Polymers against Microorganisms

The significant advances on the control and prevention of infectious diseases carried out during the first decades of the twentieth century produced an optimist sensation about the possibility to completely eradicate any illness. But this optimistic vision rapidly changed as a result of the reemerging of new and in some cases antimicrobial-resistant infections. Examples of the novel/old/appearing/reappearing infectious diseases include the Ebola virus, HIV, or Legionnaire’s disease that are still a public health problem in the twenty-first century.

Within this context, two main aspects have deserved particular attention during the last decades. On the one hand, food-borne diseases are directly related to the emergence of microbial diseases. On the other hand, the emergence of antimicrobial resistance, recognized soon after the discovery of penicillin, has followed the introduction of most every new drug. As will be depicted in this chapter, synthetic macromolecular antimicrobials have emerged as a highly promising class of therapeutics with immense potential for combating multidrug-resistant microbes. In effect, the polymers mode of action relies on physiochemical parameters such as hydrophobicity and cationic charge, rather than specific receptor-mediated interactions, the activity of the polymers can be modulated by tuning key structural parameters. Taking into account the action mechanism, polymers exhibit in comparison with other materials, important advantages that have motivated their investigation as antibacterial materials. These include that polymers do not provide toxicity to the environment, do not develop resistance, and have an enhanced antimicrobial action.

A principal challenge defying current medicine in the twenty-first century is the large occurrence of antibiotic resistance, as well as, the risk posed by drug-resistant superbugs. In spite of this, progresses on the development of novel antibiotics to combat this problem are quite limited. It appears necessary to carry out a more concerted effort to advance in the discovery of novel therapeutic agents with excellent activity and unique mechanisms of action to overcome the problem of drug resistance. In this context, macromolecular antimicrobials with a different interaction with bacteria may offer an interesting alternative to current strategies in order to successfully prevent resistance. Furthermore, biofilm-forming bacteria are recognized to be gradually resistant to the action of antibiotics and are a leading cause of mortality or morbidity in nosocomial infections.

This chapter will, thus, describe the bacterial structure and summarize the mechanisms involved in the interaction between antibiotics and bacteria as well as the resistance mechanisms developed. In addition, the proposed models of interaction between macromolecular antimicrobials and bacteria will be analyzed.

The second part of this chapter is devoted to implant-associated infections produced by the formation of a biofilms at the surface of biomaterials. More precisely, the steps involved in biofilm formation and its particular properties that reduce the antimicrobial activity will be discussed. Finally, preliminary concepts on the use of polymers to overcome this limitation are depicted.

Until the early 1980s, low-molecular weight substances were mainly employed for their antimicrobial activity. However, the discovery of antimicrobial peptides (AMPs) carried out by dramatically changed this situation. This group demonstrated that macromolecular peptides were able to kill Gram-positive bacteria, Gram-negative bacteria, and fungi. AMPs have been extensively developed and today an Antimicrobial Peptide Database (APD). Based on this finding and around the same time antimicrobial polymers known under the name “polymer disinfectants” started to be investigated. As a result, studies on syntheses of polymeric biocides have been started to develop a new utilization field of polymer materials from 1980s. In particular, synthetic polymers have been widely investigated as a new molecular platform to create antimicrobial agents that are active against drug-resistant bacteria.

As will be depicted throughout this chapter, a variety of synthetic polymers with different chemical structures have been utilized to prepare antimicrobial polymers, and some polymers with high efficacy have been reported. In addition, a thorough analysis of the chemical characteristics of antimicrobial polymers and the different strategies to prepare them will be provided.

Pioneer strategies to combat infectious diseases focused on the improvement of pharmacokinetics of the antibiotics by prolonging their blood circulation. These initial approaches permitted the antibiotic to reach difficult-to-target sites of infection and, as a consequence, to reduce dose frequency of antibiotics and more interestingly to reduce undesired rapid clearance of therapeutic agents. However, this strategy can only be accomplished in combination of the advancement of the appropriate techniques both in chemical synthesis and the understanding of macromolecular chemistry.

This chapter describes the alternatives to fabricate nanometer scale polymeric structures with antimicrobial properties. In particular, we will describe the different alternatives developed to produce efficient antimicrobial polymer nanostructures in solution.

