Surpassing nature: rational design of sterile-surface materials

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The rise of multidrug-resistant pathogens and recalcitrance of biofilm infections present a formidable challenge to combating infectious diseases. There are numerous disinfectants and antiseptics for treating materials in hospitals and community settings, and devices such as catheters impregnated with anti-infectives have been introduced into practice. However, there are many limitations of materials impregnated with a leaching antibacterial agent. Recently, non-leaching, permanent, sterile-surface materials have been developed in which one end of a long-chained hydrophobic polycation containing antimicrobial monomers is attached covalently to the surface of a material, for example, cotton or plastic. The polymeric chain allows the antimicrobial moieties to permeate into, and kill, the cells of the pathogen. These sterile-surface materials kill both air- and waterborne pathogens and are not susceptible to existing resistance mechanisms.

Introduction

We live surrounded by pathogens, and infectious diseases have been the main cause of mortality over millennia. The introduction of antibiotics in the 1940s promised to eradicate infectious diseases. Unfortunately, early successes in antibiotic development were followed by a daunting dual problem – the rise of resistant pathogens and a halt in novel antibiotic discovery. We are now confronted by Staphylococcus aureus, Enterococcus faecalis and Mycobacterium tuberculosis pathogens that are resistant to almost all currently available antibiotics 1, 2. Similarly, current measures against biofilm infections of indwelling devices are inadequate and the therapy of choice is often re-operation and removal of a prostheses or a catheter 3, 4, 5.

The last new class of broad-spectrum antibiotics (the fluoroquinolones) was discovered in the 1960s [6]. Culturable microorganisms, the source of most antibiotics, make up only ∼1% of the total number of microbial species and their over-mining largely accounts for the end of the ‘golden era’ of antibiotic discovery [7]. Synthetic compounds thus far have failed to replace natural antibiotics, despite the combined efforts of genomics, combinatorial chemistry and high-throughput screening, because they are invariably pumped out across the outer membrane barrier of gram negative bacteria by multidrug-resistance pumps (MDRs) 8, 9.

Encouraging recent developments should be noted – for example, a method for growing previously unculturable bacteria [10] and discovery of MDR inhibitors that might lead to dual-compound therapies based on a synthetic anti-infective and a pump inhibitor [8]. But for now we find ourselves close to where we started – in the pre-antibiotic era.

Successful pathogen counter-measures began not with systemic anti-infectives, but with the introduction of preventative public health measures a century ago. Countering the spread of infection has dramatically improved human health and increased longevity, surpassing the benefits of the subsequently introduced antibiotics [11]. At present, the spread of pathogens in hospitals has become a main cause of mortality from infectious diseases. Each year, ∼90 000 people die in the USA alone from nosocomial infections [12] by pathogens such as ‘MRSA’ S. aureus. Attacking the spread of infection with novel technologies promises to stem both nosocomial and community-acquired diseases. Creation of materials lethal to pathogens will also address another unmet need – prevention of biofilm infections on indwelling devices.

A host of disinfectants, antiseptics and antibiotics has been developed to fight pathogens and biofouling with a leachable anti-infective usually incorporated into a polymeric surface coating [13]. This approach, however, suffers from problems – release of the active compound is temporary, a toxic substance leaches into the environment, and the gradually decreasing level of the released compound provides perfect conditions for resistance development. The ideal approach would create a permanently sterile, non-leaching material by covalently functionalizing its surface with an antimicrobial compound 14, 15, 16, 17, 18, 19, 20, 21 and this emerging field is the subject of this article.

Section snippets

Designing sterile-surface materials

There is an obvious problem in designing a permanently sterile material – once attached to a surface, an antimicrobial molecule loses much of its mobility and, being unable to penetrate into the cell, becomes inactive. A possible solution is to link the antimicrobial agent to a long, flexible polymeric chain anchored covalently to the surface of a material.

