A high-throughput microarray method for the identification of polymers which reduce bacterial surface binding on medical devices is described.
Medical devices are often associated with hospital-acquired infections, which place enormous strain on patients and the healthcare system as well as contributing to antimicrobial resistance. One possible avenue for the reduction of device-associated infections is the identification of bacteria-repellent polymer coatings for these devices, which would prevent bacterial binding at the initial attachment step. A method for the identification of such repellent polymers, based on the parallel screening of hundreds of polymers using a microarray, is described here. This high-throughput method resulted in the identification of a range of promising polymers that resisted binding of various clinically relevant bacterial species individually and also as multi-species communities. One polymer, PA13 (poly(methylmethacrylate-co-dimethylacrylamide)), demonstrated significant reduction in attachment of a number of hospital isolates when coated onto two commercially available central venous catheters. The method described could be applied to identify polymers for a wide range of applications in which modification of bacterial attachment is important.
Polymer microarrays are miniaturized high-throughput platforms in which up to 7,000 polymers1 are printed onto glass slides for parallel analysis with prokaryotic or eukaryotic cells2. The method presented here builds on that which we first described in 20103. This screening system has been applied to numerous cell types including human hepatocytes4, stem cells5, renal tubular epithelial cells2, bacteria3,6 and protozoan pathogens7. In each case, polymers that promote or resist binding of the cells under study were identified8. Complexes of DNA with synthetic polycationic polymers have also been used in the microarray format for high-throughput screening of gene transfection candidates9. As well as screening for cell-substrate interactions, polymer microarrays have also been used for evaluating material properties10.
The ability of synthetic polymers to modulate attachment of bacteria to a surface is well established3,6,11. Numerous factors including charge, hydrophobicity and surface roughness of the polymer surface are known to affect bacterial binding. The conventional approaches of discovering biomaterials that resist binding of bacteria through sequentially or empirically designing and testing one material at a time are labor-intensive, costly and time-consuming processes. Polymer microarrays offer an attractive alternative for circumventing such limitations.
Surface-associated bacteria grow as a complex population termed a biofilm — such biofilms are highly resistant to many environmental stresses and antibiotics. This is in part due to their dense extracellular matrix (composed of proteins, polysaccharides and nucleic acids)12 and in part due to the increased presence of robust "persistor" cells in biofilms13. Although the precise mechanisms of surface association and subsequent biofilm formation are difficult to characterize, it is generally believed that there are three different stages of surface growth14–16. Initial, reversible attachment is followed by stronger adhesion of cells, the establishment of a biofilm by production of an extracellular protein and polysaccharide matrix and cell proliferation. Finally, the mature biofilm releases free-living planktonic cells, which can initiate new infections elsewhere. Bacteria-repelling polymers that prevent the initial attachment of bacteria, and hence prevent early stages of biofilm formation, potentially represent an excellent solution for minimizing infections. Given the rise of antibiotic resistance (and also the intrinsically greater resistance of surface-associated bacteria12), antibiotic-free means of reducing infections are of particular interest. In a hospital setting, bacteria-repelling polymer coatings can have a direct medical application in the reduction of nosocomial infections, which commonly form around implanted devices17.
Here, a high-throughput method for the screening of 381 polymers for repellent activity against a range of pathogenic bacteria associated with nosocomial infections, followed by hit validation and subsequent coating and assay of central venous catheter materials, is described (Figure 1). Briefly, the polymers were spotted onto agarose-coated glass slides by contact printing and, after drying and sterilization, the miniaturized arrays were incubated with clinically important bacterial cultures. After incubation, the microarrays were gently washed and adherent bacterial cells were stained and visualized by fluorescence. Subsequently, polymers which inhibited bacterial binding were investigated on a larger scale by coating onto glass cover slips and visualized by electron microscopy. Selected repellent polymers were then coated onto commercial catheters and shown to reduce attachment of bacteria by almost 100 fold.
1. Preparation of Agarose Coated Slides
NOTE: Before fabricating the polymer microarrays, aminoalkylsilane-coated glass slides are coated with agarose I-B to minimize non-specific background binding and allow evaluation of polymers that bind or repel bacteria6. Silane coating facilitates the binding of agarose to the slides.
