Biomedical Engineering Reference
In-Depth Information
rational basis for splitting a group of organisms into differently named species
required, at the very least, heritable functional or morphologic differences
among the members of the group. Gene sequencing has changed the rules for
assigning new species. For example, various organisms with subtle differences
from Bacteroides fragilis have been elevated to the level of species based
on DNA homology studies. These include Bacteroides distasonis, Bacteroides
ovatus, Bacteroides thetaiotaomicron,andBacteroides vulgatus [10].
For the sake of discussion, let us accept that there are 50 million species
of organisms on earth (a gross underestimate by some accounts). There have
been about 1400 pathogenic organisms reported in the medical literature.
This means that if you should stumble randomly upon a member of one
of the species of life on earth, the probability that it is an infectious pathogen
is about 0.000028.
Of the approximately 1400 infectious organisms that have been
recorded somewhere in the medical literature, the vast majority of these are
“case report” items; instances of diseases that have, to the best of anyone's
knowledge, occurred once or a handful of times. They are important to
epidemiologists because today's object of medical curiosity may emerge as
tomorrow's global epidemic. Very few of these ultra-rare causes of human
disease ever gain entry to a clinical microbiology textbook. Textbooks,
even the most comprehensive, cover about three hundred organisms
(excluding viruses) that are considered clinically important. In this topic, we
will cover about 350 living organisms and about 150 viruses within the main
text. The Appendix lists about 1400 organisms (common, rare, and ultra-rare).
This may seem like way too much to learn, but do not despair. Infectious
agents fall into a scant 40 biological classes (32 classes of living organisms
plus seven classes of viruses plus one class of prions). When you've learned
the basic biology of the major taxonomic divisions that contain all the infec-
tious organisms, you will understand the fundamental biological features that
characterize every clinical organism. Almost everything else you need to learn
can be acquired from web resources.
