Gram negative bacterium

Gram-negative bacterium

Gram negative bacterium Resources
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Gram-negative bacteria are a group of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell wall structure, which is more complex than that of Gram-positive bacteria.

Cell Wall Structure

The cell wall of Gram-negative bacteria is composed of a thin layer of peptidoglycan surrounded by an outer membrane. This outer membrane contains lipopolysaccharides (LPS), which are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core, and inner core joined by a covalent bond.

Peptidoglycan Layer

The peptidoglycan layer in Gram-negative bacteria is much thinner than in Gram-positive bacteria, typically only 1-2 nanometers thick. This thin layer is located between the inner cytoplasmic membrane and the outer membrane.

Outer Membrane

The outer membrane of Gram-negative bacteria is unique and serves as a protective barrier. It is asymmetrical, with the inner leaflet composed of phospholipids and the outer leaflet containing lipopolysaccharides. The presence of LPS is a distinguishing feature of Gram-negative bacteria and contributes to their pathogenicity.

Lipopolysaccharides (LPS)

Lipopolysaccharides are endotoxins and play a crucial role in the immune response. They are recognized by the immune system and can trigger a strong immune response. The LPS molecule is divided into three parts:

• Lipid A: This is the lipid component and is responsible for the toxic effects of LPS.
• Core oligosaccharide: This is attached to Lipid A and is composed of sugars.
• O-antigen: This is a polysaccharide chain that extends outward from the core oligosaccharide and is highly variable among different species.

Pathogenicity

Many Gram-negative bacteria are pathogenic and can cause diseases in humans and animals. Some well-known Gram-negative pathogens include:

• Escherichia coli
• Salmonella
• Neisseria gonorrhoeae
• Pseudomonas aeruginosa

These bacteria can cause a range of infections, from urinary tract infections to more severe diseases like sepsis.

Antibiotic Resistance

Gram-negative bacteria are often more resistant to antibiotics than Gram-positive bacteria. This resistance is due to the outer membrane, which acts as a barrier to many antibiotics, and the presence of efflux pumps that expel antibiotics from the cell. Additionally, Gram-negative bacteria can acquire resistance genes through horizontal gene transfer.

Clinical Implications

Infections caused by Gram-negative bacteria can be challenging to treat due to their resistance mechanisms. It is crucial to identify the specific type of Gram-negative bacterium causing an infection to select the appropriate antibiotic therapy.

Laboratory Identification

Gram-negative bacteria can be identified in the laboratory using the Gram stain technique, which will show them as pink or red under a microscope. Further identification can be done using biochemical tests, molecular methods, and susceptibility testing.

See Also

• Gram-positive bacteria
• Antibiotic resistance
• Bacterial cell wall

External Links

• Centers for Disease Control and Prevention
• World Health Organization