There are numerous antimicrobials suitable for immobilizing to a surface. Quaternary ammonium compounds (QACs) seemed attractive because

Mechanism of action

Is the antimicrobial polycation really delivered into the cell of a pathogen? Varying the molecular weight of the immobilized polymer should directly test the hypothesis of antimicrobial mobilization by flexible polycations. To this end, N-alkylated PEIs of different molecular weights were covalently attached to amino-glass slides. Immobilized 750 kD and 25 kD PEIs were highly lethal to airborne S. aureus. By contrast, their 2 kD and 0.8 kD counterparts had negligible, if any, antibacterial

Resistance

Resistance development by pathogens is the crucial limitation of existing antimicrobial agents and the main driving force behind the anti-infective drug discovery effort [2]. The most obvious application for sterile surfaces is to stem the spread of nosocomial diseases, such as those caused by the MRSA strains (note that although MRSA stands for ‘methicillin-resistant S. aureus’, these organisms actually carry plasmids conferring resistance to a whole slew of commonly used antibiotics [28]). We

Conclusions and perspectives

The emerging area of non-leaching sterile surfaces has achieved several important milestones demonstrating the feasibility of this technology:

(i) Proof-of-principle: surfaces modified with covalently attached polycations kill both airborne and waterborne microorganisms.

(ii) Action spectrum: sterile surfaces kill a broad range of pathogens – gram positive and gram negative bacteria, as well as fungi.

(iii) Mechanism of action: flexible polymers apparently reach across the microbial cell envelope,

Acknowledgements

Our studies reviewed herein were supported by NIH grants GM59903 and GM061162 (to K.L.) and by the U.S. Army through the Institute for Soldier Nanotechnologies under contract DAAD-19–02-D-0002 with the U.S. Army Research Office (to A.M.K.). The content does not necessarily reflect the position of the Government and no official endorsement should be inferred.

References (38)

  • P.N. Danese

    Antibiofilm approaches: prevention of catheter colonization

    Chem. Biol.

    (2002)
  • S.B. Levy et al.

    Antibacterial resistance worldwide: causes, challenges and responses

    Nat. Med.

    (2004)
  • K. Lewis

    Bacterial Resistance to Antimicrobials: Mechanisms, Genetics, Medical Practice and Public Health

    (2002)
  • J.W. Costerton

    Bacterial biofilms: A common cause of persistent infections

    Science

    (1999)
  • K. Lewis

    Riddle of biofilm resistance

    Antimicrob. Agents Chemother.

    (2001)
  • M.R. Parsek et al.

    Bacterial biofilms: an emerging link to disease pathogenesis

    Annu. Rev. Microbiol.

    (2003)
  • C. Walsh

    Where will new antibiotics come from?

    Nat. Rev. Microbiol.

    (2003)
  • M.S. Osburne

    Tapping into microbial diversity fornatural products drug discovery

    ASM News

    (2000)
  • K. Lewis et al.

    Drug Efflux

  • X.Z. Li et al.

    Efflux-mediated drug resistance in bacteria

    Drugs

    (2004)
  • T. Kaeberlein

    Isolating “uncultivable” microorganisms in pure culture in a simulated natural environment

    Science

    (2002)
  • J. Lederberg

    Infectious history

    Science

    (2000)
  • R.A. Weinstein

    Nosocomial infection update

    Emerg. Infect. Dis.

    (1998)
  • J.C. Tiller

    Designing surfaces that kill bacteria on contact

    Proc. Natl. Acad. Sci. U. S. A.

    (2001)
  • J. Lin

    Making thin polymeric materials, including fabrics, microbicidal and also water-repellent

    Biotechnol. Lett.

    (2003)
  • L. Cen

    Surface functionalization technique for conferring antibacterial properties to polymeric and cellulosic surfaces

    Langmuir

    (2003)
  • J. Lin

    Bactericidal properties of flat surfaces and nanoparticles derivatized with alkylated polyethylenimines

    Biotechnol. Prog.

    (2002)
  • J. Lin

    Mechanism of bactericidal and fungicidal activities of textiles covalently modified with alkylated polyethylenimine

    Biotechnol. Bioeng.

    (2003)
  • J. Lin

    Insights into bactericidal action of surface-attached poly(vinyl-N-hexylpyridinium) chains

    Biotechnol. Lett.

    (2002)
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