Biomedical Engineering Reference
In-Depth Information
cell walls (and the genes coding for cell walls), mollicutes have shrunk to
about 0.2 microns; and they have a remarkably small genome (under 1000
kilobases).
The mollicutes, with no cell wall to absorb the Gram stain, are technic-
ally not Gram-positive. Nonetheless, the mollicutes are usually counted
among the Gram-positive organisms, as they lack the outer membrane that
is characteristic of Gram-negative organisms. Sequence similarities for ribo-
somal RNA and a low G
C ratio (about 35%), suggest that the mollicutes
are close relatives of Class Bacilli and Class Clostridia (Chapter 12). For this
reason, Class Mollicutes is often included, along with Class Bacilli and
Class Clostridia, as a subclass of Class Firmicutes, the Gram-positive low
G
1
C bacteria.
The mollicutes have exempted themselves from the strict, universal code
that controls the translation of RNA into protein. Virtually every organism
on earth uses the triplet codon UGA to code for “stop,” thus signaling an
interruption in RNA translation. Mollicutes are the exception, using UGA as
a codon for tryptophan. It has been suggested that the low G
1
C content of
mollicutes encourages the evolution of synonymous codons containing the
overly abundant A (Adenine) or U (Uracil). In cells other than mollicutes,
there are three synonymous “stop” codons: UAA, UAG, and UGA; while
there is only one tryptophan codon: UGG. Mollicutes supplement UGG,
the normal codon for tryptophan, with a codon that contains adenine and
uracil, UGA.
Though it is relatively easy to detect the presence of mollicute species in
human tissues, using PCR (polymerase chain reaction) techniques, these
organisms cannot be cultured with any regular success. Consequently, med-
ical scientists have implicated numerous members of Class Mollicutes as
human pathogens, without fulfilling all of the rigorous studies that fully
establish disease causation (i.e., they have not produced disease in humans
or animals by inoculating the cultured organisms). In this chapter, we cover
some of the less controversial mollicute pathogens. There are three genera of
Class Mollicutes that cause diseases in humans: Erysipelothrix, Mycoplasma,
and Ureoplasma.
1
Mollicutes
Anaeroplasmatales
*Erysipelothrix
Genus Erysipelothrix contains one infectious species; Erysipelothrix rhusio-
pathiae, the cause of erysipeloid, a type of cellulitis (subcutaneous infection).
Erysipeloid is usually a mild condition, typically occurring on the hands of
workers who are exposed to the bacteria when they handle infected fish
or meat. Students should not confuse erysipeloid with the similar-sounding
disease, erysipelas. Both erysipeloid and erysipelas are types of cellulitis.
Erysipelas is caused by members of Genus Streptococcus (Class Bacilli,
