsular polysaccharide is considered a fundamental genetic trait for pneumococcal virulence, and these observations of changes in the expression of genes involved in capsule biosynthesis were later extended to the capsular genes of pneumococcal serotypes 8 and 37 (74). The ability of pneumococci to regulate capsule expression, as demonstrated on Sorbarod biofilms, could play a fundamental role in the transition from the carrier state to invasive disease (75). Recently, Allegrucci and coworkers reported that the biofilm development process correlates not only with differential production of proteins but also with a dramatic increase in the number of detectable proteins (2).

Additional evidence for a link between the pneumococcus and the biofilm mode of growth comes from study of the asymptomatic carrier state associated with S. pneumoniae colonization of the throat or nasopharynx. This condition is found mainly in children, 60% of whom may be colonized (3), but progression to a disease state occurs only under appropriate conditions (50). It may thus be the case that, in the carrier state, S. pneumoniae constitutively adopts the sessile mode of growth in order to survive for long periods of time. To ascertain whether S. pneumoniae benefits from forming an interactive community in which sessile growth is not the result of a random accretion of bacterial cells but, rather, of the formation of a bacterial community that cooperates to form a well-defined structure, laboratory conditions were established that favored biofilm formation, and the structural peculiarities of these biological entities were studied.

Current evidence suggests that pneumococci are able to form biofilms on abiotic surfaces. In the present study, this ability provided the basis for applying a genetic approach to studying biofilm accretion, i.e., the obligatory attachment of S. pneumoniae to a surface before an ordered three-dimensional (3D) structure can be constructed and the conditions that lead to defective biofilm formation. Our analyses revealed a role for several gene products in biofilm development and suggested that DNA and certain proteins contribute to the formation of an extracellular matrix in this system. The experimental data are important steps in furthering our understanding of the putative role of biofilms in some stages of the pneumococcal infection.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Strains, media, plasmid, and growth conditions. The streptococcal strains used in this study are listed in Table 1. Routinely, pneumococcal strains were grown in C medium (38) either supplemented with 0.08% yeast extract (C+Y medium) or nonsupplemented. For assays of biofilm formation, in addition to C medium, the following media were used: Todd-Hewitt broth supplemented with 0.5% yeast extract, casein tryptone (CAT) medium (59), and the chemically defined media Cden (70) and CDM (72) supplemented with choline chloride (5 µg ml^–1), asparagine (50 µg ml^–1), and sodium pyruvate (250 µg ml^–1). CDM was purchased from JRH Biosciences (Lenexa, Kansas). Where indicated, Cden and CDM media were supplemented with either 2% choline chloride or ethanolamine (EA) (50 µg ml^–1). Cells were incubated at 37°C without shaking, and growth was monitored by measuring the A₅₉₅. When used, antibiotics were added at the following concentrations: erythromycin, 0.5 µg ml^–1; kanamycin, 250 µg ml^–1; tetracycline, 0.5 to 1 µg ml^–1. Plasmid pMV158GFP, kindly provided by C. Nieto, is a derivative of the natural streptococcal plasmid pMV158, which harbors the gene encoding green fluorescent protein (GFP) under the control of a maltose-inducible promoter (53).

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TABLE 1. Characteristics of the streptococcal strains used in this study

Transformation procedures. Chromosomal DNA from pneumococcal isolates was prepared as previously described (21). S. pneumoniae was transformed with chromosomal or plasmid DNA by treating precompetent cells with 100 ng of synthetic competence-stimulating pheromone 1 (CSP1) ml^–1 at 37°C for 10 min to induce competence (48, 51), followed by incubation at 30°C during DNA uptake. Transformants were selected by plating in CAT agar supplemented with 3% (vol/vol) defibrinated sheep blood and then by challenge with a 10-ml overlay containing the appropriate antibiotic after phenotypic expression for 120 min at 37°C (51). Capsulated transformants of strain M11 were enriched, when required, by successive transfers of the transformed culture to C medium containing 0.08% bovine serum albumin and supplemented with 1 µg of anti-R antiserum ml^–1 before plating. Anti-R (antisomatic) antiserum contains group-specific agglutinins, which, at the proper dilution, agglutinate only unencapsulated pneumococci.

