Biomedical Engineering Reference
In-Depth Information
With a few exceptions, the Gram-positive cells have a single membrane,
and the Gram-negative cells have an inner plus an outer membrane. Most of the
Gram-positive cells that are round or rod-shaped fall into Class Bacilli
(Chapter 12) or Class Clostridia (Chapter 12). The mollicutes (Chapter 11) are
bacteria that have no cell wall to absorb or exclude the Gram stain, but they are
traditionally counted among the Gram-positive bacteria because they lack an
outer membrane and share a close ancestor with Class Bacilli and Class
Clostridia. Among the filamentous bacteria, Class Actinobacteria (Chapter 14)
is also Gram-positive.
All bacterial species with an inner and outer membrane are Gram-negative,
including all members of Class Proteobacteria (Chapters 5
8). Class
Proteobacteria accounts for the majority of Gram-negative pathogens. Other
Gram-negative classes are Class Chlamydiae (Chapter 13), Class Spirochaetes
(Chapter 9), Class Fusobacteria (Chapter 10), and Class Bacterioidetes
(Chapter 10).
Endotoxins are structural molecules found on bacterial cells, that produce
generalized inflammatory reactions when injected into humans (e.g., fever,
drop in blood pressure, activation of the inflammation and blood cascades).
Most endotoxins are lipopolysaccharides found on the outer membrane of
bacterial cells; hence, most endotoxins come from Gram-negative bacteria.
A classic example of an endotoxin is found in meningococcemia, due to
infection with Neisseria meningitidis (Beta Proteobacteria, Chapter 6).
The G
C ratio is a measurement of the proportion of Guanine and Cytosine
nucleotide bases in an organism's genome. Because all four nucleotide bases
(Guanine, Cytosine, Adenine, and Thymidine) are essential constituents of the
genetic code, the G
1
C ratio of most organisms lies in a somewhat restricted
range, close to 50%. For certain classes of Gram-positive bacteria, the G
1
C
ratio is used as a useful taxonomic feature. Members of Class Actinobacteria
(filamentous cells) have a high G
1
C ratio, with the ratio of some species
exceeding 70%. Members of other Gram-positive classes (Class Bacilli and
Class Clostridia) tend to have low G
1
C ratios (under 50%), though exceptional
1
species of Class Bacilli have G
C ratios exceeding 50% (e.g., Bacillus thermo-
catenulatus). Isolated DNA with a high G
1
C ratio happens to be more
1
stable than low G
C DNA, and is more resistant to high temperatures. The sig-
1
nificance of high G
C content to living organisms, however, is not understood
1
at this time.
Biologists know a great deal about bacterial organisms: their metabolism,
biosynthetic capabilities, virulence factors, the sequence of their genomes,
how they replicate, and so on (see Glossary item, Virulence factor). Their
origin is still somewhat of a mystery. We will re-visit this mystery in
Chapter 38 (Overview of Viruses), when we attempt to understand biogenic
molecules and their role in the origins of viruses, bacteria, and eukaryotes.