Bacterial Pathogens

Bacterial Classification

Bacteria are classified within two major groups, Gram-positive or Gram-negative, based on their cell wall composition. Peptidoglycan is a main component of the cell wall that surrounds the cell membrane, provides protection from osmotic changes, and determines the bacterial cell shape. The rigid mesh-like peptidoglycan layer consists of two types of glycans, N-acetylglucosamine and N-acetylmuramic acid, connected by short peptides. In Gram-positive bacteria the peptidoglycan mesh is wider, consisting of multiple layers and associated teichoic acids, polysaccharides, and peptidoglycolipids. In contrast, Gram-negative bacteria have a thin cell wall with less peptidoglycan content and cross-linking. Gram-negative bacteria are additionally surrounded by an outer membrane containing lipopolysacharides (LPS) and lipoproteins.

Mycoplasma and Mycobacterium: How are they different?

Mycoplasma species are Gram-negative bacteria which lack a cell wall. The Mycoplasma genus contains over a 100 species, some of which lead to human disease including Mycoplasma pneumoniae and Mycoplasma genitalium. In contrast, Mycobaterium species are characterized by a complex cell wall, consisting of a mycolyl-arabinoglactan-peptidoglycan complex, which sets them apart from other Gram positive bacteria. The presence of an outer membrane “myco-membrane” is a feature that Mycobacterium species share with Gram negative bacteria. Close to 200 Mycobacterium species have been identified including important human pathogens such as Mycobacterium tuberculosis and Mycobacterium leprae.

Fluorescent Probes to Image Gram-Positive and Gram-Negative Bacteria

Incorporation of fluorescently-labeled metabolic precursors by live bacteria results in the labeling of specific cell wall and envelope components such as peptidoglycans and trehalose glycolipids, respectively. In metabolic labeling, fluorescent reporters target specific cell surface structures allowing investigators to study bacterial growth and responses to pharmacological agents. Combined with super-resolution microscopy, fluorescent metabolic labeling enables the analysis of cell wall and envelope dynamics.

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Antibiotics

Treatment of bacterial infections relies primarily on the use of antibiotics, which are chemicals that inhibit bacterial growth or kill bacteria. Based on their main mechanism of action, antibiotics are classified as bacteriostatic or bactericidal, depending on whether they stop bacterial growth or induce cell death, respectively. Beyond these major mechanisms, bactericidal antibiotics may induce cell death by targeting and inhibiting the synthesis of DNA, RNA, protein or cell wall components.

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Antibiotic Resistance

Development of antibiotic resistant bacteria is currently a major public health concern. Antibiotic resistance mechanisms are diverse and encoded by bacterial genomic programs and extra-chromosomal or plasmid DNA. Several in vitro methods allow assessment of bacterial resistance to antibiotics including:

Disc diffusion testing- Involves culture of bacterial specimens on agar plates, followed by exposure to antibiotics through placement of discs containing antibiotic drug. Inhibition of growth around the antibiotic containing disc is used as a measure of antibiotic susceptibility.

Minimum inhibitory concentration- Involves inoculation of bacteria in serial dilutions of an antibiotic, typically broth, to determine the minimal antibiotic concentration which inhibits visible bacterial growth. This is based on the evaluation of bacterial growth following overnight culture.

Mechanisms of Antibiotic Resistance

Bacterial antibiotic resistance may be acquired by spontaneous genetic mutations or transfer of genetic material from other bacteria or bacteriophages. Various mechanisms of antibiotic resistance have been identified and bacteria may exhibit a single or multiple antibiotic resistance traits.

Major Bacterial Human Pathogens

In 2017 WHO published a list of "priority pathogens" which includes twelve bacterial families with increased pathogenic potential to humans due to their increased incidence of antibiotic resistance. The list classifies pathogens as critical, high and medium based on the global need for the development of new antibiotics.

*From WHO priority pathogens list for research and development of new antibiotics.

Helicobacter pylori binds to the gastric epithelium inducing inflammatory responses. Urase produced by H. pylori supports bacterial colonization and survival within the gastric mucosa. Urease also functions as a virulence factor and is implicated in the inflammatory process through various mechanisms (e.g., activation of CD74 in gastric epithelium and induction of IL8 production). 

Anti-virulence Strategies to Target Multi-Drug Resistant Pathogens

Quorum sensing (QS) signaling is a mechanism of bacterial communication, which allows regulation of virulence factors, biofilm formation, colonization, and environmental adaptation. Bacterial communication is underscored by the release and extracellular accumulation of autoinducer molecules or AIs (e.g., autoinducer-2, acylated homoserine lactones (acyl-HSLs), oligopeptides, Pseudomonas quinolone signal molecule, diffusible signal factor, γ-butyrolactone, and 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde). Increase of extracellular AI concentration is dependent on bacterial density and leads to the activation of signaling pathways which regulate the expression of bacterial virulence genes. Targeting QS signaling has been proposed as a potential anti-bacterial strategy which may circumvent the development of bacterial resistance. Various strategies to inhibit QS depend on inhibition of AI expression or synthesis, inhibition of AI extracellular accumulation, and blockade of AI detection.

View our Quorum Sensing Signaling modulators.