Organic (based on polymers) or hybrid inorganic/organic nanostructures have peculiar properties that distinguish them from materials structured at the micro scale. In particular, their large surface area to volume ratio may enhance the interaction of the nanostructured material with a given microbe as a result of a larger number of functional sites. The most studied antimicrobial nanostructures in solution are nanoparticles and within nanoparticles those made of silver have been extensively explored.

Moreover, antimicrobial polymers and, in particular, the nanostructures resulting from the self-assembly processes in solution has been recently demonstrated to be of interest for different applications including animal and human health care. Of particular interest are those cases in which the polymers form self-assembled nanostructures with a large concentration of antimicrobial moieties. Moreover, these self-assembled structures are able to incorporate other additional antimicrobials such as silver nanoparticles.

A major issue in the use of biomaterials in natural environments and in particular in hospitals is related to the microorganism adhesion to the biomaterial surface. In this context, the focus of scientists and biomedical manufacturers turned to the development of coatings capable of resisting bacterial colonization and that can be placed on the surfaces of medical devices.

In this chapter, a variety of concepts and approaches are currently being explored in order to produce materials with anti-infective properties that could be employed for biorelated applications will be described. As will be depicted, the strategies are proposed to either reduce or prevent bacterial adhesion. They basically can be divided into two different methodologies: the first type of methodologies include those strategies that either involve chemical modification to introduce antimicrobial activity or are intrinsically antimicrobial. The second type refers to those methodologies that resort to the formation of micro/nanostructures at the biomaterial surface. This chapter will focus on the first group, i.e., the description of the different strategies to chemically modify the polymer surface to improve their antifouling properties or to provide antimicrobial activity.

However, prior to the description of the different methodologies to fabricate antimicrobial surfaces the approaches that are available in order to modify the chemical composition of a particular surface will be first analyzed.

Today, it is well accepted that micro- and nanoscale surface topographical features can play a key role in controlling bacterial attachment. For instance, surface roughness has been directly related to the reduction of the initial surface contamination that can thus improve the reduction of biofilm formation. Thus, in addition to the chemical surface modification depicted in the previous chapter, in this chapter alternative attractive strategies to reduce bacterial adhesion would be simply acting on the features of the biomaterial surface will be described.

Moreover, the particularly astonishing advances in nanotechnologies permit today the controlled fabrication of surfaces with higher resolutions down to the nanometer scale. This area has currently become an area of intense research. The interest in the preparation of nanometer size features on material surfaces relies on the fact that they have been demonstrated to alter the 3D conformation of adsorbed proteins. As a result, it is expected that this behavior could potentially have an effect also on host adhesins which are the base of biofilm formation at biomaterial surfaces. This chapter provides an overview over the different strategies employed to fabricate micro/nanostructures and the effects observed when in contact with microorganisms. Equally, examples in which an additional surface functionalization supposes a significant improvement of the antibacterial/antifouling properties of the micro/nanostructured surfaces are depicted.

Nanotechnology and nanoscience involve different aspects including the manipulation, control, and assembly of nanoscale components to produce materials, systems, and/or devices. In this context, the fabrication of micro/nanofibers has attracted huge interest. In particular, micro/nanofibers have different properties such as high porosity, small pore size, high surface area, and compatibility with functionalizing additives that enables their use in multiple applications. These include their use as enzyme carriers, membranes for filtration purposes, as barriers to liquid penetration, sensors, delivery purposes, and catalysts. Polymer fibers have also been explored in a large variety of medical applications such as tissue engineering or in regenerative medicine.

In this chapter, we will provide an overview of the most extended fabrication approaches and their use in medical applications, in particular to prevent microbial contamination. The fabrication of fibers treated with antimicrobials is today a standard finish for many different textile products employed in such uses as medical, institutional, and hygienic. More recently, antimicrobial fibers have been extended to other applications including women’s wear, sportswear, and aesthetic clothing to impart anti-odor or biostatic properties.

Hydrogels are usually defined as a class of materials fabricated from natural or synthetic polymers with, among others, two unique characteristics. On the one hand, they possess three-dimensional (3D) networks with variable physical properties composed of cross-linked hydrophilic polymer chains. On the other hand, they are able to incorporate an extremely large amount of water within the structure. These materials have found multiple applications such as drug delivery, surface coatings for implants, healing of chronic and traumatic wounds, encapsulation of cells for three-dimensional cell culture and tissue engineering. To mimic natural tissues, in addition to the mechanical and chemical properties, hydrogels require also cell biocompatibility. In this context, many current synthetic strategies focused on tuning the biological and physical attributes of hydrogels in order to reach specific interactions and responses from cellular systems. Nevertheless, for many of the above-mentioned biorelated applications, microbial infections still remain a serious limitation for the use of hydrogels. In this context, to overcome this issue, different approaches have been developed to fabricate antimicrobial/antiviral hydrogels.