mariner mutagenesis of the cbpA, pcpA, and pspA genes. Insertions of the spc, cat, or kan gene cassettes in the cbpA, pcpA, and pspA genes were generated by in vitro mariner mutagenesis as previously described (48). Plasmids used as a source for minitransposons are listed in Table 2. Briefly, plasmid DNA (1 µg) was incubated in the presence of purified Himar1 transposase (39) (in a total volume of 40 µl), with a PCR fragment (cbpAmar5-cbpAmar3 for cbpA; pcpAmar5-pcpAmar3 for pcpA; pspAmar5-pspAmar3 for pspA) leading to a random insertion of the minitransposon into the fragment. Gaps in transposition products were repaired as previously described (1), and the resulting in vitro generated transposon insertion library was used to transform S. pneumoniae. Location and orientation of each mariner cassette were determined by PCRs using primers MP127 or MP128 (Table 2) in combination with either one of the two primers used to generate each mutagenized PCR fragment. The cassette-chromosome junctions were sequenced using primer MP128. The cbpA::spc2A (position 1537 with respect to the ATG of cbpA) (strain P064), pcpA::cat1A (position 1323 with respect to the ATG of pcpA) (strain P065), and pspA::kan1C (position 1145 with respect to the ATG of pspA) (strain P066) insertions were used for this study (Table 1).

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TABLE 2. Characteristics of the plasmids and primers used in this study

Biofilm formation assay and quantification. Biofilm formation was determined by the ability of cells to adhere to the walls and base of 96-well (flat-bottom) polystyrene (PST) microtiter dishes (Costar 3595; Corning Incorporated), using a modification of a previously reported protocol (55). Unless stated otherwise, cells grown in C+Y medium to an A₅₉₅ of 0.5 to 0.6 were sedimented by centrifugation, resuspended in an equal volume of the indicated prewarmed medium, diluted 1/10 or 1/100, and then dispensed at a concentration of 200 µl per well. Plates were incubated at 34°C for 6 h, and bacterial growth was determined by measuring the A₅₉₅ using a plate reader (microplate absorbance reader 2020; Anthos Labtec Instruments GmbH). Fifty microliters of a 1% solution of crystal violet (CV) was added to each well. The plates were then incubated at room temperature for approximately 15 min, rinsed three times with 200 µl of distilled water, and air dried. CV-stained biofilm formation was quantified by solubilizing the biofilm with 95% ethanol (200 µl/well) and then determining the A₅₉₅. Under these conditions, approximately 2.4 x 10⁸ and 1.5 x 10⁸ CFU per ml grew as planktonic or sessile cells, respectively.

CLSM. For observation of S. pneumoniae biofilms by confocal laser scanning microscopy (CLSM), pneumococcal strains containing pMV158GFP (Table 2) were grown on glass-bottom dishes (WillCo-dish; WillCo Wells B. V., The Netherlands) in 2 ml of C medium containing 1% maltose and 1 µg of tetracycline ml^–1 for 10 to 12 h at 34°C. The culture medium was removed, and the pneumococcal biofilms were rinsed with phosphate-buffered saline (PBS) to remove nonadherent bacteria, followed by submersion in 0.5 ml of PBS. S. pneumoniae biofilms were observed using a Leica TCS-SP2-AOBS-UV CLSM equipped with an argon ion laser. The excitation wavelength was 488 nm, and the magnification was x100. Images were analyzed using the LEICA software LCS. Projections through the x-y plane (individual scans at 0.5-µm intervals) and the x-z plane (images at 3-µm intervals) were obtained.

A LIVE/DEAD BacLight bacterial viability kit L-13152 (Invitrogen-Molecular Probes) was used for monitoring the viability of bacterial populations as a function of the membrane integrity of the cell (16). Cells with a compromised membrane that are considered to be dead or dying will stain red, whereas cells with an intact membrane will stain green.

