Eubacteria are a vast domain of life, commonly known as bacteria. These organisms are prokaryotes, meaning their cellular structure lacks a membrane-bound nucleus and other specialized compartments. Eubacteria are found in nearly every environment on Earth, from the deepest oceans to the highest mountains, and within other living organisms. Their presence highlights their role in global ecosystems.
What Are Eubacteria?
Eubacteria have prokaryotic cell organization, with genetic material, typically a single circular chromosome, floating freely in the cytoplasm. Their cells are smaller than eukaryotic cells, ranging from 0.2 to over 5 micrometers. A defining feature is their rigid cell wall, primarily composed of peptidoglycan, which provides structural support and protection. Some eubacteria also possess flagella, whip-like appendages for movement, or pili, small hair-like structures involved in attachment and gene transfer.
These organisms exhibit metabolic diversity, utilizing a wide array of energy sources and nutrients. Some eubacteria are heterotrophs, consuming organic compounds, while others are autotrophs, producing their own food through photosynthesis or chemosynthesis. This adaptability allows them to thrive in diverse habitats, including soil, water, and extreme environments like hot springs or highly saline conditions. Examples include Escherichia coli, found in the intestines, and Lactobacillus, used in fermented foods.
The Challenge of Counting Bacterial Species
Determining the exact number of eubacterial species presents a significant scientific challenge, due to their microscopic nature and microbial biology complexities. A primary hurdle is that most eubacteria remain unculturable in laboratory settings, preventing scientists from growing and identifying them. This limitation means traditional species identification methods, which rely on culturing, are inadequate for capturing their full diversity. Even for named species, many have not been extensively studied.
Another difficulty arises from the definition of a “species” in microbiology. Unlike sexually reproducing plants and animals, bacteria primarily multiply through asexual reproduction like binary fission. They also engage in horizontal gene transfer, where genetic material is exchanged directly between unrelated bacteria, blurring traditional species boundaries. This genomic plasticity makes it challenging to delineate distinct species based on genetic similarity alone. While concepts like Average Nucleotide Identity (ANI) are used to define bacterial species, there is no universally accepted standard, with thresholds often set around 95% ANI for strains within a species.
Eubacteria inhabit a wide range of environmental niches, each with unique conditions that select for different bacterial communities. From the human gut to deep-sea vents, environmental diversity further complicates efforts to comprehensively catalog all existing species. The morphological similarity among many bacterial types also means visual inspection is not a reliable method for distinguishing species. These factors collectively contribute to the difficulty in accurately counting the total number of eubacterial species on Earth.
Estimating the Number of Eubacteria
Given the challenges in direct enumeration, scientists rely on various methodologies to estimate the total number of eubacterial species. These estimates vary widely, reflecting the ongoing nature of discovery and the different approaches used. Early estimates based on indirect analyses, such as 16S rRNA gene sequence analysis, suggested there could be between 10 million and 1 billion species. More recent analyses, which incorporate larger datasets and ecological models, propose even higher figures.
One study, combining extensive surveys of bacteria and other microorganisms from diverse global locations, estimated that Earth might be home to between 100 billion and 1 trillion microbial species. This figure suggests that an overwhelming majority, possibly around 99.999 percent, of microbial species have yet to be discovered and characterized. Metagenomics, a technique involving sequencing genetic material directly from environmental samples without culturing, has revolutionized these estimations. By analyzing environmental DNA (eDNA), researchers identify numerous species that cannot be grown in the lab, providing a more comprehensive picture of microbial diversity. While approximately 43,000 bacterial species have been formally named, this number represents only a tiny fraction of the estimated total, indicating much remains unknown about eubacterial diversity.
Why This Diversity Matters
The vast and largely uncharacterized diversity of eubacteria holds significance for Earth’s ecosystems, human health, and technological advancements. In ecosystems, eubacteria are essential for nutrient cycling, converting elements like nitrogen, carbon, and phosphorus into forms usable by other organisms. They are primary agents in decomposition, breaking down dead organic matter and recycling essential nutrients back into the environment, which supports the growth of plants and other life forms. Their presence contributes to ecosystem resilience, enabling communities to adapt and recover from environmental disturbances.
Eubacteria also influence human health. The human gut microbiome, for instance, comprises trillions of bacterial cells that outnumber human cells and play a role in digestion, nutrient absorption, and immune system regulation. An imbalance in gut bacteria can contribute to various health issues, including metabolic disorders and inflammatory conditions. Beyond the gut, bacteria also reside on the skin, in the mouth, and in other body parts, forming complex microbial communities that protect against harmful pathogens.
In biotechnology, eubacteria are harnessed for numerous applications. They are used in bioremediation to clean up contaminated environments, breaking down pollutants like oil spills and toxic chemicals. In industrial processes, bacteria produce enzymes, biofuels, and bioplastics. Furthermore, bacteria are important in medicine for the production of antibiotics, vaccines, and other therapeutic agents, and their genetic material is manipulated for genetic engineering. The undiscovered diversity of eubacteria represents a vast reservoir of potential new biological processes and compounds that could address future challenges in health, energy, and environmental sustainability.