17
Monera

Unlike viruses and subviruses, which are not cellular, the members of the kingdom Monera, which includes bacteria and blue-green bacteria (sometimes called cyanobacteria, or blue-green algae), are composed of true cells. Monerans are all prokaryotic; that is, their cells lack most organelles, they do not have a membrane-bound nucleus, and most occur as single-celled organisms (see Figures 17.1 and 17.2). Monera is usually broken into two distinct kingdoms. The Archaebacteria (also called Archaea; such terms are sometimes capitalized, and sometimes not) and Eubacteria (or bacteria). (See Glossary for more information about these.)

Of the 15,000 described species, many exist as a series of cells occurring in long filaments or as more complex colonies. Scientists are discovering bacteria that form complex communities, hunt prey in groups, and secrete chemical trails for the directed movement of thousands of individual bacterial cells.

In comparison to most single-celled eukaryotes, individual bacterial cells are smaller and far more abundant, representing a remarkably important component of nearly all ecosystems. Without bacteria, life on earth could not exist as we know it. Bacteria represent some of the most important groups of decomposers; without them, dead organisms would not decay properly. Many nutrients would remain locked up in corpses forever. Geochemical recycling of the earth's nitrogen, carbon, and sulfur, which are critical to life, would not occur without bacteria. Chemicals such as nitrates, which certain plants use for protein synthesis, are produced by some species of bacteria. Certain bacteria are heterotrophic; that is, they procure their food by feeding on organic material formed by other organisms. Other species of bacteria are photosynthetic, capable of synthesizing their organic molecules from inorganic components, using the energy from the sun. One group of bacteria, the mycoplasmas, are the smallest known cells that grow and reproduce without needing a living host. Their diameters range from 1.12 to 0.25 micrometers.

Probably because of the small size of most types of bacteria, their rapid rate of cell division, their remarkable metabolic versatility, and their ability to live practically anywhere, they are the most numerous organisms on earth. Under optimal conditions, a population can double in size every 20 or 30 minutes. Species of bacteria are found thriving on icebergs, in hot springs, at the bottom of the oceans, in fresh water, on land, in the soil, and even in aviation fuel.

Image described by caption.

Figure 17.1 Diagram of a bacterial cell, illustrating a prokaryotic cell. Most prokaryotes have few, if any, membranous organelles within the cell, no nuclear membrane, no mitochondria, and no endoplasmic reticulum. Generally, there is a cell wall outside the plasma membrane. Flagelli, when present, are composed of protein, and do not have any microtubules.

Although most bacteria use oxygen in their metabolic processes, there are many species that use alternative pathways, surviving perfectly well without any oxygen. Some species have the ability to form spores, which are inactive, thick-walled forms that survive for long periods without water or nutrients in what otherwise would be unfavorable conditions.

Bacteria were first discovered in 1676. In the nineteenth century, Louis Pasteur studied viruses as far as was possible without the aid of the subsequently developed electron microscope or advanced biochemical techniques, which enabled later researchers to study these small organisms in considerably more detail.

Being prokaryotes, bacteria have cells that differ from eukaryotes in the following ways.

  1. Cell walls. Prokaryotic cell walls are composed of a polymer of glucose derivatives attached to amino acids. This substance is termed a mucocomplex. Some bacteria have an additional outer layer of a polymer composed of lipid and sugar monomers, which is termed lipopolysaccharide. Many bacteria can secrete polysaccharides that allow them to stick to things. Cell walls of cyanobacteria (blue-green bacteria) tend to be covered with gelatinous material.
    Diagram of (left) a composite bacterium that occur as a single-celled organism that does not have a membrane-bound nucleus, and (right) an electron micrograph of a bacterium with many flagella.

    Figure 17.2 (a) Composite bacterium; (b) An electron micrograph of a bacterium with many flagella.