LTSEM. Low-temperature scanning electron microscopy (LTSEM) is useful for studying the in vivo organization of cells without being influenced by artifacts due to fixation, dehydration, and drying (33). Also, LTSEM allows the localization of water in the biofilm, and microorganisms can be examined without invasive treatments and without apparently disturbing the biofilm organization. Pneumococcal biofilms formed on glass surfaces were examined by LTSEM as previously described (15). Biofilms of S. pneumoniae R6 strain were grown on 14-mm glass coverslips that had been placed in each well of a 24-well PST microtiter dish (Falcon 3047; Becton Dickinson Labware). One milliliter of C medium was added, and the dishes were incubated for 10 to 12 h at 34°C. The coverslips were rinsed with PBS to remove nonattached cells. Small fragments were mechanically cut off, fixed onto the specimen holder of a cryotransfer system, plunged into liquid nitrogen, and then transferred to a preparation unit via an air lock transfer device. The frozen specimens were cryofractured and transferred directly via a second air lock to the microscope cold stage, where they were etched for 2 min at –90°C. After ice sublimation, the etched surfaces were sputter coated with gold in the preparation unit and then transferred onto the cold stage of the scanning electron microscope chamber. Fractured surfaces were observed at –135°C with a DSM 960 Zeiss scanning electron microscope.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Adherence of S. pneumoniae to abiotic surfaces and the environmental factors affecting biofilm development. The initial step in biofilm formation is the adherence of cells to a surface where organic nutrients concentrate. This observation can be extrapolated to organisms such as S. pneumoniae, which colonizes the human nasopharynx to establish a host carrier state. To determine the optimal conditions for biofilm formation of S. pneumoniae on abiotic surfaces, biofilm assays were carried out under various conditions. First, the ability of unencapsulated pneumococcus to form biofilms on polyvinylchloride, PST, or glass was tested following basically the procedure described by O'Toole and Kolter (55). Biofilm formation was measured by staining with CV. The best results were obtained on PST microplates and on glass-bottom dishes. Several culture media and supplements were then tested for biofilm development on PST microplates. Figure 1A and B show that either chemically defined (Cden or CDM) or semisynthetic (C) medium supported strong biofilm formation, whereas growth in complex medium, such as CAT or Todd-Hewitt broth supplemented with 0.5% yeast extract, resulted in weak (if any) biofilms. Moreover, culture of the cells in rich medium alone led to the production of a highly colored background material (Fig. 1B), whereas pneumococci incubated in minimal biofilm medium supplemented with glucose (0.8%, wt/vol) or casamino acids (0.2%, wt/vol), which is optimal for Streptococcus gordonii biofilm formation (43), failed to grow and attach to the abiotic surface. In addition, there were no significant differences in growth and biofilm formation in medium supplemented with 0.5% sucrose or 0.1% catalase (data not shown). When the osmolarity of the C medium was varied between 0 and 0.5 M NaCl, biofilm formation was clearly inhibited above 0.2 M (Fig. 1C). In addition, optimal biofilm development was observed when the starting pH of the medium was adjusted to between 7.0 and 8.0 (Fig. 1D).

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FIG. 1. Some factors influencing biofilm formation in S. pneumoniae R6 strain. (A) Effect of the culture medium. (B) Biofilm formation by S. pneumoniae on PST microtiter plates in different media for 6 h at 34°C. Cells were stained with CV and the plates were washed to remove unattached cells. As a control, wells with medium but without bacteria were also stained with CV. The results of a representative experiment are shown. (C) Effect of ionic strength. (D) The influence of the initial pH of the medium. In all cases, filled and open bars indicate bacterial growth and biofilm formation, respectively. The data represent the average of six samples. Standard error bars are shown.

The influence of inoculum size on the adherence of S. pneumoniae to PST plates and C medium or CDM was subsequently investigated (data not shown). In C medium, the number of adherent cells reached a maximum after 8 h of incubation at 34°C, irrespective of the initial cell density, except when the initial inoculum was very small, in which case optimal biofilm formation was only achieved after 16 h of incubation. After 16 h of incubation in C medium, the number of adherent cells diminished, probably for reasons related to the typical autolytic behavior of S. pneumoniae. In favor of this hypothesis is the finding that no such decrease in biofilm formation was found when S. pneumoniae R6 cells were incubated in CDM instead of C medium (not shown). As the glucose concentration in CDM is eight times higher than in C medium (10 mg ml^–1 versus 1.24 mg ml^–1), it allows higher cell growth. Consequently, the pH of the medium drops below 6.0 at the stationary phase (unpublished observations); at this low pH, the peptidoglycan hydrolytic activity of the major pneumococcal autolysin (LytA) is completely prevented (46), and important biological activities of LytA, such as progeny-phage release, are hindered at nonpermissive pH values (>8.0 or < 5.8) (46, 60). Consequently, we decided to study the viability of pneumococcal biofilms in C medium supplemented with 10 mg of glucose ml^–1 to prevent the lysis of the culture (Fig. 2). Interestingly, although autolysis was inhibited, the number of dead cells in the biofilm markedly increased after prolonged incubation (Fig. 2).

