The term “germs” is often used as a catch-all for any microscopic entity, but in biology, precision is important. Scientists use “real bacteria” to refer to the specific and ancient lineage of life known as the domain Bacteria. These organisms represent one of the three major branches of life on Earth. They are a cohesive group defined by a shared evolutionary history and a distinct set of biological characteristics that set them apart from all other forms of life.
Defining Characteristics of Bacteria
A bacterium’s cellular organization is a primary characteristic. Bacteria are prokaryotes, meaning their cells lack a true nucleus, which can be visualized as a simple layout compared to the complex structure of a eukaryotic cell. Instead of being housed in a nucleus, their genetic material exists within the cytoplasm in a region called the nucleoid. This genetic information is typically contained on a single, circular chromosome.
The cell wall is another defining feature that serves as a primary identifier for the domain Bacteria. Most bacterial cell walls are made of peptidoglycan, a mesh-like polymer of sugars and amino acids. This unique material provides structural support and protection, and its presence is a key distinction from other domains of life. While a cell membrane encloses the cell’s contents, the peptidoglycan wall is the signature feature.
The cytoplasm contains all the necessary components for life. Ribosomes are responsible for protein synthesis, and while they perform the same function as in eukaryotic cells, their structure is different. Bacteria may also have storage granules for nutrients. They lack the complex, membrane-bound organelles—such as mitochondria or chloroplasts—that characterize eukaryotic cells.
Distinguishing Bacteria from Other Microbes
Bacteria share the prokaryotic cell structure with Archaea, but the two groups are distinct. The most significant difference is their cell wall composition; archaeal cell walls lack peptidoglycan. Furthermore, the lipids in their cell membranes are chemically different. Bacterial membranes have unbranched fatty acid chains with ester linkages, while archaeal membranes feature branched hydrocarbon chains with ether linkages, a difference that helps many archaea survive in extreme environments.
The distinction between bacteria and viruses is more profound. Bacteria are living, cellular organisms capable of independent reproduction through processes like binary fission. In contrast, viruses are not cells but non-cellular entities composed of genetic material—either DNA or RNA—enclosed within a protein coat called a capsid. As obligate parasites, viruses cannot replicate on their own. To reproduce, a virus must infect a living host cell and hijack its cellular machinery to create copies of itself.
Bacterial Diversity and Classification
The bacterial domain is incredibly diverse, with organisms varying in size, habitat, and metabolism. One basic way to classify bacteria is by their shape. The three most common shapes are cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped).
The arrangement of bacterial cells after division is also used for classification. Some bacteria form distinct groupings indicated by a prefix. For instance, the prefix staphylo- describes cells that group in clusters resembling grapes, as in Staphylococcus. The prefix strepto- refers to cells that remain linked in chains, such as Streptococcus.
Bacterial diversity also extends to how they interact with oxygen. Aerobic bacteria require oxygen to survive, while anaerobic bacteria do not and may even be harmed by it. This metabolic flexibility allows different bacterial species to thrive in a vast range of environments, from oxygen-rich soils to the oxygen-deprived depths of the ocean.
The Role of Bacteria in Ecosystems
Bacteria are fundamental to every ecosystem on Earth, primarily as decomposers. By breaking down dead organic material, such as fallen leaves and dead animals, bacteria release nutrients like carbon and nitrogen back into the environment. This recycling process makes these nutrients available for plants and other organisms.
Some bacteria are also primary producers that create their own food. Cyanobacteria, for example, perform photosynthesis, using sunlight to convert carbon dioxide into organic compounds. This process forms the foundation of many aquatic food webs and produces a significant portion of the Earth’s oxygen.
Many bacteria exist in symbiotic relationships with other organisms. The human gut microbiome is a well-known example where bacteria aid in digestion and support the immune system. While some bacteria are pathogens that cause disease, the vast majority play beneficial or neutral roles in the biosphere.