GRAM NEGATIVE BACILLI: CHARACTERISTICS, STRUCTURE, TYPES - BIOLOGY - 2025

The Gram negative bacteria (GNB) are rod-shaped bacteria that give negative Gram stain. This is due to the biochemistry of its cell wall. The term bacteria refers to all cells with prokaryotic characteristics and that belong to the Bacteria domain, also called Eubacteria.

Prokaryotes are characterized by the absence of nuclei and membrane-enclosed compartments. This characteristic is also possessed by members of the Archaeobacteria domain, in which there are also Gram-negative species. The cell wall of the members of both domains and other characteristics are different.

Source: Photo by Eric Erbe, digital colorization by Christopher Pooley, both of USDA, ARS, EMU.

General characteristics

The characteristics shared by species that are Gram negative bacilli are: 1) they are rod-shaped; 2) are negative to Gram stain, so they appear red. The latter is produced by the structure of the cell wall, which is different from that of Gram positive bacteria.

Gram negative rods differ greatly from one another in terms of mobility (flagella mobile; sliding mobile; nonmobile), nutrition, and physiology (phototrophic, chemoorganotrophic, ability to use oxygen, etc.), among other factors. Therefore, it is easier to specify the general characteristics of Eubacteria and Archaea.

They measure on average 3 µm long and 1 µm wide. They can be bigger or smaller. They lack a membrane that surrounds DNA, and organelles such as mitochondria and chloroplasts.

They have a cell wall that surrounds the plasma membrane. They have only one circular chromosome and plasmids. DNA lacks introns.

Some members of the Eubacteria are pathogens for animals and plants, while members of the Archaeobacteria are not pathogens.

Eubacteria can be sensitive to antibiotics (penicillin, kanamycin, streptomycin, etc.), fix nitrogen, form capsules, have chlorophyll-dependent photosynthesis, do not have a photosystem with rhodopsin, and be non-methanogenic. Archaeobacteria are the opposite.

Structure of the cell wall Gram-negative bacteria

All Gram-negative bacteria, regardless of their cell shape, share the same structural characteristics of the cell wall.

The cell wall of Gram negative bacteria is a multi-layered structure, which is composed of peptidoglycan. They have an outer membrane that surrounds the peptidoglycan. Between the outer membrane and the cell wall, there is a narrow space called the periplasmic space.

The cell wall measures 30 Å. It is composed of peptidoglycan (murein), which is made up of N-acetylglucosamine (gluNAC) molecules that alternate with N-acetylmuramic acid (murNAc) molecules and form chains. Peptidoglycan makes up 10% of the wall, the rest is outer membrane.

The gluNAC and murNAc chains are crossed by tetrapeptides, chains of four amino acid residues. Often the third amino acid residue, in the tetrapeptide, is diaminopimelic acid. Two tetrapeptides form a covalent bond with each other and are directly connected to the gluNAC and murNAc chains.

The outer membrane is a lipid bilayer that is covalently attached to the peptidoglycan layer by lipoprotein molecules. This membrane has porins that form channels through the outer membrane.

Structure of the cell wall of Gram negative Archaea

From a structural and biochemical point of view, the cell envelope of Archaea differs greatly from that of Eubacteria. The cell wall of Archaea contains only 10% peptidoglycan. The outer membrane typically present in Gram negative bacteria is absent in Archaea.

In Gram negative Archaea there is an S-layer that surrounds the plasma membrane. While Gram positive Archaea there is an envelope surrounding the S-layer.

In species of both domains, Eubacteria and Archaea, the S-layer is composed of glycoproteins, which are proteins linked, by covalent bonds, to carbohydrates. The latter are repetitive subunits that can be linear or branched, constituting from 1% to 20% of the total mass of glycoproteins.

Glycoproteins are rich (between 40% and 50%) in hydrophobic amino acid residues. Its cysteine ​​and methionine content is low. They have 10% lysine, glutamic acid and aspartic acid. Because of this, the extracellular surface is very hydrophobic.

Types of bacteria that are Gram negative rods

Bacillus-shaped Gram-negative bacteria are found in various taxonomic groups. Within the same genus there may be Gram negative bacteria that can have different shapes.

For example: 1) the genus Chorobium, in which the green sulfurous bacteria are found, shaped like bacilli and curved rods; 2) the genus Pasteurella, which has pleomorphic species (of multiple forms).

There are heterogeneous groups, such as the "gliding bacteria" and the sulfur bacteria, whose members are generally Gram negative, and may have the shape of a bacillus, or other forms.

Slippery bacteria are phylogenetically very diverse, with various motility mechanisms. They are bacilli and lack flagella. Sulfur bacteria are a very diverse group of microorganisms that can be curved rods or rods.

