GRAM NEGATIVE BACTERIA: CHARACTERISTICS, STRUCTURE, DISEASES - BIOLOGY - 2024

The Gram negative bacteria are bacteria that do not retain crystal violet dye when Gram stain is used. Because of this, they stain red or faint pink, rather than blue or purple as Gram-positive bacteria do. The color acquired by Gram negative ones is due to another dye that can be fuchsin or safranin.

The difference in coloration is related to the nature of the cell wall in both groups of organisms. During staining, crystal violet binds to the bacterial cell wall and also forms a water-insoluble complex with the iodine contained in lugol (another compound used in Gram staining).

Pink Gram negative bacteria. Taken and edited from: Microrao.

The cell wall of Gram positive bacteria is thick and complex in nature, being able to retain the complex formed by crystal violet and iodine, when treated with alcohol or acetone. The cell wall of Gram negative bacteria is simpler and does not retain crystal violet.

These bacteria are ubiquitous and are present in practically every environment on earth capable of supporting life. The group contains pathogenic species for humans that are associated with urinary, respiratory and gastrointestinal infections; some of which are acquired in the same health centers (nosocomial).

General characteristics

The main characteristic of this group of bacteria is that they stain red or pink with the Gram stain technique. During this differential staining technique, all bacteria stain blue or violet, but only Gram positive ones are able to retain such staining after being treated with alcohol or acetone.

These bacteria have two lipid membranes, one of them represented by the cytoplasmic membrane and an additional one external to the cell wall. It occurs exclusively in Gram negative ones and is generally composed of phospholipids and lipopolysaccharides.

The lipid component of the outer membrane lipopolysaccharides is known as lipid A and acts as an endotoxin that, upon entering the circulatory system, causes a toxic reaction that can even trigger endotoxic shock.

The cell wall is composed of peptidoglycans forming a thin layer, and lacks teichoic and lipoteichoic acids, contrary to what happens in Gram-positive ones, which are composed of several layers of peptidoglycans and present both types of acids. These serve to anchor the nuclear envelope to the cytoplasmic membrane.

The flagella of Gram negative bacteria, when present, have four supporting rings (two for each membrane), while in Gram positive bacteria they have only two supporting rings, since they only have one membrane.

Structure

Average prokaryotic cell.

All bacteria lack a nucleus and the chromosomal material is not separated from the cytoplasm by a nuclear membrane. Ribosomes are also embedded in the cytoplasm.

A Gram negative bacterium consists of two cell membranes and a cell wall in between. Between the cell wall and the inner membrane is the periplasm. In addition to this, it can sometimes present flagella, fimbriae, capsule and very exceptionally spores.

Cellular wall

The cell wall is mainly composed of peptidoglycan, a polymer made up of repeating units of N-acetyl Glucosamine and N-acetylmuramic acid that form a thin layer.

The outer membrane is tightly bound to peptidoglycan and is part of the cell wall of Gram negative bacteria.

Outer membrane

The outer membrane contains phospholipids, lipopolysaccharides, and membrane proteins. Lipopolysaccharides are made up of lipid A (lipid portion), polysaccharide O and, between them, another chain of polysaccharides. The lipid A is immersed in the membrane and the polysaccharide O in the external face.

Lipipopolysaccharide (LPS) is an endotoxin, whose toxic potential is due to lipid A. As this lipid is immersed in the membrane, its toxic effect appears only when the bacterial cell is lysed.

Periplasm

A gelatinous substance made up of water and high concentrations of proteins and peptidoglycan. Of great importance in the energy metabolism of Gram negative bacteria, it is located between the cell wall and the cytoplasmic membrane (periplasmic space).

Microscope image (X1000 magnification) of Gram-positive (Staphylococcus aureus, in purple) and Gram-negative (Escherichia coli, in pink) bacteria. Taken and edited from: Y tambe.

Cellular membrane

The cell membrane is the structure that delimits the intracellular space from the external environment. It is a semi-permeable membrane, of about 8 nm, lipid in nature, similar in constitution to that of other bacteria and that differs from the eukaryotic membrane because the latter has sterols, while the bacterial one does not.

Cytoplasm

It is the cell content delimited by the cytoplasmic membrane and containing ribosomes, nucleoids and various macromolecules in suspension.

