Infectious Disease: Connecting Innate Immunity to Biocidal Polymers

Abstract

Infectious disease is a critically important global healthcare issue. In the U.S. alone there are 2 million new cases of hospital-acquired infections annually leading to 90,000 deaths and 5 billion dollars of added healthcare costs. Couple these numbers with the appearance of new antibiotic resistant bacterial strains and the increasing occurrences of community-type outbreaks, and clearly this is an important problem. Our review attempts to bridge the research areas of natural host defense peptides (HDPs), a component of the innate immune system, and biocidal cationic polymers. Recently discovered peptidomimetics and other synthetic mimics of HDPs, that can be short oligomers as well as polymeric macromolecules, provide a unique link between these two areas. An emerging class of these mimics are the facially amphiphilic polymers that aim to emulate the physicochemical properties of HDPs but take advantage of the synthetic ease of polymers. These mimics have been designed with antimicrobial activity and, importantly, selectivity that rivals natural HDPs. In addition to providing some perspective on HDPs, selective mimics, and biocidal polymers, focus is given to the arsenal of biophysical techniques available to study their mode of action and interactions with phospholipid membranes. The issue of lipid type is highlighted and the important role of negative curvature lipids is illustrated. Finally, materials applications (for instance, in the development of permanently antibacterial surfaces) are discussed as this is an important part of controlling the spread of infectious disease.

Fig. 1

Collection of chemical structures to illustrate the terms “selective” AMMs and polymer “biocides” as determined by MIC and HC experiments. For the cited cases, the MIC is the minimum concentration at which E. coli growth is inhibited 90 – 100%. HC is the hemolytic concentration to lyse 50%, as convention, of a RBC solution. Polymers that have been traditionally studied for biocidal activity have usually not been subjected to HC experiments so that some may in fact be “selective” by other criteria. A classic example is polyhexamethylene biguanides (PHMB), a polymer well-accepted as a “disinfectant” but is non-toxic at the concentration used in contact lens solution (~ 0.0001 wt%).

Fig. 2

1 = Selective antimicrobial β-peptides [39]. 2 = β²/β³ peptide [40, 41]. 3 = “β-17” β-peptide [42].

Fig. 3

4 = Promising hit from peptoid combinatorial library [52]. 5 = Selective peptoid mimic of magainin-II [53]. 6 = Peptidomimetic for Ti surface modification [54].

Fig. 4

7 = General structure of FA arylamide oligomers [11]. 8 = Selective arylamide [57]. 9 = Pyrimidine arylamide oligomer [60].

Fig. 5

10 = General structure of phenylene ethynylenes studied [62, 63]. 11 = Trimer derivatives with distinct activities [38].

Fig. 6

12 = Set of polynorbornes whose activities are relatively MW independent [68]. 13 = Guanidinium functionalized polynorbornene [69]. 14 = Design to access copolymer series with a range of hydrophobicities [69].

Fig. 7

15 = Widely used contact lens disinfectant, PHMB [74]. 16 = Methacrylate monomer containing biguanide [7].

Fig. 8

17 = DABCO-based quaternary ammonium polymers [78]. 18 = Quaternary ammonium polymers quaternized > 90% after polymerization [79]. 19 = Copolymer of pyridinium containing methacrylamide and NIPAAm [81].

Fig. 9

20 = Quaternary ammonium containing block copolymer [85]. 21 = Ammonium and phosphonium polymers synthesized from a common reactive backbone [87]. 22 = Polystyrenes with quaternary ammonium groups [8, 88].

Fig. 10

23 = Polyamide with pendant quaternary pyridinium groups [89]. 24 = Poly(benzylvinylalkyl pyridinium bromide)s [90]. 25 = Random copolymer of acrylamide and quaternized vinyl pyridine [91]. 26 = Crosslinked polystyrene-r-quaternary pyridinium-type polymers [93].

Fig. 11

Cell wall components of Gram-positive and Gram-negative bacteria, taken from http://filebox.vt.edu/users/chagedor/biol_4684/Methods/cellwalls.html.

Fig. 12

(A) General structure of the common phospholipids and cholesterol. All the lipids have a polar phosphate head group and hydrophobic fatty acyl tails R ₁, R ₂ (R₁ = R₂ for symmetric lipid or R ₁ ? R₂ for asymmetric lipid). ^a C₀ > 0 or C₀ ~ 0, ^b C₀ < 0, ^c C₀ < 0 when bound to Ca ²⁺. (B) Lamellar (top) and hexagonal (bottom) phases promoted by intrinsic curvature of the lipid, C₀ ~ 0 (e.g. PC) and C₀ < 0 (e.g. PE), respectively. The free energy (F_H) per unit area in the lipid monolayer of the hexagonal phase is approximated by the above equation, where k is the bending modulus for the monolayer, R is the radius of a pivotal plane, and R₀ is the radius of intrinsic curvature describing the lipid assembly in a stress-free state with the minimum energy. Fig. 12(B) reproduced with permission from Biophysical Journal [106].

Fig. 13

High-sensitivity DSC heating scans illustrating the effect of the presence of increasing quantities of gramicidin S (GS) on the thermotropic phase behavior of DMPC MLVs. The top scan is of DMPC alone and the DMPC/GS molar ratios of the lower scans are indicated on the figure itself. Reproduced with permission from Biochimica et Biophysica Acta, Biomembranes [129].

Fig. 14

Binding isotherms for binding of GS to various LUVs derived from the ITC measurements at 25 °C. The degree of binding (X_bⁱ) is plotted as a function of free peptide concentration ( ${\text{c}}_{\text{f}}^{\text{i}}$). Each data point represents an individual titration step. The solid lines represent theoretical fits according to the one-site binding mo del. Reproduced with permission from Biochemistry [186].

Fig. 15

SEM micrgraphs of E. coli in contact with neat LDPE (A) and modified LDPE (B–D) after 15 (B), 30 (C), and 60 min (A, D) of contact time. Reproduced with permission from Biomacromolecules [202].

Fig. 16

Photographs of amino glass slide (Left) and a hexyl-PVP-modified slide (Right) onto which aqueous suspensions (10⁶ cells/mL of distilled water) of S. aureus cells were sprayed, air dried for 2 min, and incubated under 0.7% agar in a bacterial growth medium at 37 °C overnight. Reproduced with permission from Proceedings of the National Academy of Sciences [218].

Fig. 17

Photographs of modified medical grade PVC from catheter tubing (blended with AMM), (Left), and unmodified PVC, (Right), after spraying with aqueous suspensions of S. aureus cells (10⁵ cells/mL), air drying for 3 min, and incubating under rich growth media at 37 °C for 24 h.

Fig. 18

Untreated (left) and treated (right) PU film. The treated sample was able to completely kill E. coli whereas the untreated surface allowed bacterial colonization. Reproduced with permission Journal of Industrial Microbiology & Biotechnology [67].

Scheme 1

Synthetic pathways for the formation of quaternized PVP on glass surfaces.

Scheme 2

Synthetic route for the ATRP and quaternization of DMAEMA on solid surfaces.