2. Preparation of Polymer Solutions for Printing
3. Printing Polymer Microarrays Using a Contact Printer
NOTE: Printing of microarrays was performed using a contact printer. Specific instructions regarding polymer microarray printing are given below. For general guidelines on using the printer and safety recommendations, follow the user manual from the manufacturer. Although we use a contact printer, a suitable manual stamping device could also be used.
4. Inoculation of Polymer Microarrays with Bacteria
NOTE: Ensure good aseptic technique. All handling of cultures should be performed in a sterile environment: either using a Bunsen burner or in a flow hood. Cultures should be grown with oxygen availability, growth medium, and temperature adjusted to the requirements of each species.
5. Microarray Imaging and Analysis
6. Coating of Cover Slips for 'Hit' Validation
7. Attachment and Analysis of Cover Slip Using Scanning Electron Microscope (SEM)
8. Selection of a Solvent for Coating Catheters
9. Analysis of Bacterial Attachment on Catheters by Confocal Microscopy
10. Analysis of Bacterial Attachment on Catheters by SEM
Figure 2 shows bacterial attachment (normalized fluorescence intensity) to a number of polymers as determined by microarray analysis. Spots printed without polymer (NMP only) are the negative control, as agarose strongly resists bacterial binding6 — recorded fluorescence is very low. The polymers displayed are all low-binding although in most cases, the repellent properties of a polymer varies significantly between bacterial species tested. This reflects the wide differences between attachment mechanisms across different species. The selection of an appropriate polymer is therefore dependent on the application, but the low-binding polymers are easily identified by comparison with agarose. High-binding polymers are likely to be identified in an array, and can be used as positive controls for subsequent experiments. For polymers that do not show auto-fluorescence in the DAPI channel, qualitative comparison of the spot images can be made visually (as shown in Figure 3, highlighting one representative high-binding polymer and three example low-binding polymers). Such comparison, while providing less information than the statistical analysis, can be a useful visual validation of the intensity values calculated.
With 22 appropriate polymers identified using the microarray, scale-up experiments are carried out to confirm their bacteria-repellent property when used for coating larger surfaces. Shown are the best-performing examples, used to coat glass cover slips (analyzed by SEM (Figures 4 and 5)) and catheter slices (analyzed by confocal microscopy (Figure 6) and SEM (Figure 7)). Both microscopic methods have the benefit of allowing direct cell counts on the surface, providing unambiguous data; reduction in cell attachment is easily visible. Those coatings which best retained their repellent properties on scale-up, as well as being amenable to large-scale coating techniques, were investigated further. The results shown illustrate the best-performing polymers from each stage.
Figure 1: Steps involved in the identification of bacteria repellent polymers through polymer microarrays for biomedical applications. SEM = scanning electron microscopy.
Figure 2: Bacterial binding on a series of polymers, determined by microarray analysis. Low bacterial binding is seen on a number of polyacrylates/acrylamides (PA) and polyurethanes (PU). Results from polymer microarrays probed with several bacterial species (C. jejuni, C. difficile, C. perfringens, S. mutans, and two consortia; BacMix-1 and BacMix-2) are shown. Bacterial binding expressed as background corrected mean DAPI fluorescence intensity (normalized). Error bars represent standard deviation. Adapted from reference6. Please click here to view a larger version of this figure.
Figure 3: Example images of polymer spots. Fluorescence (DAPI) and brightfield microscopy images of BacMix-2 showing bacterial attachment on representative polymers. A strongly-binding polymer is included for comparison to several non-binding polymers (PU-20, PA-336 and PU-179). Scale bar = 100 µm. Adapted from reference6. Please click here to view a larger version of this figure.
Figure 4: Scale-up experiments with glass cover slips. SEM images showing bacterial attachment on uncoated glass and glass coated with 'hit' polymers (examples chosen from Table 1). Scale bars = 20 µm.
Figure 5: Bacterial binding quantified by cell counts from SEM images. Comparison of cells attached per unit area of surfaces coated with the best 'hit' polymers. Agarose was used as a control 'non-binding' surface, with glass as a control binding surface. Error bars represent standard deviation.