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FIG. 2. CLSM image of the viability of biofilm-grown S. pneumoniae as a function of the incubation time. S. pneumoniae R6 cells were incubated in C medium to an A595 value of 0.5. The culture was centrifuged, and the cells were resuspended in an equal volume of prewarmed C medium. A portion of the culture was diluted 1/1,000 in the indicated medium. One percent glucose stimulates cell growth and the pH of the medium drops to below 6.0; this pH is nonpermissive for autolytic activity. After incubation at 34°C for 12 h (A) or 24 h (B), the cells in the biofilms were stained with the BacLight kit showing living (green fluorescence) and dead (red fluorescence) bacteria. Images are horizontal 3D reconstructions of 25 scans in the x-y plane.

Biofilm formation in species genetically related to S. pneumoniae. The biofilm-forming capacity of several streptococci of the mitis group was assayed and quantified using the optimal conditions determined for pneumococcus (see Materials and Methods). With the exception of Streptococcus mitis^T, the other three streptococcal type strains were able to form biofilms in C medium (Fig. 3). To the best of our knowledge, this is the first demonstration of biofilm formation by Streptococcus pseudopneumoniae. It should be mentioned, however, that variations in the ability to form biofilms among strains of the same species have been previously reported (43).

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FIG. 3. Biofilm formation capacity of various streptococcal species of the mitis group. Smi, Sor, Sgo, and Sps indicate, respectively, the type strain of S. mitis, S. oralis, S. gordonii, and S. pseudopneumoniae. The S. pneumoniae R6 strain (Spn) was used as a control. Cells were grown in C medium for 6 h at 34°C. Filled and open bars indicate growth and biofilm formation, respectively. The results are the average of three independent experiments.

Role of choline and choline-binding proteins in biofilm formation capacity. In most bacteria, surface molecules appear to be important for biofilm development. We were interested in examining the initial stages of the infective process of S. pneumoniae by identifying specific microbial components that might affect biofilm formation and, presumably, pneumococcal colonization of the human nasopharynx. In this sense, we paid special attention to the pneumococcal surface proteins, particularly choline-binding proteins (CBPs), involved in adhesion to and invasion of the host cells (as the adhesins CbpA and PcpA) and in the establishment of nasopharynx colonization (as the murein hydrolases LytB and LytC). Choline is an essential component of pneumococcal teichoic and lipoteichoic acids (22) and is required for anchoring the CBPs that play important roles in pneumococcal infection (44). Likewise, it seems that choline mediates bacterial adherence to the host cells, apparently through its interaction with the human platelet-activating factor receptor (22). Some CBPs are released from the surface of the pneumococcus upon incubation with high choline concentrations (>1%). Under these conditions, choline also inhibits those CBPs that have cell wall hydrolase activity (45). Accordingly, there was a notable reduction in biofilm formation by the R6 strain when incubated in C medium containing 2% choline chloride (Fig. 4A). This effect might be attributed to the excess of choline and/or to the inhibition of one or more CBPs. These possibilities were investigated by analyzing the effects on biofilm formation of mutations in several CBPs, namely, the murein hydrolases LytA (SP1937) (an N-acetylmuramoyl-L-alanine amidase; the main pneumococcal autolysin), LytB (SP0965) (a possible glucosaminidase involved in cell separation), and autolytic LytC lysozyme (SP2190), as well as the phosphocholinesterase Pce (SP0930) and CbpA major adhesin (SP2190), CbpD putative amidase (SP2201), PcpA possible adhesin (SP2136), and PspA (SP0117) (pneumococcal surface protein A) (for recent reviews, see references 45 and 66). As shown in Fig. 4B, inactivation of LytA, LytB, LytC, CbpA, PcpA, or PspA diminished biofilm formation on PST plates, whereas a mutation in the pce or cbpD genes had no detectable effect. We also analyzed the capacity of several pneumococcal mutants affected simultaneously in more than one CBP to form biofilms (Fig. 4B). Notably, biofilm formation was inhibited by 70% when a lytA-lytB-lytC triple mutant was used (Fig. 4B). Pneumococcal strains lacking an active LytB enzyme (either lytB mutants or lytB⁺ strains incubated in 2% choline chloride or in EA-containing medium; see below) grow as long chains of cells (14). Consequently, it was tempting to speculate that chaining may hinder biofilm formation. However, chain formation by itself does not appear to be the determining factor in the reduced adherence of either the lytB mutant or the R6 strain cultivated with 2% choline chloride, since several streptococci of the mitis group that grow as long chains (e.g., strains 1078 or 782) (52) formed a biofilm virtually identical to that of the R6 strain (unpublished observations).

