Eubacteria: Definition, Types & Characteristics

Eubacteria Definition

Eubacteria, or “true” bacteria, are single-celled prokaryotic microorganisms that have a range of characteristics and are found in various conditions throughout all parts of the world. All types of bacteria fall under this title, except for archaebacteria. Since eubacteria are so common, it comprises one of the three domains of life; the three domains are Archaea, Bacteria, and Eukarya.

Characteristics of Eubacteria

Eubacteria, or microorganisms lacking a defined membrane nucleus, have several general characteristics. All are enclosed by a cellular wall, which is made up of peptidoglycans in a cross-linked chain pattern. This gives the wall of the bacteria the strength needed to maintain its shape and size during changing environments. Small molecules can diffuse through the cell wall, but often a proton gradient must first be established in order for molecule exchange to properly occur. Unlike eukaryotes, bacteria have cholesterol present in the membrane to enhance permeability properties of the membrane and increase stiffness. Similar to eukaryotes, bacteria also have a plasma membrane within the cell wall. Some bacteria may have a flagella, or a projection composed of protein filaments, that is used for movement. Other bacteria may have pili, which are small projections all over the outside of the cell, and are used for sticking to surfaces and transferring DNA. When a large amount of bacteria are attached to a surface and are surrounded by a polysaccharide sac, this is referred to as a biofilm. This complex has high antimicrobial resistance.

Within the plasma membrane of the bacteria is the cytoplasm, or the intracellular milieu. It is composed of mainly water (approximately 80%), but has a gel-like consistency. Bacteria do not have membrane-bound organelles, unlike eukaryotes, which have mitochondria and chloroplasts. There are ribosomes, however, which are organelles that are composed of RNA and used for protein synthesis. Free floating within the cytoplasm is also the genome, or bacterial DNA, found in the nucleoid. Bacterial chromosomes are often circular but can also be linear in shape. This shape comes in handy when a bacterium is undergoing replication. Bacteria can asexually reproduce through binary fission or budding. Binary fission is when two equal progeny cells are produced. Bacteria that undergo binary fission must first elongate before separating. Budding is when there is growth off the parent cell. Binary fission produces two equal daughter cells, while budding produces a new cell while the parent cell remains.

During times of extreme conditions not conducive to replication, such as starvation, eubacteria have the ability to become endospores. In this state, the bacteria can tolerate exceedingly high and low temperatures, acidic and basic conditions, and large amounts of radiation. Endospores are extremely hard to kill. Surprisingly, they can be boiled for hours and still survive. Endospores can only be made by Gram-positive bacteria. Within the endospore remains the bacterial DNA, but the cytoplasm has a decreased water concentration. This is thought to help in protecting against high heat. The bacteria will take on a tough coating composed of calcium and dipicolinic acid, creating a dense and impregnable barrier to stabilize the DNA within the cell. DNA repair enzymes are also still active, aiding in the resistance of the endospore.

Plasmids are also found within bacteria separate from the bacteria’s circular DNA. Also referred to as “replicons”, plasmids are autonomous replicating DNA molecules. Not all plasmids replicate in bacteria, though. These elements allow for horizontal gene transfer, which is a way for a bacterium to gain new genes and therefore traits. They primarily aid in the rapid mutation in bacteria to several factors. Similar to the other genetic material, the plasmids can be passed onto daughter cells during replication. They are the common DNA structure used in research because they are relatively easy to manipulate, implant and measure.

Types of Eubacteria

Eubacteria are typically classified into five different phylums: Chlamydias, Cyanobacteria (Blue-green algae), Gram-positive bacteria, Proteobacteria, and Spirochetes. Chlamydias are often parasitic bacteria. Cyanobacteria are most commonly known to be aquatic and obtain energy via photosynthesis. Some, but not all, bacteria have an additional layer enclosing the cell wall referred to as the lipopolysaccharide or LPS layer. This extra layer cannot be dyed with a Gram stain that is often used to classify bacteria by researchers and are thus referred to as Gram-negative bacteria or Proteobacteria. Proteobacteria make up the second largest group of bacteria and they can be stained by the dye. These bacteria are referred to as Gram-positive. Spirochetes are long, thin, spiral shaped bacteria that are known to cause Lyme disease. They are distinct from the other types of bacteria due to their helical shape and movement. They typically move by spinning along their axis.

Bacteria commonly take on one of three shapes: bacilli, cocci, and spirilli. Bacilli have a rod shape, cocci have a spherical shape, and spirilli have a spiral or wave shape. Their shape was often used as a classification system until recently. Bacteria may stay linked after division, forming other shapes such as clusters, filaments, and tight coils.

Examples of Eubacteria

Escherichia coli, abbreviated to E. coli, belongs to the Eubacteria domain. It is classified into the Proteobacteria phylum. It is rod-shaped and Gram-negative so it has the additional membrane. E. coli is commonly found in the gut of many different types of warm-blooded hosts, including humans. Most strains are harmless, but some can cause food poisoning and other illnesses. The bacteria can only survive outside of a host for a limited time.

Streptococcus pneumoniae, abbreviated to S. pneumoniae is another common eubacteria. It belongs to the Firmicutes phylum. It is cocci shape, as its name implies, and Gram-positive. S. pneumoniae can be found on healthy hosts in the respiratory tract, nasal cavity, and sinuses. However, the bacteria can become pathogenic and spread to other parts of the body, often causing pneumonia and meningitis in immunocompromised hosts. The bacteria in high amounts can cause other illnesses as well, including, but not limited to, bronchitis, acute sinusitis, and sepsis.