Biomedical Engineering Reference
In-Depth Information
2.1.5. Prokaryotes
The sizes of most prokaryotes vary from 0.5 to 3
m in equivalent radius. Different species
have different shapes, such as spherical or coccus (e.g. Staphylococci), cylindrical or bacillus
(E. coli), or spiral or spirillum (Rhodospirillum). Prokaryotic cells grow rapidly, with typical
doubling times of half an hour to several hours. Also, prokaryotes can utilize a variety of
nutrients as carbon sources, including carbohydrates, hydrocarbons, proteins, and CO 2 .
m
2.1.5.1. Eubacteria
The Eubacteria can be divided into several different groups. One distinction is based on
the gram stain (developed by Hans Christian Gram in 1884). The staining procedure first
requires fixing the cells by heating. The basic dye, crystal violet, is added; all bacteria will
stain purple. Next, iodine is added, followed by the addition of ethanol. Gram-positive cells
remain purple, while gram-negative cells become colorless. Finally, counterstaining with
safranin leaves gram-positive cells purple, while gram-negative cells turn red. Cell reactions
to the gram stain reveal intrinsic differences in the structure of the cell envelope.
A typical gram-negative cell is E. coli. It has an outer membrane supported by a thin pepti-
doglycan layer, as shown in Fig. 2.5 . Peptidoglycan is a complex polysaccharide with amino
acids and forms a structure somewhat analogous to a chain-link fence. A second membrane
(the inner or cytoplasmic membrane) exists and is separated from the outer membrane by the
periplasmic space. The cytoplasmic membrane contains about 50% protein, 30% lipids, and
20% carbohydrates. The cell envelope serves to retain important cellular compounds and
to preferentially exclude undesirable compounds in the environment. Loss of membrane
integrity leads to cell lysis (cells breaking open) and cell death. The cell envelope is crucial
to the transport of selected material in and out of the cell.
A typical gram-positive cell is Bacillus subtilis. Gram-positive cells do not have an outer
membrane. Rather, they have a very thick, rigid cell wall withmultiple layers of peptidoglycan.
Gram-positive cells also contain teichoic acids covalently bonded to the peptidoglycan. Because
gram-positive bacteria have only a cytoplasmic membrane, they are better suited to excretion of
proteins. Excretion is technologically advantageous when the protein is a desired product.
Some bacteria are neither gram-positive nor gram-negative. For example, Mycoplasma has no
cell walls. These bacteria are important not only clinically (e.g. primary atypical pneumonia) but
also because they commonly contaminate media used industrially for animal cell culture.
Actinomycetes are bacteria but morphologically resemble molds with their long and
highly branched hyphae. However, the lack of a nuclear membrane and the composition
of the cell wall require classification as bacteria. Actinomycetes are important sources of anti-
biotics. Certain Actinomycetes possess amylolytic and cellulolytic enzymes and are effective
in the enzymatic hydrolysis of starch and cellulose. Actinomyces, Thermomonospora, and Strep-
tomyces are examples of genera belonging to this group.
Other distinctions within the eubacteria can be made based on cellular nutrition and
energy metabolism. One important example is photosynthesis. Cyanobacteria (formerly
called blue-green algae) have chlorophyll and fix CO 2 into sugars. Anoxygenic photosyn-
thetic bacteria (the purple and green bacteria) have light-gathering pigments called bacterio-
chlorophyll. Unlike true photosynthesis, the purple and green bacteria do not obtain reduction
energy from the splitting of water and do not form oxygen.
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