**1. Introduction**

The increasing occurrence of infectious disease is a global issue. Emerging pathogens with increasing levels of drug resistance are a continuing danger to both public health and agricul‐ ture. Accurate and rapid detection of pathogens is critical to implement preventative measures to mitigate this problem. Despite this urgent need, conventional methods for bacterial detection require cell culture and serology, which can take several weeks. As new pathogens emerge, it is even more important that our detection technologies evolve to keep pace with the need to

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discriminate pathogen from host flora. This requires an understanding of pathogen biology, the types of samples they occur in, and their mechanism of immune interaction within the hosts [1].

Herein, we present a comprehensive description of the structural and biochemical properties of LPS, current methods for its detection, and potential approaches to overcome the current

Lipopolysaccharides have been the subject of intense study for over half a century [37–39]. LPS is the prototypical lipoglycan with an overall net negative charge [40–42], and is the primary component of the outer membrane of nearly all Gram‐negative bacteria [11]. The

 glycerophospholipid molecules, comprising approximately three‐quarters of the outer membrane [43–45]. Thus, there are approximately 62 pg of LPS per cell (for *E*. *coli* in log phase growth) [46]. LPS has an amphipathic tripartite structure (**Figure 1**). Lipid A is the most conserved portion of the LPS molecule, and consists of six, sometimes seven, fatty acid tails (*E*. *coli* and *Salmonella*, respectively), which gives the molecule its hydrophobic properties [10, 43, 45]. Lipid A is also called endotoxin [43], and is responsible for the biological effects of LPS

**Figure 1.** Representative structure of the molecular components of smooth LPS. The hypervariable O‐polysaccharide

antigen, core polysaccharide, and the hydrophobic lipid A group. Reprinted with permission from Ref. [74].

lipid A moieties and

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Detection Methods for Lipopolysaccharides: Past and Present

http://dx.doi.org/10.5772/intechopen.68311

limitations for direct detection of the molecule in physiological matrices.

bacterial membrane of each *E*. *coli* cell is composed of approximately 106

**3. Lipopolysaccharide structures and conformations**

107

The innate immune system is able to discriminate pathogens from nonpathogens, and rap‐ idly sense pathogen biomarkers in the complex milieu of the host. Exploiting this recognition via measurement of pathogen signatures, can provide an optimal strategy for discriminatory biodetection. A primary category of such biomarkers is virulence signatures termed pathogen‐ associated molecular patterns (PAMPs) [2]. PAMPs are evolutionarily conserved molecules that bind pattern‐recognition receptors in the host, and activate the innate immune response [2, 3], providing a means for both early and specific pathogen detection. Biochemically, PAMPs are a diverse array of proteins, lipopeptides, lipoglycans, peptidoglycans, teichoic acids, and nucleic acids [4]. However, many detection methods have largely focused on proteins and nucleic acids [1, 5], ignoring other categories of PAMPs [2, 6–8]. Also, their small size, biochem‐ istry, and low concentration in hosts make them difficult to target in detection assays [8, 9].

Classified as a lipogylcan, lipopolysaccharides (LPS) are small amphiphilic molecules that are associated with Gram‐negative bacteria [7, 10]. LPS is an indicator of active infection, is sero‐ group‐specific [11–13], more stable than its protein counterparts, and is released early in infec‐ tion, making it an ideal candidate for detection and diagnostics. LPS serves as a biomarker that aids in serological discrimination of Gram‐negative bacteria; this allows for identification and characterization of pathotypes that are essential for timely mitigation and treatment of infec‐ tions. Since LPS is a pathogen‐specific biomarker, it is an indicator of acute infection, which is an advantage over serological assays. In addition to medical diagnostics, LPS detection pro‐ vides a method for detecting *Escherichia coli* in the food‐industry, which is often associated with food‐borne illnesses. Finally, LPS is also a virulence factor whose structure and function deter‐ mines *E*. *coli* serogroup, a factor which has ramifications on vaccine design and therapeutic interventions. While many methods for LPS detection exist, most of them are not optimized for amphiphilic detection in physiological samples. An ideal measurement for LPS should be sen‐ sitive enough to detect low concentrations of the amphiphile in aqueous physiological milieu (e.g., blood), and use antibodies or ligands that provide serogroup selectivity [14]. Coupling sensitive detection platforms with surfaces designed to maximize the binding of amphiphilic PAMPs is a potential solution to achieve such an ideal.

### **2. Sources of lipopolysaccharides**

Bacteria are classified into Gram‐negative and Gram‐positive [15], which release amphiphi‐ lic virulence factors such as LPS, lipoarabinomannan (LAM), and lipoteichoic acid (LTA) in the host. Species of pathogenic Gram‐negative bacteria of concern to human health, include *Acinetobacter* [16], *Burkholderia* [17], *Bordetella* [18], *Campylobacter* [19–21], *Chlamydia* [22, 23], *E*. *coli* [20, 24], *Helicobacter* [25, 26], *Hemophilius* [27], *Klebsiella* [28], *Legionella* [20, 29], *Moraxella* [30], *Neisseria* [31], *Pseudomonas* [32], *Proteus* [33], *Salmonella* [20, 34], *Shigella* [35], *Yersinia* [36], and others, grouped into the Enterobacteriaceae family. These pathogens are contaminants in food, water, and soil, used as agents of bioterrorism, and can cause nosocomial infections [5]. Detection of these organisms, particularly *E*. *coli*, is an important aspect for epidemiology, disease control, and treatment.

Herein, we present a comprehensive description of the structural and biochemical properties of LPS, current methods for its detection, and potential approaches to overcome the current limitations for direct detection of the molecule in physiological matrices.
