Kingdom Monera: The Prokaryotes

The Monerans are the most numerous and widespread organisms on earth. They comprise the only kingdom of prokaryotic organisms, those which lack a nucleus or other membrane-bounded organelles. External to the plasma membrane, most bacteria have a cell wall partially composed of peptidoglycan, a complex structural molecule not found in eukaryotic cells. Let's have a look at the basic flavors of bacteria.


ARCHAEBACTERIA

There are three types of archaebacteria, the most ancient of all living things. The thermoacidophiles live in the extremely hot, acidic water and moist areas within and surrounding sulfur hot springs. So closely adapted are they to their bubbly environment that they die of cold at temperatures of 55oC (131oF)!

Methanogens are obligate anaerobes (free oxygen kills them) which oxidize CO2 during cellular respiration to produce methane (CH4) as a waste product. Although RNA sequencing suggests that all ten known species are evolutionarily related, they exist in environments as diverse as scalding volcanic deep-sea vents and the intestines of mammals. The reason you can light a puff of flatulence (should you choose to go into show business) is because of the symbiotic methanogens inside your guts.

Strict halophiles live in extremely salty solutions such as the Dead Sea, the Great Salt Lake and that can of pickled herring you left open in the cupboard. Their pink carotenoid pigments make them conspicuous when the bacteria are present in large concentrations, as they are on the shores of some salty, land-locked lakes.

EUBACTERIA

The "true bacteria" are classified on the basis of several characteristics, of which perhaps the most familiar is the Gram Stain method.

  • Gram negative Eubacteria

    • About 75% of known eubacteria are gram negative. They include the gliding bacteria, the spirochetes, the curved (vibrios) and spiral (spirillae) bacteria, gram-negative rods, gram-negative cocci, rickettsias, chlamydias and the photosynthetic cyanobacteria. Gram negative bacteria form an extremely diverse group. The fact that they are all gram-negative does not necessarily imply that they comprise a monophyletic taxon.

  • Gram positive Eubacteria

    • Not as diverse as the gram-negative bacteria, the gram-positives still make up an impressively varied group. This division includes the gram-positive rods, gram-positive cocci, and the actinomycetes, which exhibit superficial similarity and function (but no evolutionary relationship) to the (eukaryotic) fungi.

    MYCOPLASMAS

    These are the smallest living cells ever discovered, and are believed to have the minimum amount of DNA needed to code for a functioning cell. They lack the cell wall characteristic of the other three types of bacteria.

    Most mycoplasmas exist as intracellular plant or animal parasites, a life history which protects them from environmental osmotic stresses as long as the host cell is functioning properly. Penicillin, an antibiotic lethal to most other bacteria because it interferes will cell wall formation, is not effective against the naked little mycoplasmas.

    The Many Shapes of Bacteria


    As you already know, bacteria come in a vast array of shapes and sizes, and there are several taxonomically distinct groups. Take a slide to your station and observe under the compound microscope. Remember: bacteria are extremely small. Focus with extreme care, on low power first, and don't break the slide!

    For many years, the evolutionary relationships of bacteria were so poorly understood that they were classified only on the basis of their shape and staining characteristics. These characters can still be useful in the early stages of identification, but more recent advances in DNA and RNA sequencing give us a more accurate idea of origins and relationships among these tiny, vital organisms.

    Each of these slides has three separate smears, each with a different shape of bacteria. Rod-shaped bacilli (sing., bacillus) are the most common. Escherichia coli (our mammalian gut symbiont), Lactobacillus spp. (which may be agents of tooth decay or ingredients in yogurt) and Bacillus anthracis (a pathogen causing anthrax in sheep and humans) are examples.

    Spherical cocci (sing., coccus) are also common. Streptococcus spp. are chain-forming cocci responsible for ailments such as strep throat in humans. Staphylococcus spp. form clusters reminiscent of tiny bunches of grapes (staphylo is Greek for "cluster"), and are responsible for those nasty "staph" infections (and often, gangrene) found in untreated puncture wounds.

    Spiral-shaped spirilla (sing., spirillum) are the largest of these three types, and the simplest to identify. Maybe you should start with those. . .

    Asexual Reproduction in Prokaryotes

    You are probably most familiar with mitosis as the mode by which cells reproduce themselves. Because prokaryotes have a single, circular chromosome rather than the sets of chromosomes found in the more familiar eukaryotes, mitosis does not occur in prokaryotes. Instead, most replicate via a process of binary fission.

    Bacterial Locomotion

    Bacteria exhibit various modes of locomotion, including "squirming", gliding and propulsion via flagella. The flagellum of a bacterium is quite different from the flagellum of a eukaryote. It is composed of a protein called flagellin, not found in eukaryotes, whereas the eukaryotic flagellum is composed of a symmetrically arranged series of microtubules. Unlike the eukaryotic flagellum, which beats with a wavelike motion, the bacterial flagellum rotates to propel the little beastie through its substrate.
    Here are a few images of bacterial flagella...

    Close up of the flagellum of Spirilla volutans
    The spectacular Proteus vulgaris, a ubiquitous and non-pathogenic bacterium.

    The Economic Importance of Bacteria

    Bacteria affect the lives of your average Homo sapiens in countless ways. They may be pathogens, such as these Clostridium tetani. These bacilli are the pathogens responsible for causing tetanus in humans.
    Other organisms may serve as vectors to spread bacteria. Flies, cockroaches, biting insects, rodents and other animals get a lot of the blame for transmitting diseases to humans. But if the truth be told, you're in a lot more danger of contracting somethign dangerous from personal contact with your fellow Homo sapiens than you are from being licked by a fly (or your dog!).
    Nitrogen-fixing bacteria (such as these Rhizobiumsp.) inhabit the root cells of plants in the legume family (Fabaceae). These moneran symbionts convert gaseous nitrogen from the atmosphere (N2) into usable "fixed" nitrogen (ammonia, nitrite and nitrate) which can be absorbed by the roots and used by the plant to manufacture protein and nucleic acids.
    Other bacteria, such as these Streptomyces spp., are sources of life-saving medicines. This genus yields the powerful antibiotic known as streptomycin. Actinomycetes are the source of actinomycin.
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