Promising applications of D-amino acids in periprosthetic joint infection

Abstract

Due to the rise in our aging population, a disproportionate demand for total joint arthroplasty (TJA) in the elderly is forecast. Periprosthetic joint infection (PJI) represents one of the most challenging complications that can occur following TJA, and as the number of primary and revision TJAs continues to rise, an increasing PJI burden is projected. Despite advances in operating room sterility, antiseptic protocols, and surgical techniques, approaches to prevent and treat PJI remain difficult, primarily due to the formation of microbial biofilms. This difficulty motivates researchers to continue searching for an effective antimicrobial strategy. The dextrorotatory-isoforms of amino acids (D-AAs) are essential components of peptidoglycan within the bacterial cell wall, providing strength and structural integrity in a diverse range of species. Among many tasks, D-AAs regulate cell morphology, spore germination, and bacterial survival, evasion, subversion, and adhesion in the host immune system. When administered exogenously, accumulating data have demonstrated that D-AAs play a pivotal role against bacterial adhesion to abiotic surfaces and subsequent biofilm formation; furthermore, D-AAs have substantial efficacy in promoting biofilm disassembly. This presents D-AAs as promising and novel targets for future therapeutic approaches. Despite their emerging antibacterial efficacy, their role in disrupting PJI biofilm formation, the disassembly of established TJA biofilm, and the host bone tissue response remains largely unexplored. This review aims to examine the role of D-AAs in the context of TJAs. Data to date suggest that D-AA bioengineering may serve as a promising future strategy in the prevention and treatment of PJI.

Fig. 1

A schematic presenting examples of the various pathogens associated with orthopedic PJIs and their estimated incidence.^,, Mono- and polymicrobial infections are associated with PJI, with up to 15% of all cases being comprised of multiple bacterial species (spp.). Staphylococcus and coagulase-negative staphylococci are involved in 50%–60% of PJIs

Fig. 2

S. aureus and aerobic gram-negative bacilli together contribute to ~60% of early-onset (<3 months) infections. Polymicrobial infections are also higher at this time. Typically, delayed-onset PJI (3 months to 12–24 months postsurgery) occurs at the time of surgery and due to inoculation with less virulent microorganisms. At this stage, coagulase-negative staphylococci and enterococci are more common. Late-onset PJI (>12 to 24 months postsurgery) occurs mostly following hematogenous seeding from a primary infection located elsewhere in the body; S. aureus predominates in this situation. Late-onset PJI is less common and is often due to inoculation with relatively avirulent microorganisms peri-surgically

Fig. 3

A comparative heatmap of data compiled from the literature showing the various gram-positive and gram-negative microorganisms associated with PJI and the efficacy of D-AAs to inhibit pathogenic bacterial adhesion to an abiotic surface. Based on the D-AAs examined thus far, studies suggest a beneficial role. There has been a dominant focus on exploring the effects of D-AAs on glass or polypropylene surfaces. Future studies that examine the response of pathogenic bacteria to D-AAs when exposed to orthopedic-related materials, e.g., medical grade titanium alloy, ceramics, polyetheretherketone, and highly crosslinked polyethylene, are warranted. Furthermore, to date, few studies have investigated the effect of D-AAs on many of the gram-positive species associated with PJI

Fig. 4

A comparative heatmap of data compiled from the literature showing the various gram-positive and gram-negative microorganisms associated with PJI and the efficacy of D-AAs to inhibit pathogenic biofilm formation and/or augment the disassembly of mature biofilms. Variability in the response to D-AAs for both gram-positive and gram-negative species was found. For example, all studies reported thus far have shown D-AA efficacy against A. baumannii biofilms; however, many of the D-AAs investigated against S. mutans were reported to be ineffective

Fig. 5

An example schematic showing the life cycle of S. aureus biofilm formation as follows: (1) adhesion to an abiotic surface, (2) the development of a monolayer, (3) microcolony formation, (4) biofilm maturation, and (5) dispersal. Initial S. aureus adhesion is reversible via van der Waals forces, gravitational forces, surface electrostatic charge, hydrophobic interactions, Lewis acid–base interactions, and hydrogen bonding. S. aureus surface adhesion occurs via recognition of adhesive matrix molecules (MSCRAMMs), including FnBPa, FnBPB, and ClfA, by microbial surface compounds and via cell surface proteins, e.g., Bap, SasC, SasG, and Atl. D-AAs may be able to inhibit initial adhesion, reduce cell adhesion efficiency, and block bacterial attachment and growth from the foci of the monolayer. Robust aggregations composed of eDNA, amyloid fibers, polysaccharide intercellular adhesin/poly-ß (1-6)-N-acetylglucosamine (PIA/PNAG), polysoluble modulins (PSMs), and other proteins are formed. Some D-AAs may prevent this cell‒cell adhesion, leading to structural complications in the mature biofilm. During maturation, activated Agr-mediated quorum sensing (QS) initiates biofilm matrix modulation, and the EPS matrix is fully developed. QS can be activated either through PSM production or protease activation. PSMs maintain biofilm structure, and persister cells develop. D-AAs may interfere at this stage by incorporating into the peptidoglycan bond, inhibiting protein binding to the cell wall and disrupting cell‒cell and cell-surface interactions, thereby disassembling biofilm structure in areas of high concentration and/or preventing protein synthesis that is necessary for biofilm maintenance. This may enhance the effects of antibiotics. During dispersal, Agr-mediated QS initiates the dispersal of a segment of biofilm cells. This action is dependent on cell density signal molecules, namely, autoinducers. Autoinducing peptide (AIP) binds and activates histidine kinase (AgrC), which in turn phosphorylates AgrA. AgrA activates the transcription and production of a regulatory RNA molecule that impacts cell‒cell adhesion. This involves the release of PSMs, proteases and nucleases that aid dispersion. When the segment of cells becomes detached from the biofilm, they become planktonic and repeat the cycle, thereby infecting distant sites. During this phase, D-AAs may be capable of decreasing the metabolic activity and growth of planktonic cells

Fig. 6

A schematic showing contemporary understanding of the role of exogenously applied D-AAs in regulating the bacterial species (e.g., S. aureus) associated with PJI in osteocompetent cells and bone tissue. D-AAs are not bactericidal but prevent pathogenic adhesion to surfaces and host cells, spore germination, and biofilm formation and are able to disassemble established biofilms. The mechanism(s) for this remains unknown. One route in which D-AAs may prevent bacterial adhesion is by reducing the number of hydrogen bonds (H-bonds) and thus adhesive forces to the surface. Furthermore, the mechanism of D-AA incorporation into the peptidoglycan bond may alter cell wall chemistry, density, thickness, and strength, thereby disrupting surface protein numbers and locations. This in turn may disrupt cell‒cell and cell-surface adhesion to abiotic surfaces and to eukaryotic host cells. The impairment of surface proteins may also decrease cell interconnectivity, thereby preventing biofilm assembly while also promoting biofilm disassembly. D-AAs have been shown to be nontoxic to host cells at lower concentrations that are able to regulate pathogenic activity. In terms of bone, D-AAs directly regulate collagen type I, osteocalcin, alkaline phosphatase (ALP), osteoblasts, and osteoclast activity in vitro. In vivo studies have reported a beneficial response to D-AAs in terms of bone volume, density, and architecture