Figure 6: Comparison of bacterial binding on coated and uncoated catheter slices. Confocal images comparing untreated catheter (cath-1) (A) and catheter coated with PA13 (B) after incubation with BacMix-2. Images taken with a 40X objective (Scale bar = 20 µm). Adapted from reference6.
Figure 7: Comparison of bacterial binding on coated and uncoated catheter slices. SEM images show comparison of untreated catheter (cath-1) (A) and catheter coated with PA13 (B) after incubation with a bacterial cocktail consisting of BacMix-2. Scale bars = 10 µm. Adapted from reference6.
Polymer | Monomer 1 | Monomer 2 | Monomer 3 | Ratio of Monomers | ||
PA465 | MEMA | DEAEMA | HEA | 8 | 1 | 1 |
PA475 | MEMA | DEAEA | HEMA | 6 | 1 | 3 |
PA513 | MEMA | DEAEMA | MMA | 8 | 1 | 1 |
PA515 | MEMA | DEAEA | MMA | 6 | 1 | 3 |
PA13 | MMA | DMAA | – | 9 | 1 | – |
PU1 | PEG2000 | HDI | – | 4.9 | 5.2 | – |
PU16 | PEG2000 | MDI | – | 4.9 | 5.2 | – |
PU161 | PEG2000 | MDI | BD | 2.5 | 5.2 | 2.3 |
PU7 | PEG900 | BICH | – | 4.9 | 5.2 | – |
PU83 | PEG900 | HMDI | BD | 2.5 | 5.2 | 2.3 |
PU227 | PPG-PEG-1900 | HDI | – | 4.9 | 5.2 | – |
PU129 | PPG425 | BICH | DMAPD | 2.5 | 5.2 | 2.3 |
PU10 | PTMG2000 | BICH | – | 4.9 | 5.2 | – |
PU179 | PTMG2000 | HDI | NMAPD | 2.5 | 5.2 | 2.3 |
PU20 | PTMG2000 | MDI | – | 4.9 | 5.2 | – |
PU5 | PTMG2000 | HDI | – | 4.9 | 5.2 | – |
Table 1: Composition of the bacteria non-binding 'hit' polymers in Figure 2.
Attachment of bacteria to a surface is a complex process determined by a wide range of factors dependent on the bacterial species, the properties of the surface, the surrounding medium and the physical environment. Although certain chemical groups are known to affect bacterial binding (polyglycols, for instance, typically resist attachment11), correlating the biological impact of polymers with their chemical structures is difficult, making rational design of polymers for specific functions challenging. In the absence of detailed attachment mechanisms, other studies have attempted to mimic naturally-occurring repellent surfaces, with lengthy and extensive optimization processes21. The miniaturized high-throughput method presented here overcomes these challenges by facilitating parallel screening of hundreds of polymers to identify leads for further study.
Results from the microarray method principally serve to identify likely lead candidates. Figure 2 illustrates 22 candidates with low binding of at least one species, while Figure 3 demonstrates the clear reduction in binding capacity. All 22 low-binding polymers shown in in Figure 2 were taken forward into scale-up experiments, during which the best (in terms of repellence and coating properties) were determined to be PU83, PA13, and PA515 (Figures 4 and 5). Polyacrylates offer greater flexibility in terms of polymerization methods and so the lowest-binding polyacrylate, PA13, was chosen for catheter coating studies (Figures 6 and 7). More detailed further work on other candidates was carried out and has been reported elsewhere6.
Through a number of experimental iterations we found a number of minor steps were key to success and reproducibility. As well as facilitating the adhesion of the polymers to the glass slides, using an agarose under-coating provides a clean background, as agarose is highly resistant to bacterial colonization. Likewise consistency in the polymer spots themselves, both within the same array and between arrays, is vital and therefore the printing of the arrays must be carefully controlled. Careful adjustment of the pins in the print-head and also uniform filling of the 384-well plate are required to ensure uniform spotting. As some of the polymers we used exhibited a degree of autofluorescence, taking background fluorescence data for each slide before incubation with bacteria was vital. To account for variations and to obtain robust data replicates of microarrays are advised.
