There is growing evidence that the role of lipids in innate immunity is more important than previously realized. How lipids interact with bacteria to achieve a level of protection, however, is still poorly understood. To begin to address the mechanisms of antibacterial activity, we determined MICs and minimum bactericidal concentrations (MBCs) of lipids common to the skin and oral cavity—the sphingoid basesd-sphingosine, phytosphingosine, and dihydrosphingosine and the fatty acids sapienic acid and lauric acid—against four Gram-negative bacteria and seven Gram-positive bacteria. Exact Kruskal-Wallis tests of these values showed differences among lipid treatments (P< 0.0001) for each bacterial species exceptSerratia marcescensandPseudomonas aeruginosa.d-Sphingosine (MBC range, 0.3 to 19.6 μg/ml), dihydrosphingosine (MBC range, 0.6 to 39.1 μg/ml), and phytosphingosine (MBC range, 3.3 to 62.5 μg/ml) were active against all bacteria exceptS. marcescensandP. aeruginosa(MBC > 500 μg/ml). Sapienic acid (MBC range, 31.3 to 375.0 μg/ml) was active againstStreptococcus sanguinis,Streptococcus mitis, andFusobacterium nucleatumbut not active againstEscherichia coli,Staphylococcus aureus,S. marcescens,P. aeruginosa,Corynebacterium bovis,Corynebacterium striatum, andCorynebacterium jeikeium(MBC > 500 μg/ml). Lauric acid (MBC range, 6.8 to 375.0 μg/ml) was active against all bacteria exceptE. coli,S. marcescens, andP. aeruginosa(MBC > 500 μg/ml). Complete killing was achieved as early as 0.5 h for some lipids but took as long as 24 h for others. Hence, sphingoid bases and fatty acids have different antibacterial activities and may have potential for prophylactic or therapeutic intervention in infection.

Host-derived lipids are increasingly recognized as antimicrobial molecules that function in innate immune activities along with antimicrobial peptides. Sphingoid bases and fatty acids found on the skin, in saliva and other body fluids, and on all mucosal surfaces, including oral mucosa, exhibit antimicrobial activity against a variety of Gram positive and Gram negative bacteria, viruses, and fungi, and reduce inflammation in animal models. Multiple studies demonstrate that the antimicrobial activity of lipids is both specific and selective. There are indications that the site of action of antimicrobial fatty acids is the bacterial membrane, while the long-chain bases may inhibit cell wall synthesis as well as interacting with bacterial membranes. Research in this area, although still sporadic, has slowly increased in the last few decades; however, we still have much to learn about antimicrobial lipid mechanisms of activity and their potential use in novel drugs or topical treatments. One important potential benefit for the use of innate antimicrobial lipids (AMLs) as antimicrobial agents is the decreased likelihood side effects with treatment. Multiple studies report that endogenous AML treatments do not induce damage to cells or tissues, often decrease inflammation, and are active against biofilms. The present review summarizes the history of antimicrobial lipids from the skin surface, including both fatty acids and sphingoid bases, in multiple human body systems and summarizes their relative activity against various microorganisms. The range of antibacterial activities of lipids present at the skin surface and in saliva is presented. Some observations relevant to mechanisms of actions are discussed, but are largely still unknown. Multiple recent studies examine the therapeutic and prophylactic uses of AMLs. Although these lipids have been repeatedly demonstrated to act as innate effector molecules, they are not yet widely accepted as such. These compiled data further support fatty acid and sphingoid base inclusion as innate effector molecules.

MicroRNAs (miRs) regulate inflammation and BMP antagonists, thus they have potential uses as therapeutic reagents. However, the molecular function of miR-200c in modulating proinflammatory and bone metabolic mediators and osteogenic differentiation is not known. After miR-200c was transduced into a human embryonic palatal mesenchyme (HEPM) (a cell line of preosteoblasts), using lentiviral vectors, the resulting miR-200c overexpression increased osteogenic differentiation biomarkers, including osteocalcin (OCN) transcripts and calcium content. miR-200c expression also down-regulated interleukin (IL)-6, IL-8, and chemokine (C-C motif) ligand (CCL)-5 under lipopolysaccharide (LPS) stimulation and increased osteoprotegerin (OPG) in these cells. miR-200c directly regulates the expression of IL-6, IL-8 and CCL-5 transcripts by binding to their 3’UTRs. A plasmid-based miR-200c inhibitor effectively reduces their binding activities. Additionally, miR-200c delivered using polyethylenimine (PEI) nanoparticles effectively inhibits IL-6, IL-8 and CCL-5 in primary human periodontal ligament fibroblasts and increases the biomarkers of osteogenic differentiation in human bone marrow mesenchymal stem cells (MSCs), including calcium content, ALP, and Runx2. These data demonstrate that miR-200c represses IL-6, IL-8 and CCL-5 and improves osteogenic differentiation. miR-200c may potentially be used as an effective means to prevent periodontitis-associated bone loss by arresting inflammation and osteoclastogenesis and enhancing bone regeneration.

Synopsis: As one moves from the skin across the vermilion region of the lip and into the oral cavity, the oral mucosa is encountered. The oral mucosa consists of connective tissue known as the lamina propria covered by a stratified squamous epithelium. In the regions of the hard palate and gingiva, the epithelium is keratinized like the epidermis. In the buccal region, the floor of the mouth and the underside of the tongue, the epithelium is non‐keratinized. The epithelium on the dorsum of the tongue is a specialized epithelium, but can be approximated as a mosaic of keratinized and non‐keratinized epithelia. The non‐keratinized epithelial regions do not produce a stratum corneum. Nuclei with intact DNA are retained in the superficial cells. In all regions, the outer portions of the epithelium provide a protective permeability barrier, which varies regionally. Antimicrobial lipids at the surfaces of the oral mucosa are an integral part of innate immunity.

Skin is complex and comprised of distinct layers, each layer with unique architecture and immunologic functions. Cells within these layers produce differing amounts of antimicrobial peptides and lipids (sphingoid bases and sebaceous fatty acids) that limit colonization of commensal and opportunistic microorganisms. Furthermore, antimicrobial peptides and lipids have distinct, concentration-dependent ancillary innate and adaptive immune functions. At 0.1–2.0 µM, antimicrobial peptides induce cell migration and adaptive immune responses to coadministered antigens. At 2.0–6.0 µM, they induce cell proliferation and enhance wound healing. At 6.0–12.0 µM, they can regulate chemokine and cytokine production and at their highest concentrations of 15.0–30.0 µM, antimicrobial peptides can be cytotoxic. At 1–100 nM, lipids enhance cell migration induced by chemokines, suppress apoptosis, and optimize T cell cytotoxicity, and at 0.3–1.0 µM they inhibit cell migration and attenuate chemokine and pro-inflammatory cytokine responses. Recently, many antimicrobial peptides and lipids at 0.1–2.0 µM have been found to attenuate the production of chemokines and pro-inflammatory cytokines to microbial antigens. Together, both the antimicrobial and the anti-inflammatory activities of these peptides and lipids may serve to create a strong, overlapping immunologic barrier that not only controls the concentrations of cutaneous commensal flora but also the extent to which they induce a localized inflammatory response.