Biomedical Engineering Reference
In-Depth Information
Class Proteobacteria contains more human pathogens than any other
bacterial class. The human pathogens fall into four biologically distinctive,
phylogenetic subclasses (alpha, beta, gamma, and epsilon), discovered
through ribosomal RNA sequence analyses [21]. Woese had his own name
for Class Proteobacteria; Purple bacteria, based on their descent from a com-
mon ancestor that was capable of a photochemical reaction that yielded a
photochrome that conferred a purple tinge to bacterial colonies. In addition,
all members of Class Proteobacteria are Gram-negative. An outer cell mem-
brane encloses the cell wall and excludes the Gram stain. Class Proteobacteria
accounts for the majority of the Gram-negative bacteria.
The Alpha Proteobacteria are characterized by their small size, their Gram
negativity, and their intimate associations with eukaryotic cells. The Alpha
Proteobacteria live as sybionts, endosymbionts, or as intracellular parasites.
This close relationship between Alpha Proteobacteria and Class Eukaryota
may extend back to the very first eukaryotic cell. Based on sequence similar-
ities between the Alpha Proteobacteria and eukaryotic mitochondria, it has
been proposed that eukaryotic mitochondria evolved from an endosymbiotic
member of Class Alpha Proteobacteria.
There are two major subclasses of Class Alpha Proteobacteria: Rhizobiales
and Rickettsiales. Class Rhizobiales contains nitrogen-fixing bacteria that live
in a symbiotic relationship with plant roots. Without class Rhizobiales, life
on earth, as we know it, would cease to exist. Class Rhizobiales contains two
human pathogenic genera: Bartonella and Brucella.
Alpha Proteobacteria
Rhizobiales
Bartonellaceae
*Bartonella
Brucellaceae
*Brucella
Genus Bartonella, formerly known as Rochalimaea, is a group of facultative
intracellular organisms that produce a wide range of diseases, but which
seem to share a common life cycle. The pathogenic species of Genus
Bartonella are injected into humans from the bite of a blood-feeding vector:
fleas, lice, sandflies, and possibly ticks. The Bartonella organism infects the
endothelial cells that line blood vessels. Later, the organism leaves the endo-
thelial cell and infects erythrocytes. A blood-feeding vector extracts infected
red blood cells from an infected human or from an animal reservoir. The
cycle repeats.
The diseases caused by Bartonella species vary, to some extent, on the
most favored host cell (endothelial cell, erythrocyte, lymphocyte, monocyte,
or granulocyte), on the numbers of infectious organisms, on the chronicity
of the infection, and on the pathogenic properties of individual species of
the organism (e.g., does it cause red blood cell lysis?). Though Bartonella
