Eubacteria represent one of the three primary domains of life, alongside Archaea and Eukarya. These single-celled organisms are prokaryotes, meaning their cellular structure lacks a nucleus and other membrane-bound internal compartments. They are the most widespread life forms on Earth, inhabiting nearly every environment, from the deep ocean floor to the digestive systems of animals. The following sections will define Eubacteria and provide examples of their beneficial and harmful impacts.
Defining True Bacteria
The Eubacteria domain is defined by its cellular architecture. Unlike eukaryotic cells, Eubacteria do not enclose their genetic material within a membrane-bound nucleus; their single, circular chromosome is located in the cytoplasm’s nucleoid region. A key structural feature differentiating Eubacteria from Archaea is the chemical composition of the cell wall. Eubacterial cell walls are primarily composed of peptidoglycan, a sturdy polymer that provides structural rigidity and protection. Archaea, by contrast, possess cell walls made of different materials, such as pseudomurein, and lack true peptidoglycan.
Classification Methods
Scientists employ several methods to categorize Eubacteria, often relying on physical characteristics that can be observed microscopically. The most basic classification is morphological, which sorts bacteria into three main shapes. Cocci are spherical, Bacilli are rod-shaped, and Spirilla are bacteria with a spiral or corkscrew shape.
A more powerful diagnostic tool is the Gram stain, which exploits the differences in cell wall structure. Gram-positive bacteria have a thick layer of peptidoglycan that retains the crystal violet dye, causing them to appear purple under a microscope. Gram-negative bacteria have a much thinner peptidoglycan layer sandwiched between two membranes, which prevents them from retaining the purple dye and causes them to take on the pink or red color of the counterstain. This staining property is crucial because it often correlates with differences in antibiotic susceptibility and pathogenicity.
Beneficial and Environmental Examples
Many Eubacteria play positive roles in both ecosystem function and human health. Cyanobacteria are a prime example, as they are photosynthetic organisms. These bacteria were responsible for the initial oxygenation of the Earth’s atmosphere and continue to contribute to global oxygen production.
Other species are vital for nutrient cycling, such as nitrogen-fixing bacteria like Rhizobium. These bacteria form a symbiotic relationship within the root nodules of legumes, converting inert atmospheric nitrogen gas into ammonia, a form that plants can readily absorb to build proteins and DNA. This process is paramount to soil fertility and sustainable agriculture. In the human body, beneficial bacteria like Lactobacillus and Bifidobacterium reside in the gut, where they aid in the breakdown of complex carbohydrates and synthesize compounds like Vitamin K and B vitamins. These members of the gut flora also help maintain the intestinal barrier function and prevent the colonization of harmful microbes.
Pathogenic Examples
While many Eubacteria are harmless or beneficial, a significant number of species are pathogenic, meaning they cause disease in their hosts. One well-known example is Mycobacterium tuberculosis, the slow-growing bacterium responsible for tuberculosis, a serious lung infection that remains a major global health concern. This bacterium can survive and multiply inside the host’s immune cells, making it difficult to eliminate.
Other common pathogens include Streptococcus pyogenes, which is responsible for illnesses ranging from strep throat to more severe infections like necrotizing fasciitis. Foodborne illnesses are frequently caused by Eubacteria such as Salmonella enteritidis, which is often ingested through contaminated food products like poultry and eggs. Salmonella causes gastroenteritis by invading the intestinal lining, leading to symptoms like diarrhea, fever, and abdominal pain. These harmful bacteria typically cause disease either by directly damaging host cells or by releasing potent toxins that interfere with normal bodily functions.