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8.6A: Common Bacterial Traits

  • Page ID
    9734
  • Bacteria are a subset of prokaryotes and while very different, they still have some common features.

    Learning Objectives

    • Identify common bacterial traits

    Key Points

    • Bacteria vary from species to species, thus assigning many common traits to bacteria is difficult. Bacterial species are typified by their diversity.
    • There are three notable common traits of bacteria, 1) lack of membrane-bound organelles, 2) unicellular and 3) small (usually microscopic) size.
    • Not all prokaryotes are bacteria, some are archaea, which although they share common physicals features to bacteria, are ancestrally different from bacteria.

    Key Terms

    • archaea: a taxonomic domain of single-celled organisms lacking nuclei, formerly called archaebacteria but now known to differ fundamentally from bacteria.
    • binary fission: a form of asexual reproduction and cell division used by all prokaryotes, (bacteria and archaebacteria)

    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:

    1. A lack of membrane-bound organelles
    2. Unicellularity and thus division by binary-fission
    3. 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”.

    image
    Figure: Bacterial structures: Cell structure of a Gram-positive prokaryote.

    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.

    image
    Figure: Binary fission: Many bacteria reproduce through binary fission.

    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. 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.

    image
    Figure: Archaea and other domains: Phylogenetic tree showing the relationship between the Archaea and other domains of life. Eukaryotes are colored red, archaea green and bacteria blue.