The relationship between two living organisms can be
classified as parasitic, symbiotic, or commensal.1–3 This same
classification scheme can be used to describe relationships
between microorganisms and more complex living organisms
that act as hosts. The term parasite is used here in its broad
sense to mean a microorganism interacting with another organism
(either vertebrate or invertebrate) in the same ecologic
niche.
The following definitions are used in this chapter:
Parasitism: Association between two different organisms
wherein one benefits at the expense of the other. All infectious
agents causing illness belong to this category.
Commensalism: Association between two organisms in which
one derives benefit from the other without causing it any
harm. This intermediate category is not uniformly accepted.
Often, upon detailed analysis, the relationship turns out to be
either parasitic or symbiotic.2
Symbiosis or mutualism: Both organisms benefit from the
relationship. The type of relationship also depends on host
factors. For example, bacteria normally inhabiting the
bowel live in an apparent commensal or (by inhibiting
potential pathogens) symbiotic relationship with humans.
However, in cases of cirrhosis with consequent hepatic
insufficiency, bacteria can become a dangerous source
of ammonia that leads to hepatic encephalopathy. A commensal
relationship can be transformed into a potentially
harmful one. In malnourished people with borderline
deficiencies of B-complex vitamins, clinical beriberi can
be triggered by administration of broad-spectrum antibiotics.
Normally, in this situation bacteria play a symbiotic
role by supplying a significant amount of B-complex
vitamins.2
MICROBIAL FACTORS
Principles of Microbial Evolution and Classification
The earth is approximately 4.5 to 5 billion years old.
There is good fossil evidence of microbial life approximately
3.5 billion years ago. Microbial life (stromatolites) was mostly
photosynthetic, unicellular, and anaerobic.1,4 Eukaryotes, bacteria,
and archaea evolved from a still hypothetical universal
common ancestor.5–7 Eukaryotes then evolved into protozoans,
metazoans, plants, and animals, as we know them today.
Moreover, there is strong evidence that primitive eukaryotic
cells established relationships with bacterial organisms that
later evolved into cytoplasmic organelles, such as chloroplasts
in plants and mitochondria in animals.8
To put things into perspective, approximately five-sixths
of the history of life on Earth has been exclusively microbial.
Human beings appeared on the planet only 2 million years
ago as very late newcomers to the biosphere. Life was initially
anaerobic, but with the appearance of photosynthetic organisms
and chloroplasts, oxygen was released into the atmosphere for
the first time.9 Radiation in the upper atmosphere created the
ozone layer from molecular oxygen, which then shielded the
earth’s surface from dangerous radiation. Nucleic acids were
therefore protected from harmful mutations. Organisms had
to evolve to survive in the presence of oxygen. A few of the
ancient anaerobes were able to survive in the highly oxidant
atmosphere, and they represent the anaerobes as we know
them today.
The phylogeny of living organisms is based on molecular
approaches, particularly analysis of ribosomal RNA.5,6,10
Because of the antiquity of the protein synthesis machinery,
these molecules appear to be excellent evolutionary clocks.
For prokaryotes, the 16S subunit of ribosomes appears to be
the most useful for classification purposes. The number of
microbes in the world is tremendous, and relatively few are
pathogenic to humans.
Viruses deserve special comment because of their molecular
simplicity and at the same time their importance as human
pathogens and as possible agents of hereditary changes and
cancer