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FIG. 4. Influence of choline and CBPs on biofilm formation and CSLMs of S. pneumoniae biofilms. (A) Influence of choline and EA on biofilm formation by the R6 strain. Filled and open bars indicate growth and biofilm formation, respectively. (B) Biofilm formation capacity of several mutant strains. In this experiment, the following strains were incubated in C medium for 6 h: R6, M32 (lytA), R6B (lytB), R6C (lytC), R6D (pce), M31B (lytA lytB), P042 (lytA lytC), R6BC (lytB lytC), M31BC (lytA lytB lytC), M31BCD (lytA lytB lytC pce), P064 (cbpA), R1582 (cbpD), P065 (pcpA), and P066 (pspA). The values of biofilm formation were normalized for absorbance, and the percentages were calculated in relation to strain R6. Shown are CLSM images of pneumococcal strains harboring pMV158GFP: P040 (R6), P049 (pce), and P050 (lytA lytC). (C, D, G, and H) Horizontal 3D reconstructions of 55 scans in the x-y plane. (E, F, I, and J) Vertical 3D reconstructions of 65 scans in the x-z plane. In all of the images, the bar corresponds to 30 µm.

When choline is replaced by its structural analogue EA in a chemically defined medium, teichoic acids contain EA instead of choline (69), and, phenotypically, EA-grown cells are identical to those incubated in the presence of 2% choline (7). A near 50% reduction in biofilm formation was found when the R6 strain was incubated in either Cden medium or CDM in the presence of EA (Fig. 4A). Moreover, this inhibition became even more marked when 2% (about 140 mM) choline chloride was used instead of EA. This behavior may be related to the previous finding that an increase in the ionic strength of the medium reduced development of the biofilm (Fig. 1C).

Biofilm architecture of some pneumococcal strains. Biofilms formed by GFP-expressing S. pneumoniae cells incubated in C medium for 10 to 12 h at 34°C were examined by CLSM and exhibited the 3D multicellular structure typical of a biofilm (Fig. 4C and E). The adherent cells formed a mat of cells of significant depth (20 to 30 µm), which, as expected, was much more crowded and thick than the mat formed by the same strain incubated in medium containing 2% choline chloride (Fig. 4D and F). It should be mentioned that biofilms formed in the presence of a high choline concentration were not tightly attached to the PST or glass surfaces, and extreme care was employed during washes to avoid a complete loss of the attached cells (not shown). Confocal micrographs also showed that the biofilms formed by a lytA lytC mutant (strain P050) (or, simply, by a lytA mutant) were consistently thinner (15 to 20 µm) (Fig. 4G and I) than those formed by the P040 (lytA⁺) strain (Table 1). However, and in agreement with the results mentioned above, no detectable differences were found between the biofilms formed by strain P040 and a pce mutant (strain P049) (Fig. 4H and J).

The ultrastructure of biofilms formed by S. pneumoniae R6 was observed using LTSEM (Fig. 5). The micrographs confirmed that this strain formed a biofilm about 25 µm deep, in which many pneumococcal cells lay on the surface of the coverslip. Large void spaces (originally occupied by water) could be observed on transverse sections of biofilms (Fig. 5B to D). The cells were interconnected by small, thin filaments that linked the cells to each other and/or bound the cells to the intercellular matrix (Fig. 5D). It is well established that, as ice crystals grow during cryofixation, they sweep the solutes from the medium into a eutectic boundary formed between neighboring ice crystals (33). That boundary corresponds to the scaffold-like material, which could also be observed in the absence of cells (not shown). Figure 5E shows an apical view of several microcolonies, revealing an irregular and discontinuous surface that coated the cells and contained holes of different sizes which might represent channels between the cell clusters.

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FIG. 5. LTSEM of S. pneumoniae R6 biofilm. (A) General view of the biofilm formed on the surface of a glass coverslip. (B) Magnification of the area indicated by a rectangle in panel A. (C and D) Two different views. Filamentous material (indicated by arrows) links pneumococcal cells to each other and to the intercellular matrix. (E) Apical view of the irregular surface of a pneumococcal biofilm. Microcolonies of different sizes can be seen. In all micrographs, the bar indicates 20 µm.