  2. Plasma membrane. Inside the cell wall of some bacteria is a plasma membrane that coils and loops, creating a unique structure known as the mesosome, which may be important in cell division.
  3. Other internal membranous structures. Some prokaryotes have internal membranous structures containing photosynthetic pigments and related enzymes. Of the aerobic bacteria, those that use molecular oxygen for cellular respiration, some have internal membrane systems containing respiratory enzymes.
  4. Ribosomes. The only organelles that consistently occur in prokaryotes are ribosomes, on which messenger RNA (mRNA) is found. The mRNA carries instructions from the genes to the ribosomes, where protein synthesis occurs. Prokaryotic ribosomes are smaller than those found in eukaryotic cells.
  5. Flagella. Some bacteria are flagellated, meaning they have whip-like appendages, extending singly or in tufts, that propel the cells. The flagella of higher organisms consist of a hollow cylinder containing nine pairs of fibrils surrounding two central fibrils. A bacterial flagellum consists of a single fibril of contractile protein.

PROKARYOTIC DNA

Prokaryotic DNA (deoxyribonucleic acid) differs from eukaryotic DNA in that it is associated with different proteins. It also differs from eukaryotic DNA in that it is not paired, but is circular. Circular DNA molecules consist of only about one-thousandth of the DNA found in eukaryotic cells.

REPRODUCTION

Most bacterial cells reproduce by the simple cell division, binary fission. Neither mitosis nor meiosis ever occur in prokaryotic cells; however, some prokaryotes have a sexual process that transfers material between cells. Occasionally these bacterial cells will transfer DNA to another cell, after which some of the new DNA will replace the recipient's DNA. To date, nothing analogous to a sexual system has been observed in any of the cyanobacteria. There are three methods by which genetic material may be transferred between bacteria.

  1. Transformation. One bacterial cell breaks; its DNA can be taken up by another bacterial cell.
  2. Conjugation. Two bacterial cells come together and are joined by a protein bridge, a pilus, through which DNA fragments pass from cell to cell.
  3. Transduction. A bacteria-attacking virus, known as a bacterial virus, or bacteriophage, carries bacterial DNA from one bacterial cell to another.

Each of the three methods can result in the transfer of DNA fragments from one bacterial cell to another. During the transfer, sometimes homologous DNA fragments, those containing the same type of genetic information, are substituted in the recipient's circular DNA without a net increase or decrease in the total amount of circular DNA.

It is not certain how important genetic recombination is for prokaryotic evolution. However, despite the fact that mutations (inheritable changes in the organism's genetic material) occur infrequently, prokaryotes do have a high degree of genetic variability and therefore evolve quickly. When it exists, their rapid rate of evolution is usually attributed to their great numbers and their incredible reproductive rate, as well as to mutations and genetic recombinations. Knowledge of such DNA recombination led to research using viruses that transmit DNA fragments to other types of organisms. This research led to human gene therapy.

In addition, techniques are now available to edit genes to produce genetically modified organisms (GMOs). CRISPR is one of the tools used to edit genes. CRISPR is an acronym for clusters of regularly interspaced short palindromic repeats. This method enables genetic engineers to edit genes while the genes are still inside living organisms. The technique involves inserting customized genes into strands of DNA without having to cut anything. Scientists are using this method to treat disease and develop products. This technique was originally developed when scientists were studying viruses that infect prokaryotic cells of bacteria and archaea. The techniques that have been developed are now used to inactivate genes, to treat people with genetic disorders, and to change insects so they cannot transmit diseases. In the future, it is hoped that such techniques will lead to new treatments for cancer, heart disease, mental illness, and human immunodeficiency virus (HIV) infection. (See Glossary for more about genetic engineering, CRISPR, genetically modified organisms, GMOs.)

SPORES

Many prokaryotes are also capable of producing a dormant stage known as a spore. Unlike the spores of other organisms, this is not a reproductive unit. Rather, bacterial spores function wholly as units that contain stored food and are highly resistant to desiccation as well as extremely hot and cold temperatures. Bacterial spores have been shown to survive temperatures as cold as −252 °C (−421 °F), and some may be able to live for thousands of years. When conditions become favorable, the bacterial spore germinates into a new cell.

HETEROTROPHIC BACTERIA

Most bacteria selectively absorb organic molecules through the cell wall, rather than manufacturing all their organic nutrients internally, or autotrophically. These heterotrophic bacteria, along with fungi, are important decomposers because they secrete enzymes that digest large organic molecules into smaller molecules that can then be absorbed.