Photoautotrophic bacteria are Gram negative and have a variety of forms, such as bacilli and cocci. They comprise a single phylogenetic branch within the Eubacteria domain.

There are taxonomic groups whose members are Gram negative and in which their only form is that of a bacillus, namely:

Enterobacteria (order Enterobacteriales, family Enterobacteriaceae), Pseudomonas (order Pseudomonadales, class Gammaproteobacteria), Azotobacter (class Gammaproteobacteria), and Bacteroides (phylum Bacteroidetes, class Bacteroidia).

Types of Arcaheobacteria that are Gram negative rods

Members of Haloarchaea (Halobacteria) are Gram negative rods. They belong to the order Halobacteriales and the family Halobacteriacea. This family has 19 genera and some 57 species. The Haloarchaea are a monophyletic group, that is, they have an exclusive ancestor.

A relatively close group to the Haloarchaea are the methanogenic archaebacteria, which can be Gram negative or Gram positive. They have varied shapes. Approximately 160 different species are known, belonging to 29 genera, 14 families and six orders.

Examples of Gram negative rods in Eubacteria

Enterobacteria group

Representative genera: Escherichia (gastroenteritis), Enterobacter (rarely pathogenic), Serratia (rarely pathogenic), Salmonella (enteritis), Proteus (urinary tract infection), Yersinia (plague), Klebsiella (pneumonia). They belong to the gammaproteobacteria.

Pseudomonas Group

Representative genera: Pseudomonas, Burkholderia, Zymomonas and Xanthomonas. They can be straight or curved bacilli. Several species are pathogenic to animals and plants. For example, Pseudomonas auruginosa is a wound and burn colonizer.

Sulfur oxidizing bacteria group

Representative genus: Thiobacillus. This genus is the best known of the chemolytotrophs. Thiobacillus species are scattered among the subdivisions (alpha, beta, and gamma) of proteobacteria.

Acetic acid bacteria group

Representative genera: Acetobacter and Gluconobacter. They carry out the incomplete oxidation of alcohols and sugars. When the substrate is ethanol, they form acetic acid. They are particularly useful in the alcoholic beverage industry.

Nitrogen fixing bacteria group

Representative genera: Azotobacter and Zomonas. They fix nitrogen aerobically. Most belong to the alpha or gammaproteobacteria. Bacteria of the genus Azotobacter are particularly large bacilli.

Hyperthermophilic bacteria group

Representative genera: Thermotoga and Thermodesulfobacterium. They are bacillary hyperthermophiles that grow at temperatures above 70 ºC. They have been isolated from terrestrial habitats, hot springs and underwater sources.

Examples of Gram negative rods in Archaea

Halobacterium salinarum

It is mobile, lives in environments with a high concentration of salt (> 4 M). It can use light as a source of energy because it has bacteriorhodopsin, a light-dependent proton pump. This pump allows capturing light and creating an electrochemical gradient across the membrane.

The energy of the electrochemical gradient is used to synthesize ATP by ATP synthase.

Metanomicrobium movile

It is a Gram negative rod. Layer S shows a hexagonal organization. S-layer proteins have low isoelectric points, indicating that they are rich in acidic amino acid residues. The percentage of hydrophobic residues is low.

It lives in the rumen of sheep. Produces methane via carbon dioxide reduction using H ₂ or formate. It cannot metabolize acetate, methylamines, or methanol. It exhibits an optimal pH in the range between 6.5 and 8. It has an important role in rumen function and animal nutrition.

Gram negative rods that cause disease

Klebsiella pneumoniae

It is one of the causative agents of pneumonia. K. pneumoniae is an opportunistic pathogen that infects the respiratory system of humans and animals. It is not mobile. It is a Gram negative rod that produces a preponderant capsule, which protects it against phagocytosis.

Pseudomonas aeruginosa

It is a Gram negative rod. It produces diseases such as urinary tract and ear infections. It colonizes wounds and burns on the skin. Under certain conditions, it produces a polysaccharide biofilm to establish a bacterial community and protect it from the immune system.

Haemophilus

They are small Gram negative rods and sometimes pleomorphic. There are several species associated with diseases in humans, such as H. influenzae (pneumonia), H. aegyptius (conjunctivitis), H. ducrey (chancroid), and H. parainfluenzae (bacteremia and endocarditis).

Legionella

They are thin, pleomorphic Gram negative rods. They are intracellular parasites. They multiply in alveolar macrophages. It produces pneumonia and sporadic, epidemic and nosocomial infections. Legionella pneumophila is responsible for epidemics.

Salmonella

They are facultative Anerobic Gram-negative rods. They can colonize various animals, including mammals, birds, and reptiles. Most infections occur after ingestion of contaminated food, or by direct transmission through the fecal-oral route. Causes gastroenteritis.

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