Influence of the capsular polysaccharide in biofilm formation. The definition of a biofilm has evolved during the last 25 years (17), and the current definition takes into account the finding that extracellular polysaccharides are not always required for biofilm development, as recently reported in Staphylococcus aureus (68). The great variety of polysaccharide and protein components of the biofilm matrix is an emerging theme of great importance, since the expression of different extracellular molecules affects the surface properties of bacteria and, consequently, promotes (or hinders) the formation and maintenance of different types of multicellular communities (6). These observations are particularly important in the case of pneumococcus, in which 90 different types of capsular polysaccharides have been described (31). Down-regulation of capsular polysaccharide production appears to enhance attachment of S. pneumoniae to host cells and bacterial invasion of the nasopharynx during establishment of the host carrier state (77). To gain further information on the role of the capsule in the initial steps of biofilm formation, the capacities of clinical pneumococcal isolates of various serotypes to develop biofilms were compared. The results shown in Fig. 6 indicated that, with the exception of serotype 6B strains that showed a reduction of approximately 30%, the presence of a capsule reduced biofilm development by more than 60%. To ascertain whether the observed differences in the capacity to form biofilms were due exclusively to the capsular polysaccharide itself and not to other factors (e.g., CBPs) that may vary among different isolates, we constructed a series of strain M11 derivatives that showed growth rates similar to that of their parental M11 strain and expressed comparable amounts of the different capsular polysaccharides. These encapsulated transformants were also impaired in their capacity to form biofilms, and some of them formed significantly less biofilm than the clinical isolates. Again, the transformant of serotype 6B showed only a limited reduction in its capacity to form biofilms (Fig. 6). Furthermore, it should be emphasized that a nontypeable pneumococcal isolate (strain ST344) formed biofilms with an efficiency even greater than that of laboratory strain R6.

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FIG. 6. Biofilm formation by encapsulated S. pneumoniae strains. Clinical pneumococcal isolates of the indicated serotypes were grown in C medium for 6 h at 34°C. Biofilm formation (hatched bars) was quantified by staining with CV. Open bars correspond to biofilms developed by encapsulated transformants of the indicated serotype using the unencapsulated M11 strain as recipient. NT corresponds to the M11 strain, which was used as control (filled bar) and to the nontypeable pneumococcal isolate ST344 (cross-hatched bar). The values of biofilm formation by encapsulated strains were normalized for absorbance, and the percentage was presented and normalized for the control strain M11. The percentages shown are the average of three independent experiments.

Involvement of extracellular DNA and proteins in biofilm development. Extracellular DNA has been implicated as a major structural component for the initial establishment of biofilms in bacteria that specifically release DNA (78). Moreover, it has been recently observed that free DNA plays an important role in biofilm formation by S. mutans (58). In S. pneumoniae, DNA is spontaneously released into the medium in a process which is triggered by competence induction for genetic transformation and which depends on LytA and LytC (51, 63, 64). We observed that when strain R6 was grown in the presence of DNase I, biofilm formation was greatly impaired (Fig. 7A) although culture growth was not affected by addition of the enzyme (not shown). Similar experiments carried out with proteases again provided evidence of a dramatic reduction in biofilm formation. Finally, incubation of preformed R6 biofilms with either DNase or proteinase K drastically diminished the number of biofilm-associated sessile cells (Fig. 7B).

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FIG. 7. Inhibition of biofilm development in S. pneumoniae cultures in the presence of DNase or proteases. (A) S. pneumoniae R6 was grown overnight at 37°C to an A595 value of 0.5 to 0.6 in C+Y medium, centrifuged, and adjusted to an A595 of 0.6 with fresh medium. Afterwards, the cell suspension was diluted 10-fold, and 200-µl aliquots were distributed in the wells of a microtiter plate, which was then incubated for 6 h at 34°C (cross-hatched bars). Other samples received either DNase I (hatched bars), trypsin (filled bars), or proteinase K (open bars) at the indicated concentrations and were incubated as above. (B) After biofilm development (6 h at 34°C), DNase I (hatched bars), trypsin (filled bars), or proteinase K (open bars) was added at 100 µg ml–1, and incubation proceeded for an additional 1 h at 34°C before staining with CV to quantify biofilm formation.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Most microorganisms adopt biofilm formation as a lifestyle in nature, and the unique properties of many such multicellular communities have been characterized. However, molecular approaches to study these sessile forms of growth are lacking. The experiments reported in the present study provide evidence for the influence of several well-defined genetic traits and extracellular macromolecules in biofilm development by S. pneumoniae. Under the experimental conditions described here, pneumococcal cells adhered to a surface and adopted a multicellular 3D structure; i.e., they formed a biofilm.