Species of bacteria have been found thriving in just about every habitat, including inside all animals. Many of these bacterial species do no harm, but some do cause disease. Others, bacterial symbionts, are vital to their hosts; some of these live in the gut of their hosts, digesting materials otherwise difficult to digest. In the case of termites, for example, their symbiotic bacteria digest cellulose into smaller molecular constituents that are then absorbed by the cells in the termite's gut. Both the bacteria and the termites benefit from such a relationship.

Bacteria that live inside cells are known as endosymbionts. Some researchers say that the distinction between many cellular organelles and their intracellular symbionts may be a function of when the association first took place and the degree to which the different elements have become interdependent.

A pathogen is any infecting agent, such as a virus, microorganism, or other substance. Some bacteria are pathogenic, or capable of causing disease. Some do so by destroying cells and others by producing toxins, chemicals that can harm the host. Antibiotics, substances produced by some bacteria and fungi, arrest the growth of or destroy the agents of specific infectious diseases. Some antibiotics have been particularly effective in controlling diseases caused by specific species of bacteria. However, since antibiotics were first discovered during World War II, rapid bacterial evolution has favored resistant strains, as in the case of certain strains of E. coli (Escherichia coli) and some strains of sexually transmitted diseases that can no longer be controlled with the antibiotics that previously were effective.

Some bacteria have an episome, which is a DNA segment not attached to the circular DNA. The episome can be transmitted from one individual to another of the same or even different species. Plasmids are very similar to episomes, except plasmids cannot become integrated into its host's DNA. Sometimes the genes involved in drug resistance are located on the episome and are capable of being rapidly transmitted throughout the bacterial population within a relatively brief period after a new drug reaches the market. Some of the most widely known bacterial diseases are syphilis, gonorrhea, botulism, bubonic plague, diphtheria, and tetanus.

Without going into all the different categories of bacteria, it should be said that many bacteriologists classify them into two major subdivisions: the Archaebacteria and the Eubacteria. The Archaebacteria are thought by some to represent the oldest group of organisms still living. Others say they are intermediate between eubacteria and eukaryotes. They are distinctive with regard to their biochemical characteristics. Their membranes have an unusual lipid composition, their transfer RNAs (tRNA) and RNA polymerases are distinctive, and their cell walls do not contain peptidoglycan, which is found in all the Eubacteria. The two major groups of Archaebacteria are the Methanogens and the Thermoacidophiles.

The Eubacteria represent a large assemblage of species that reproduce by binary fission, the process whereby one cell divides asexually into two daughter cells. Eubacteria are often described in terms of their shape. Those that are rod-shaped are called bacilli; spherical Eubacteria are known as cocci; and spiral Eubacteria are spirilla. Some bacteria are gram-negative bacteria and others, gram-positive bacteria; these terms merely describe whether the bacteria in question retain a violet dye used in Gram's staining technique.

There are many other major bacterial groups, including cyanobacteria (blue-green bacteria), as well as the purple, brown, and green sulfur bacteria, sometimes called the pseudomonadales, spirochaetes, actinomycetes, rickettsias, and mycoplasmas.

CHEMOSYNTHETIC BACTERIA

Chemosynthesis is the process by which chemical changes and chemical reactions create new organic compounds. Bacteria are remarkably diverse with regard to their metabolic pathways. Some are aerobic. Others do not require molecular oxygen in their breakdown of food to release energy; these forms are termed anaerobic. Chemoautotrophic bacteria use energy derived from the oxidation of inorganic compounds. Instead of using energy from the sun, as photosynthetic plants do, or energy from other organisms, some chemoautotrophic bacteria utilize chemical energy in sulfur compounds to convert carbon dioxide and water into carbohydrates. Most cyanobacteria have elaborate internal membranes containing photosynthetic pigments that synthesize organic compounds from inorganic materials using light energy. Fossils closely resembling living cyanobacteria have been found that indicate oxygen-producing photosynthesis existed more than 3.3 billion years ago.

