Bacteria constitute a large domain of prokaryotic microorganisms. Bacteria were among the first life forms to appear on Earth, and are present in most habitats on the planet. Bacteria grow in soil, acidic hot springs, radioactive waste, water, and deep in the Earth's crust. In addition, they grow in organic matter and the live bodies of plants and animals, providing outstanding examples of mutualism in the digestive tracts of humans, termites, and cockroaches.
But what defines a bacteria? Bacteria as prokaryotes share many common features, such as:
- A lack of membrane-bound organelles
- Unicellularity and thus division by binary-fission
- Generally small size
Bacteria do not tend to have membrane-bound organelles in their cytoplasm and thus contain few large intracellular structures. They consequently lack a true nucleus, mitochondria, chloroplasts, and the other organelles present in eukaryotic cells, such as the Golgi apparatus and endoplasmic reticulum. Bacteria were once seen as simple bags of cytoplasm, but elements such as prokaryotic cytoskeleton, and the localization of proteins to specific locations within the cytoplasm have been found to show levels of complexity. These subcellular compartments have been called "bacterial hyperstructures" (Figure 3).
Unlike in multicellular organisms, increases in cell size (cell growth and reproduction by cell division) are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual reproduction (Figure 2).
Perhaps the most obvious structural characteristic of bacteria is (with some exceptions) their small size. For example, Escherichia coli cells, an "average" sized bacterium, are about 2 micrometres (μm) long and 0.5 μm in diameter. Small size is extremely important because it allows for a large surface area-to-volume ratio which allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes.
The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes, having the similar traits outlined above. However, molecular systematics show prokaryotic life to consist of two separate domains, originally called Eubacteria and Archaebacteria, but now called Bacteria and Archaea that evolved independently from an ancient common ancestor (Figure 1). The archaea and eukaryotes are more closely related to each other than either is to the bacteria. It should be noted that Bacteria and Archaea are similar physically, but have different ancestral origins as determined by DNA of the genomes that encode different prokaryotes.