Gram-Negative Bacteria

Gram-negative bacteria are classified as such due to their appearance after a gram stain test. They have certain physical properties, that while invisible to the naked eye, allow them to pick up particular dyes in gram staining, which belie the characteristics of their cell wall and other structures. We will examine these properties of gram-negative bacteria and, by doing so, see the differences that distinguish gram-negative bacteria from gram-positive bacteria.

Diagram of gram-negative bacteria

The contents of the cell are contained in the plasma membrane. Outside of this is the periplasmic space. Beyond that is the peptidoglycan cell wall, and lastly we have the outer membrane. We will go over each layer and its specific contents one by one, below.

Overview of gram-negative bacteria structure

Plasma membrane

• The plasma membrane surrounds the bacterial cell. Encased within its boundaries are the ribosomes, cell nucleus, mitochondria, and more.
  • The plasma membrane consists of a phospholipid bilayer.
    • The phospholipid bilayer of a gram-negative bacterial cell is like most other phospholipid bilayers, including the ones present in our cells. Phospholipids are composed of a phosphate head attached to two fatty acid tails. The tails of the top layer will intermingle with the tails of the bottom layer, and both heads will face outward towards the intracellular fluid and the extracellular fluid. Phosphate heads are hydrophilic, and fatty acid tails are hydrophobic, which facilitates this conformation (Fig. 2).

• Strewn throughout the plasma membrane are proteins.
  • Peripheral proteins are those that only rest on the inner or outer surface of the membrane bilayer.
  • Integral proteins are those that integrate into at least partial thickness of the phospholipid bilayer.
  • Transmembrane proteins are a specific type of integral protein; one that spans the full length of the phospholipid bilayer and is present on both the inner and outer surface.

Periplasmic space

• The periplasmic space is present between the inner and outer membranes of gram-negative bacteria. It presents a significant portion of the space that gram-negative bacteria occupy.

  • The periplasmic space is an environment for enzymatic activity, including:

    • Protein folding

    • Production of substances for secretion by the bacterium

• The peptidoglycan cell wall is present within the periplasmic space, but we will discuss this in its own section, below.

Peptidoglycan cell wall

• The peptidoglycan cell wall in gram-negative bacteria is quite thin, much thinner than in gram-positive bacteria. This is especially relevant with gram staining.

• Atop the peptidoglycan layer are lipoproteins called "Braun's Lipoproteins", which help attach the peptidoglycan cell wall to the outer membrane beyond it.

Outer membrane

• The outer membrane of gram-negative bacteria is made up of another lipid bilayer, uniquely present outside of its cell wall.

• Different proteins may be present in this lipid bilayer

  • The transmembrane proteins, peripheral proteins, and integral proteins can be seen as a part of the cytoplasmic membrane bilayers and may be seen here in the outer membrane again.

  • Porins - channels made up of protein subunits that facilitate the movement of substances in and out of the cell.

    • These porins act, as you could guess, as pores. When closed, they reduce permeability and, when open, increase permeability.

• Lipopolysaccharides are present as the last and most important constituent of gram-negative bacterial cell membranes (Fig. 3).

  • There are three parts of lipopolysaccharide molecules (also called LPS).

    • Two of those parts are made up of carbohydrates, and one is made up of lipids.

  • The innermost part of LPS, which intercalates within the outer leaflet of the outer membrane, is called lipid A

  • The outermost part of LPS is called the O antigen or the O-polysaccharide.

  • Between lipid A and the O-polysaccharide is the core polysaccharide.

Properties of gram-negative bacteria cell wall

What are the properties that arise from each feature of a gram-negative bacterial cell, its cell wall, and its membranes? We will examine them one by one.

• Gram-negative bacteria get their virulence from their lipopolysaccharide molecules. Particularly, the lipid A moiety is the most capable of causing immune reactions in humans.

• Gram-negative bacteria have a large periplasmic space and relatively many transport proteins and porins, that allow for the production of toxins and their transport outside the cell. These pores and channels also allow for nutrients to be brought into the cell.

• A thin peptidoglycan cell wall facilitates the color change from purple to pink, seen during gram staining. The first step of gram staining - primary stain - involves dying the cell purple. The second step - fixing - helps to fix this color to the cell's wall. However, because of the thin, highly porous cell wall in gram-negative bacteria, alongside the lipid-rich outer membrane that is easily lyzed by alcohol, the third step - decolorizing - easily washes out the purple stain and leaves the gram-negative bacterial cell colorless. Thus, in the fourth and final step - counterstaining with safranin - the gram-negative bacteria are free to pick up the pink color of the second dye. Hence, gram-negative bacteria are pink after gram staining.

Gram-negative bacteria examples

What are some examples of gram-negative bacteria? We can group them according to their morphology. Cocci are spherical or circular, bacilli are rod-shaped, and coccobacilli are some intermediate of the two. We will list them by genus.

Cocci

• Neisseria - can cause STDs and meningitis.
• Moraxella - causes the common cold.

Bacilli

• Salmonella - can cause food poisoning.
• Pseudomonas - can cause many different skin, ear, and lung infections.
• E. coli - mostly causes vomiting and diarrhea, can be more severe in young children and infants.

Spiral shaped

• Spirillum
• Helicobacter - causes stomach ulcers.
• Campylobacter - causes food poisoning.

Coccobacilli

• Bordatella - causes pertussis, a respiratory illness.
• Haemophilus - can cause pneumonia and other respiratory illnesses.
• Brucella

Difference between gram-negative and gram-positive bacteria

Gram-negative bacteria and gram-positive bacteria have quite a few differences in structure. These differences help decide what organisms, and what organs within those organisms, they can infect. These differences also help define their classification, which is important both when diagnosing and studying these pathogens. It can even help medical doctors decide what kind of antibiotic or treatment is preferable, as the activity of certain medications are stronger in gram-negative vs gram-positive bacteria, and vice versa. We will outline the differences between gram-positive and gram-negative bacteria below.

Difference #1: Gram-negative bacteria have a relatively quite thin peptidoglycan cell wall. Gram-positive bacteria, on the other hand, have a large, thick peptidoglycan cell wall.

Difference #2: Gram-negative bacteria lack both lipoteichoic acids and teichoic acids. Gram-positive bacteria can, and frequently do, have both.

Difference #3: Gram-negative bacteria have lipopolysaccharides on the outer leaflet of their outer membrane, beyond their cell walls. Gram-positive bacteria do not have an outer leaflet or lipopolysaccharides, their best version of highly virulent agents are their lipoteichoic acids.

Difference #4: Gram-negative bacteria have an outer membrane. Gram-positive bacteria, on the other hand, do not. They have only a single cell membrane, beneath their cell wall. This is the standard cytoplasmic membrane that gram-negative bacteria have in addition to their outer membrane.

Difference #5: Gram-negative bacteria are defined by the pink color they assume after the process of gram staining is complete. Gram-positive bacteria are defined by their purple color.