Lipid Fingerprints

BACLIB determines microbial species and antimicrobial resistance by measuring the lipids in microbial membranes. Each microbial species has a unique “fingerprint” pattern of membrane lipids. BACLIB detects a microbial species in a sample by detecting the unique lipid fingerprint of that species.

Evolutionary pressure for unique lipid fingerprints

Bacteria and fungi membranes are specialized for the organism’s ecological niche. Pathogens specialize their membranes to manipulate the host immune system and exploit the host. Importantly, pathogens gain an advantage by evading host immune responses that target a competitive pathogen. Consequently, pathogenic microbes exhibit unique lipid fingerprints, as has been demonstrated for a wide variety of microbes, including the most important human pathogens.

The membrane of Gram-negative bacteria is shown schematically below. Lipopolysaccharide (LPS) molecules are attached to the outer membrane. LPS consists of O antigen, core, and lipid A. O antigen is often highly variable, which aids bacteria in evading host detection. However, lipid A expression is unique to each Gram-negative species. BACLIB fingerprints for a Gram-negative bacterial species essentially comprise the lipid A expression for that species.

Schematic of the Gram-negative bacterial membrane: LPS molecules (O antigen, core and lipid A) on the outer membrane, then peptidoglycan and the inner membrane — **Fig. 1**Gram-negative bacterial membrane

Gram-positive bacteria and fungi express different membrane lipids from Gram-negative bacteria, causing the BACLIB fingerprints of Gram-negative, Gram-positive, and fungal pathogens to be highly distinct. The same evolutionary pressures cause all three groups of pathogens to have BACLIB fingerprints that are unique per species.

Lipid A is species-specific

Over the last fifteen years, lipid A has been studied for a wide variety of Gram-negative bacteria. Each species exhibits a unique lipid A pattern. Individual species typically have a characteristic lipid A molecule that is unique to that species. Below are characteristic lipid A molecules for three common Gram-negative pathogens.

Structure of a characteristic Escherichia coli lipid A molecule — **Fig. 2**Characteristic lipid A molecules of three common Gram-negative pathogens.

Structure of a characteristic Pseudomonas aeruginosa lipid A molecule — **Fig. 2**Characteristic lipid A molecules of three common Gram-negative pathogens.

Gram-positive bacteria have similar species-specific membrane patterns of lipoteichoic acid and cardiolipins. Fungi express species-specific sphingolipids and sterols in their membranes. Because BACLIB detects all of these membrane lipids, BACLIB identifies bacteria and fungi with one process.

Four species, four fingerprints

m/z1796

Escherichia coli

Gram-negative bacterium · lipid A

Gram-negativelipid A
Gram-positivecardiolipins and lipoteichoic acid
Fungisphingolipids and sterols

Fig. 3Schematic; peak positions from Sorensen et al., Sci Rep 2020. Heights are rough readings of the published figure, and minor peaks are left out. Select a species and a peak.