The stain employed here (DAPI) has no selectivity for bacterial species, binding non-specifically to DNA. Therefore, good aseptic technique is essential once bacterial cultures are introduced as contaminants may go unnoticed, confusing the interpretation of the results. The same is true of later experiments using scanning electron microscopy, where it is only possible to distinguish rods and cocci but not genus or species.
After microarray screening, promising polymers should be chosen for further validation. In the example presented here, seven polymers of interest were identified by their clear reduction in fluorescence on the microarray and their inhibition of attachment was confirmed by coating them on larger surfaces. Figures 4 and 5 show the reduction in binding achieved on glass coverslips, a practical means to test the behavior of the polymers as bulk coatings rather than as microarray spots. Subsequently, these polymers were coated on medical devices to fully quantify reduction in bacterial attachment. It is important that the solvent chosen (see protocol section 8) for these coating studies is benign to the desired substrate (here, the catheter) while retaining ability to dissolve the polymer of interest, in order to allow coating. Here, we used acetone which, as well as the properties mentioned, has a low boiling point and evaporates quickly to leave a uniform coating.
The means of validation chosen will depend on the specific application being studied. As observation of cells by electron and fluorescence microscopy allows direct quantification of individual cell attachment, we chose these techniques as a complement to the bulk staining microarray assay. Results are shown in Figures 6 and 7, which demonstrate the importance of complimentary validation methods. The confocal images in Figure 6 provide very clear images of individual cells, while the SEM has the added benefit of allowing an assessment of the surface of the polymer, which is here smooth and uniform. These methods are limited by the field of view of the microscopes used, and therefore it is important to take a series of snapshots to have confidence in the results. The method described above cannot quantitate bacterial adherence over the entire surface, only infer coverage from a number of small regions. We believe this is sufficient for the application described. Reduction in bacterial binding could be assessed by enumerating surface adhered bacteria on the entire coated and uncoated catheter pieces using methods as described elsewhere22. However such methods require the biomaterial surfaces screened to have a uniform surface area, which is difficult to maintain when assays are performed with medical devices, which often have complex geometry.
Clearly, any device intended for clinical use must go through substantial further testing to ensure safety and efficacy in humans. The method presented here represents the beginning of this process and further work must include confirmation of in vivo activity. In this case, studying venous catheters, initial work could investigate the binding of blood components and whole cells to the polymer. The effect of blood components on bacterial binding should also be considered, possibly by repeating the binding assays in the presence of inactivated serum or de-fibrinated blood23. The definitive test of the technology will be in an in vivo model such as a subcutaneous implant infection model24.
We demonstrate the potential of the polymer microarray method for screening of surface-altering polymers. Such polymers (both resisting and promoting bacterial binding) have a great number of applications in medicine, the food industry and biotechnology, meaning this method may be useful in many areas of research. Although the work here uses bacteria, the method could be adapted to other cell types and likewise other chemical microarrays.
The authors have nothing to disclose.
The authors thank EASTBIO (the East of Scotland BioScience Doctoral Training Partnership funded by the BBSRC) (S. V.) and the Medical Research Council (P.J.G) for funding.
Agarose | Sigma | 05066 | |
Silane-prep slides | Sigma | S4651 | |
Polymers | Synthesised in-house | Not applicable | |
NMP | Sigma | 494496 | |
LB Broth | Oxoid | CM1018 | |
DAPI | Thermo Fisher | D1306 | |
Tetrahydrofuran | Sigma | 401757 | |
(3-aminopropyl) triethoxysilane coated glass slides | Sigma | Silane-prep | |
Cacodylate buffer | Sigma | 97068 | |
Catheter 1 | Arrow International | CS12123E | |
Catheter 2 | Baxter Healthcare | ECS1320 | |
Osmium tetroxide | Sigma | 201030 | |
Equipment | |||
Contact printer | Genetix | Qarraymini | |
Microarray microscope | IMSTAR | Pathfinder | |
Spin Coater | Speedline Technologies | 6708D | |
Confocal microscope | Leica | SP5 | |
Image analysis software | Media Cybernetics | Image-Pro Plus | |
Scanning electron microscope | Philips | XL30CP | |
Sputter coater | Bal-Tec | SCD050 |