Eubacteria, often referred to as “true bacteria,” represent a diverse group of single-celled microorganisms. These organisms are found in nearly every environment on Earth, from the deepest oceans and hot springs to fertile soils and within other living organisms. They are fundamental to many natural processes.
Key Characteristics
Eubacteria are defined by several fundamental features that categorize them as prokaryotes. Unlike eukaryotic cells, eubacterial cells lack a membrane-bound nucleus, meaning their genetic material is not enclosed within a distinct compartment. They also do not possess other membrane-bound organelles such as mitochondria or chloroplasts. Instead, their cellular functions occur within the cytoplasm, a jelly-like substance filling the cell. The genetic information is typically organized as a single, circular chromosome located in a region called the nucleoid, though some may have additional small, circular DNA molecules called plasmids.
A distinguishing structural component of eubacteria is their rigid cell wall, primarily composed of a polymer called peptidoglycan. This protective layer provides structural stability and safeguards the cell from osmotic pressure changes. Eubacteria are generally small, typically ranging from 0.2 to 50 micrometers in size, and reproduce primarily through a process called binary fission, where one cell divides into two identical daughter cells. Many eubacteria also possess flagella, whip-like appendages that enable movement, or pili, hair-like structures that assist in attachment to surfaces or other cells.
Distinguishing Eubacteria
Understanding eubacteria involves recognizing how they differ from other major domains of life: Archaea and Eukaryotes. Both eubacteria and archaea are prokaryotes, meaning they share the characteristic absence of a membrane-bound nucleus and other complex internal compartments. However, a primary distinction lies in their cell wall composition; eubacteria possess peptidoglycan in their cell walls, a component absent in archaea. Archaea often have cell walls made of pseudopeptidoglycan or S-layers, and their membrane lipids are linked differently (ether-linked) compared to the ester-linked lipids found in eubacteria.
Archaea and eubacteria exhibit differences in their ribosomal RNA sequences and the structure of their RNA polymerases. Archaea’s genetic machinery shows more similarities to eukaryotes in some aspects, despite their prokaryotic cell structure. When compared to eukaryotes, both eubacteria and archaea are simpler in organization. Eukaryotic cells are typically much larger and possess a true nucleus, along with various membrane-bound organelles that perform specialized functions, such as mitochondria for energy production and chloroplasts for photosynthesis in plant cells.
Diverse Bacterial Groups
The domain Eubacteria encompasses a wide variety of forms and functions, often categorized based on characteristics like cell wall structure. Gram-positive bacteria, for example, have a thick layer of peptidoglycan in their cell walls, which allows them to retain a crystal violet stain used in a common laboratory test called Gram staining. Examples include Staphylococcus species, which can reside on skin, and Lactobacillus species, important in dairy fermentation.
In contrast, Gram-negative bacteria possess a thinner peptidoglycan layer and an outer membrane, causing them to not retain the crystal violet stain. This group includes organisms such as Escherichia coli (E. coli), a common inhabitant of the gut, and Salmonella, a bacterium often associated with foodborne illnesses. Cyanobacteria, often called blue-green algae, perform oxygenic photosynthesis, similar to plants. These organisms play an important role as primary producers in aquatic environments and include genera like Anabaena and Nostoc.
Spirochetes represent another distinct group of eubacteria, characterized by their helical or spiral shape. These bacteria often move with a corkscrew-like motion. A notable example is Borrelia burgdorferi, the bacterium responsible for causing Lyme disease. This diversity allows eubacteria to thrive in various habitats on Earth.
Impact on Life and Environments
Eubacteria play important roles across various ecosystems and influence life on Earth, including human well-being. They are essential in nutrient cycling, processes that transform chemical elements through the environment. For instance, some eubacteria are decomposers, breaking down dead organic matter and returning nutrients like carbon and nitrogen to the soil and atmosphere. Other eubacteria are important for nitrogen fixation, converting atmospheric nitrogen into forms usable by plants, thereby enriching soil fertility.
Many eubacteria engage in symbiotic relationships with other organisms. The human gut, for example, hosts a complex community of eubacteria that aid in digestion, synthesize certain vitamins, and help maintain a balanced microbial environment. Similarly, some eubacteria form beneficial associations with plants, residing in root nodules where they facilitate nutrient uptake. Beyond natural ecosystems, eubacteria are harnessed in various industrial applications. They are used in the production of fermented foods like yogurt and cheese, and in biotechnology for synthesizing pharmaceuticals such as antibiotics and vitamins.
While many eubacteria contribute positively to life and environments, some species can also be agents of disease. Certain eubacteria are known pathogens, capable of causing infections in humans, animals, and plants. Examples include Streptococcus pneumoniae, which can lead to pneumonia, and specific strains of E. coli that cause severe gastrointestinal issues. Their diverse metabolic capabilities highlight their importance to global biochemical cycles and the health of living systems.