Other types of bacteria have the capacity to synthesize high-energy compounds from inorganic materials without needing any light energy. These bacteria trap the energy released when oxidizing inorganic compounds. This form of autotrophic nutrition, where organic nutrients are manufactured from inorganic raw materials, involves the oxidation of various nitrogen and sulfur compounds. Even iron and molecular hydrogen are involved in certain chemosynthetic pathways. A few of the more common reactions are discussed below. Nitrification is the biological oxidation of free ammonia (NH3) or of the ammonium cation (NH4+). This process of oxidizing ammonia or ammonium into nitrite (NO2−) releases energy by the addition of oxygen.

equation

An alternative chemosynthetic process, employed by other bacteria, creates energy through oxidation when oxygen is added to nitrite (NO2−), synthesizing nitrate (NO3−).

equation

Another such process oxidizes sulfur (S) to sulfate (SO4−−).

equation

NITROGEN FIXATION

Nitrogen is an important element in many molecules and in many chemical reactions. Although 78% of the atmosphere is gaseous nitrogen (N2), it occurs in a very unreactive form. Before nitrogen becomes useful to any organism, N2 must first be broken into two atoms. This is done by organisms called nitrogen fixers, most of which are prokaryotic; many live in close association with certain plants. Some of these bacteria live in the root nodules of such plants as the legumes. Chemosynthesis of carbohydrates occurs in nitrite bacteria that oxidize ammonia and in nitrate bacteria that convert nitrous acid.

KEY TERMS

aerobic heterotrophic bacteria
anaerobic lipopolysaccharide
antibiotics mesosome
Archaebacteria methanogens
bacilli Monera
bacteria mucocomplex
bacterial virus mutations
bacteriophage mycoplasmas
binary fission nitrogen fixers
blue-green bacteria pathogen pilus
cell walls plasma membrane
chemosynthesis prokaryotic
cocci prokaryotic DNA
conjugation ribosomes
cyanobacteria spirilla
decomposers spore
endosymbionts symbionts
episome Thermoacidophiles
Eubacteria toxins
eukaryotic DNA transduction
flagella transformation
genetic recombination

SELF-TEST

Multiple-Choice Questions

  1. Genetic material may be transferred between bacteria by __________.
    1. transformation
    2. conjugation
    3. transduction
    4. none of the above
    5. all of the above
  2. Inside the prokaryotic cell wall is a plasma membrane, which in some bacteria coils and loops, creating a unique structure known as the __________, which may be important in cell division.
    1. aster
    2. centromere
    3. centriole
    4. mesosome
    5. ribosome
  3. The only organelles consistently found in prokaryotes are __________.
    1. ribosomes
    2. autosomes
    3. nuclei
    4. chloroplasts
    5. coenocytes
  4. Many prokaryotes are capable of producing a dormant stage, known as a(n) __________, which contains stored food and is highly resistant to desiccation as well as to hot and cold temperatures.
    1. seed
    2. cyst
    3. pollen granule
    4. spore
    5. egg
  5. __________ are substances derived from either fungi or bacteria that arrest the growth of or destroy the agents of specific infectious diseases.
    1. antibiotics
    2. enzymes
    3. lysosomes
    4. fungicides
    5. all of the above
  6. Some bacteria have a(n) __________, which is a DNA segment that is not attached to the circular DNA.
    1. mesosome
    2. ribosome
    3. episome
    4. lysosome
    5. chromosome
  7. The process where one cell divides asexually into two daughter cells is known as __________.
    1. mitosis
    2. meiosis
    3. transformation
    4. transduction
    5. binary fission
  8. Spherical Eubacteria are known as __________.
    1. episomes
    2. centromeres
    3. bacilli
    4. cocci
    5. spirilla

ANSWERS

  1. d
  2. a
  3. d
  4. a
  5. e
  6. c
  7. e
  8. d

Questions to Think About

  1. Give several examples of Monera, and describe the characteristics that all Monera share.
  2. What role do bacteria play in most ecosystems?
  3. Describe different modes of bacterial reproduction.
  4. How do heterotrophic and chemosynthetic bacteria differ?
  5. What is nitrogen fixation, why is it important, and how does it occur?