Chapter Capsule

Names for Procaryotes: Bacteria, Archaea, Monera, Archaebacteria, Eubacteria, Schizomycetes (the last three are older)

I. Overall Morphology

A. Procaryotic cells lack a nucleus, mitochondria, chloroplasts, or other membranous organelles.
B. The outer covering is a cell envelope composed of glycocalyx (slime layer, capsule), cell walls (in most), and cell membrane with mesosomes. Gram stain differentiates two types of cells on the basis of wall structure: Gram-positive cells have a thick layer of peptidoglycan; gram-negative cells have an outer membrane and a thin layer of peptidoglycan. The diameter of most bacteria is 1–5 mm.
C. Special appendages: Bacterial flagella that provide motility are common; fimbriae are important in cell adhesion, and pili in genetic recombination.
D. Genetic material is in a chromosome and plasmids; ribosomes synthesize proteins.
E. Other special structures include endospores that are highly resistant survival cells and inclusions and granules for storing nutrients.
F. Most bacteria are unicellular and form colonies on solid nutrients. Their shapes are distinct, including cocci, bacilli, spirilla, and spirochetes. Certain groups can assume long filaments; some show variations in shape, or pleomorphism.
G. Various arrangements based upon mode of cell division are termed diplo-, strepto-, staphylo-, or palisade.

II. Nutritional/Habitat Requirements

A. A large number of bacteria are heterotrophs that require an organic carbon source derived from dead organisms by absorption or from a live host (parasites). Several groups are capable of synthesizing nutrients using sunlight (photosynthetic).
B. Bacteria can be aerobic, facultative, or anaerobic with respect to oxygen, and most exist between 10° and 40°C. Some species live in very cold habitats, even at subfreezing temperatures, and some at high temperatures (up to 250°C).

III. Reproduction and Life Cycles

Bacteria generally have a single chromosome; reproduce primarily by asexual means, including budding and simple binary fission; and have no mitosis. Genetic exchange can occur through conjugation. The life cycle is complex in sporeformers, gliding bacteria, and appendaged bacteria.

IV. Identification

Bacteria are identified by means of microscopic morphology (Gram stain, shape, special structures), macroscopic morphology, and biochemical, molecular, and genetic characteristics.

V. Classification of Major Groups

One system divides the Kingdom Procaryotae into divisions based on the type of cell wall.

A. Gracilicutes: The largest group; contains bacteria with gram-negative cell walls; includes several medically significant microbes, such as Salmonella, Shigella, and other intestinal pathogens, the Rickettsias, and the agents of gonorrhea and syphilis. The photosynthetic, gliding, sheathed, and appendaged bacteria are also in this group.
B. Firmicutes: Gram-positive cells; examples of pathogens are in the genera Streptococcus, Staphylococcus, Mycobacterium (tuberculosis), and Clostridium (tetanus).
C. Tenericutes: Cells without cell walls; primarily in the genus Mycoplasma (one type causes pneumonia).
D. Mendosicutes (also Archaea): Unusual procaryotes different from bacteria in genetics and some structural features; they are adapted to extremes of habitat (salt, temperature, and nutrients). Halophiles live in salt concentrations of 10–30%; hyperthermophiles can live at temperatures of 60°–110°C or more. Methanogens live without oxygen and produce methane gas. No known medically important species.

VI. Ecological Importance

A. Because procaryotes are highly adaptable as a group, they are found nearly everywhere: water, soil, air, dust, food, deep sea, plants, animals, swamps, hot springs, North and South Poles.
B. They are important as decomposers of organic matter and play a role in the cycles of nitrogen, phosphorus, sulfur, and carbon.
C. The cyanobacteria contribute oxygen to the atmosphere through photosynthesis.

VII. Economic Importance

A. Bacteria are used in a number of industries, including food, drugs (production of antibiotics, vaccines, and other medicines), and biotechnology (manipulation of bacteria for making large quantities of hormones, enzymes, and other proteins).
B. Bacteria are responsible for spoilage of food and vegetables. Many plant pathogens adversely affect the agricultural industry.

VIII. Medical Importance

Bacteria are very common pathogens of humans. Approximately 200 species are known to cause disease in humans, and many normally inhabit the bodies of humans and other animals. Infections can be treated with antibiotics.

IX. Unique Features

A. Bacteria and archaea are the smallest cells in existence.
B. Bacteria and archaea have procaryotic structure, unique shapes and arrangements, and lack traditional sexual reproduction and mitosis.
C. The cell wall of most contains peptidoglycan, and the cytoplasm of some contains mesosomes.
D. Other unique features are a single chromosome and 70S ribosomes.

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