Biofilm formation by pneumococcus cultured in a semisynthetic medium suggests that sessile growth represents a survival strategy in a nutritionally limited environment, assuming that surface colonization provides certain advantages, such as an increased capacity to capture nutrients, and that the oral surface represents a nutritionally limited environment. The environment defined by the human nasopharynx environment is subjected to significant fluctuations in pH, osmolarity, and nutrient availability; it is a well-ventilated zone where the temperature (about 34°C) (36) is cooler than in the rest of the body. Values of 6.4 to 6.9 for pH (34), 82 to 91 mM for Na⁺, 82 to 108 mM for Cl^– (73), and 3.9 to 5.8 mM for glucose (20) have been reported for nasopharyngeal and/or airway surface secretions in humans. In our experiments, environmental changes, such as nutrient content of the medium, pH, and osmolarity, affected biofilm formation in vitro (Fig. 1). Thus, the conditions described here, which were analogous to those found in the nasopharynx (see above), may be suitable for studying the cellular factors and molecular mechanisms that influence the formation of pneumococcal biofilms. Furthermore, our experimental approach fulfills the prerequisites for the comprehensive analysis of biofilm formation and complements previous attempts to study the role of capsular production during in steady-state growth on Sorbarod filters (74, 75).

The biofilms formed by pneumococcal cells developed into a 3D structure made up of a mat of cells, as revealed by CLSM and LTSEM (Fig. 4 and 5). Incubation of pneumococcus in the presence of 2% choline chloride (or EA) from the initial steps of growth reduced biofilm development (Fig. 4D). It is well known that when pneumococcal cells are incubated with 2% choline, some CBPs are released into the medium, whereas those having enzymatic activity are completely inhibited (44). For example, the characteristic long-chain growth of cells incubated in a high concentration of choline is due to the inhibition of LytB glucosaminidase (14, 24). Although it could be speculated that biofilm inhibition is a direct consequence of chaining itself, this appears not to be the case in pneumococci, as several closely related streptococcal strains that grow in long chains were perfectly able to form biofilms (see above). Moreover, the use of pneumococcal strains harboring mutations in the other cell wall hydrolases, namely, LytA, LytC, and Pce, showed that these CBPs play a variable role in biofilm formation on abiotic surfaces, since lytA, lytB, or lytC mutants formed, respectively, 20, 45, and 25% less biofilm than the wild-type strain, whereas no effect was observed when a pce mutant was employed (Fig. 4B). An additive effect on biofilm inhibition was observed with a lytA lytB lytC mutant. Interestingly, a positive correlation between peptidoglycan breaks, rather than particular molecules, and biofilm-forming capacity in the gram-positive bacterium Lactococcus lactis has been established recently (49). Additionally, it should be noted that a reduced colonizing capacity of the nasopharynx was reported for lytB and lytC mutants (25).

Many aspects of biofilm formation appear to be counterintuitive; e.g., some bacteria preferentially form biofilms in environments characterized by very high shear forces, such as occurs in the heart valves (17). This is also the case for the production of capsular polysaccharides by pneumococcus in that one would expect these compounds to mediate adherence to the nasopharyngeal epithelium, thereby initiating biofilm formation. Interestingly, it has become apparent that down-regulation of pneumococcal capsule production enhances host-cell invasion in the asymptomatic carrier state, although the capsular phase must be restored for the bacteria to survive after invasion (29, 76). This observation may have important implications for experiments with S. pneumoniae in vitro. Most of the capsular polysaccharides tested appeared to inhibit biofilm formation although, for still unknown reasons, pneumococci synthesizing a type 6B capsule were only partially inhibited in their capacity to grow as a biofilm (Fig. 6). This behavior provides a model to further study the impact of capsular variation on biofilm development in a species with a large number of different capsular types. Recently, it was suggested that this is a route to better analyze the role of the bacterial extracellular matrix in cell accretion (6). Furthermore, our observations are in keeping with previous findings reporting that exopolysaccharides influence initial attachment either positively or negatively, depending upon the exopolysaccharide type and the organism (62). This may be the case for S. aureus, which without a capsule was able to better persist in the murine mammary gland (71). However, it is premature to discard the possibility that capsular polysaccharide is not an essential component of biofilm matrix, as recently illustrated in the case of S. aureus, in which biofilm-associated protein (Bap) appears to replace the sugar matrix as a cementing material (68).