This chapter aims to discuss the explored systems on the preparation of antimicrobial/antifungal hydrogels. Illustrative examples of the different methodologies will be presented as well. In particular, the antimicrobial hydrogels will be classified depending on their role as carrier or based on its inherent antimicrobial activity. Moreover, highly sophisticated systems in which the response to environmental conditions is at the base of the antimicrobial activity of the hydrogel will be discussed in detail as well.

Membranes have been typically defined as interfaces between two interfaces having as a major role to regulate the transport between two different compartment and act as selective barrier. Membranes are able to selectively allow the transport of one substance in the presence of other compounds without the use of additives or the use of elevated temperatures, thus reducing the energy consumption. They have found multiple applications in different areas ranging from separation processes but have also been employed in the fabrication of biomaterials, catalytic purposes, or even lab-on-chip devices.

Several major characteristics including the low operation cost, relatively small footprint, and complicity with environmental regulations have provoked that polymers have been extensively employed for the fabrication of membranes. Polymeric membranes do not require the use of additives. This permits these membranes to be active at low temperatures thus enabling a significant decrease of the energy employed for the separation in comparison with other processes. In addition, these membranes are easily formed and up-scaling and downscaling can be easily carried out.

This chapter will provide a brief description about polymeric membranes focusing on one of the major remaining issues, that is, their contamination by microorganisms and, in particular, by bacteria. Upon a concise analysis of the problem, the alternative approaches developed to produce antifouling/antibacterial membranes will be thoroughly analyzed. For detailed reviews on membrane fabrication and their applications, the reader is referred to the following publications.

The use of antimicrobial molecules has, unfortunately, side effects that may limit their final use. Therefore, in addition to the antibacterial performance, the evaluation of environmental and safety issues is a requirement. According to the Directive 98/8/EC of the European Parliament relative to the use of biocidal products, it has been pointed out that several conventional biocides need to be replaced. Moreover, the use of antimicrobial substances, for instance, in food-related applications requires following the FDA requirements. In particular, the ISO 10993 is related to the biocompatibility and safety standards aiming to server as framework for selecting tests to evaluate biological responses. These include cytotoxicity, primary skin irritation, dermal sensitization, and systemic toxicity. In addition to the toxicity of the material, it is also crucial to determine if there exist leachable substances and eventual degradation products. In this context, antimicrobial polymers can provide alternative solutions to current microbial contamination and biofouling issues while respecting the environmental and health regulations.

This chapter will briefly describe the environmental problems that need to be considered when using polymers in particular in those cases, where the antimicrobial employed is leached from the polymeric material. The cytotoxicity associated to the nonselective performance of antimicrobials will be discussed as well. Finally, illustrative ongoing works for the fabrication of nontoxic antimicrobial polymeric materials will be analyzed.

The use of antimicrobial polymers has been extended to many different fields mainly due to their improved quality and safety benefits in comparison to traditionally employed biocides. In effect, low-molecular weight antimicrobial agents have important disadvantages including their toxicity to the environment and/or short-term antimicrobial ability. On the contrary, the use of antimicrobial polymers may enhance the effectiveness of some of the currently employed antimicrobial agents while reducing the environmental issues accompanying conventional antimicrobial agents (typically by decreasing the residual toxicity of the agents, increasing their efficacy and selectivity, and extending the life span of the antimicrobial agents).

Taking into account the important advantages that antimicrobial polymers offer, a wide range of classes and applications can be envisaged for these materials. As will be depicted in this chapter, areas that can benefit from the use of antimicrobial polymers include the fabrication of fibers, textile sector, the design of water filtration systems, food packaging, and biomedical and pharmaceutical industries. In particular, focusing in the biomedical field, these polymers can decrease the sufferings of people improving their recovery, therefore offering better life quality.

This chapter will summarize the most important areas of applications in which polymers are at this time playing an important role or can be of potential interest in the near future. Moreover, the current limitations as well as those aspects that require both further investigation and improvements will be depicted focusing in their use for food packaging and food storage as well as for biorelated applications including the fabrication of medical devices, hygienic applications, or surgery equipment.