Ribosomes

In Gram-negative bacteria, ribosomes can occur individually in the cytoplasm, in groups or in chains associated with messenger RNA, called polyribosomes.

Bacterial DNA

As in all bacteria, bacterial DNA is not contained within a nucleus. This genetic material is made up of a double strand of DNA in a circular shape.

Fimbriae or pilis

The fimbriae or pilis are filiform structures that are mainly distributed on the external surface of Gram negative bacteria. Despite their similarity to flagella, they are not used for displacement but for other functions such as adhering to host tissues or for reproduction.

Capsule

The capsule is a structure that can be present in any type of bacteria. It is not a vital structure, since its loss does not affect the viability of the bacteria, but it is important because it protects the organism from phagocytosis. When present, it is found external to the cell wall.

It is generally made up of mucous-like polysaccharides. It constitutes a virulence factor for some bacteria.

Spores

These structures are found almost exclusively in Gram-positive rods, except for a single Gram-negative species (Coxiella burnetti) that also has them. They are formed inside the body, which is why they are called endospores and are highly resistant structures to adverse environmental factors, as well as radiation and chemical disinfectants.

Flagellum

The bacterial flagellum is a whip-shaped structure which is used for movement. Their number and arrangement on the bacterial surface varies according to the species.

Diseases

Gram negative bacteria, especially Enterobacteriaceae and non-fermentative bacteria, are of great medical importance as they are the causative agents of a wide variety of infectious diseases, among which we can mention:

Urinary tract infections

Gram-negative bacterial urinary tract infections can be divided into two types: endogenous and exogenous. Endogenous infections occur when bacteria colonize the ascending gastrointestinal tract through the urinary tract. Exogenous ones generally occur due to the use of contaminated instruments during medical care.

These diseases can present local symptoms, as in the case of cystitis and prostatitis or, on the contrary, they can present systemic symptoms which include fever and sepsis, as can occur in pyelonephritis or kidney abscess.

Intra-abdominal infections

Many Gram negative bacteria are normal members of the gastrointestinal bacterial flora, without causing any type of disease, however they are opportunistic pathogens that can cause infections if there are lesions in the digestive tract.

These infections can be local and limited to the walls of the gastrointestinal tract, as in diverticulitis or cholecystitis, or they can spread into the abdominal cavity causing peritonitis.

The peritoneum is a membrane that covers all the coelomic viscera, so an infection of this membrane can quickly spread to all organs. Normally when intra-abdominal infection occurs, especially if there are abscesses, antibiotic treatment is insufficient and surgical options must be used.

Respiratory tract infections

This type of infection is associated with high morbidity and mortality. It generally occurs in hospital patients or those who have been subjected to antibiotic treatments. Respiratory diseases caused by Gram-negative bacteria include tracheobronchitis and pneumonia.

Other diseases

Gram-negative bacteria are also associated with a wide variety of other diseases, including gonorrhea, bacterial meningitis, as well as most hospital-acquired infections.

Examples

Klebsiella pneumoniae

Klebsiella pneumoniae is a rod-shaped bacterium, which can be found individually, in pairs, in chains, or in groups. It does not have a flagellum or sporula, but it does present a well-developed capsule.

This bacterium is an opportunistic pathogen associated with various hospital infections, which can even cause death. In recent years, hypervirulent strains that are difficult to treat and associated with significant morbidities and mortalities have appeared.

Neisseria gonorrhoeae

This bacterium is a diplococcus with the adjacent sides of the cocci flattened and the major axis of each cell perpendicular to the major axis of the pair and generally has capsule and pili on its surface. It is normally found within polymorphonuclear lymphocytes. It is oxidase negative and catalase positive.

Neisseria gonorrhea is responsible for the disease known as gonorrhea or blenorrhea, which is a sexually transmitted disease.

Morganella morganii

Morganella morganii is an oxidase negative, facultative anaerobic bacterium, which is shaped like a straight rod, flagellated at temperatures below 30 ° C, but flagellated at temperatures above this. It is a common member of the intestinal tract of humans and other species.

Despite being normally found as a commensal of the digestive tract without causing disease, it is an opportunistic pathogen that can cause urinary tract infections, septicemia, bacteremia, diarrhea, pneumonia, spontaneous bacterial peritonitis or Ludwig's angina, among other diseases.

References

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