In microbial biofilms, bacterial cells aggregate on the surface in microcolonies and are embedded in an extracellular matrix whose composition is as variable as that of the biofilm. In general, the sticky matrix is composed of diverse extracellular polymeric substances, including exopolysaccharides, proteins, and DNA, and may contain other noncellular materials, depending on the environment in which the biofilm developed. Biofilm formation requires protein synthesis, and it is thought that extracytoplasmic proteins, as surface-exposed proteins, play a role in bacterial attachment to abiotic surfaces (55). Recently, using a continuous-culture biofilm system, de novo protein biosynthesis was reported to take place during biofilm formation by S. pneumoniae; more than 700 proteins that were detected in 9-day-old biofilms were apparently absent in planktonic cells (2). Our results (Fig. 7) showed that extracytoplasmic protein(s) is required for biofilm formation and maintenance. Furthermore, DNA has been identified as a key structural component of the biofilm extracellular matrix (78), and, more recently, biofilm formation was shown to involve a functional DNA-binding uptake system (58). Under the experimental conditions assayed here, DNA appears to have been released spontaneously into the medium, suggesting that it is an important component of the biofilm formed by S. pneumoniae. This conclusion is supported by the fact that there was a significant inhibitory effect on biofilm formation when DNase I was added either before or afterwards (Fig. 7). However, when either homologous or heterologous DNA was added to a pneumococcal culture, biofilm formation was not significantly affected (unpublished results). It has been shown that simultaneous inactivation of lytA and lytC abolished DNA release in liquid culture (51). In addition, Kausmally and coworkers (35) reported that DNA release is strongly attenuated in a cbpD mutant. Interestingly, we found that the two autolysins (but not CbpD) are also involved in biofilm formation (Fig. 4B, G, and I). We are currently investigating whether DNA-binding proteins up-regulated by the CSP are involved in biofilm formation in pneumococcus, as already documented for Streptococcus intermedius (57).

Given the tremendous metabolic and physiological changes that are required for the switch from a planktonic to a biofilm form of growth, it would be reasonable to assume that gene regulation facilitates this process. This switch, which is based mainly on local environmental cues that alter gene expression, seems to be crucial for microbes. Moreover, the switch serves to determine the virulence of the invading bacteria (54). It has been pointed out that, once attached to a surface, microbial cells initiate long-term interactions with each other (37). Such interactions may facilitate DNA interchange in organisms possessing natural competence, as is the case for pneumococcus.

Finally, it is well documented that competence and DNA release in pneumococcus are activated by a quorum-sensing (QS) circuit, a regulatory mechanism for bacteria to alter gene expression at very specific cell densities and to coordinate a population-type behavior in which dozens of genes are mobilized. CSP is the QS signal that mediates both competence and genetic transformation, and it has been well studied in S. pneumoniae (9). Horizontal DNA transfer is one of the major mechanisms driving the evolution of microorganisms, and related species cooperate with each other by sharing genes. This is exemplified by S. pneumoniae in its acquisition of penicillin resistance following genetic transformation with DNA released by other streptococci of the mitis group (26). QS systems have also been suggested to influence biofilm formation by S. mutans (41), although the molecular and biochemical details of this finding merit further investigation (65).

Studies of the QS system (9) and CBPs (44) are most advanced in S. pneumoniae. Thus, the availability of a feasible design for biofilm formation offers a model for genetically approaching questions regarding the social behavior of this human pathogen. In this sense, details on cell-to-cell communication will generate interesting information on survival strategies, such as passive protection against antibiotics, which appears to represent a phenotype rather than the result of genetic alterations. The goal of the results reported, to provide insight into biofilm development by pneumococcus, is thus beginning to take shape.

 
We thank C. Ascaso, F. Pinto, and S. Paniagua for their help with LTSEM, and M. C. Risueño and M. Seisdedos for the CLSM images. We are grateful to P. García and D. Llull for helpful comments and critical reading of the manuscript. We also thank E. Cano for skillful technical assistance and W. Ran for correcting the English version of the manuscript.

This work was supported by grants from the Dirección General de Investigación Científica y Técnica (BMC2003-00074) and from Redes Temáticas de Investigación Cooperativa (G03/103 and C03/14).

 
* Corresponding author. Mailing address: Departamento de Microbiología Molecular, Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, 28040 Madrid, Spain. Phone: 34 91 837 3112. Fax: 34 91 536 0432. E-mail: e.garcia{at}cib.csic.es.

Published ahead of print on 25 August 2006.

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Journal of Bacteriology, November 2006, p. 7785-7795, Vol. 188, No. 22
0021-9193/06/$08.00+0     doi:10.1128/JB.00673-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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